Magic Lantern Forum

Using Magic Lantern => Tutorials and Creative Uses => Topic started by: Audionut on May 29, 2014, 05:41:00 PM

Title: RAW based exposure feedback
Post by: Audionut on May 29, 2014, 05:41:00 PM
Magic Lantern contains a number of exposure feedback features, based from raw data.  This is very beneficial, as regular Canon exposure feedback is subject to picture style, and other JPG based processing.



These features can be found in the Overlay menu.

(https://s1.postimg.cc/6ggsz6boun/Global_draw.png)


They work in both Movie mode and Photo mode Live View, and in the Quick Image review. 

In Movie mode Live View, you must load one of the raw recording modules and have it activated for raw based exposure feedback to function.
In Photo mode (Live View or not), picture quality must be set to raw in Canon menu, and in Photo mode Live View, ExpSim must be enabled.

Spotmeter (#post_Spotmeter)
Histogram (#post_Histogram)
Simplified Histobar (#post_Histobar)
Zebras (#post_Zebras)
Dual ISO Zebras (#post_dualzebras)
RAW vs JPG (#post_JPG)



Spotmeter

(https://s1.postimg.cc/7jgia29vlr/Spotmeter.png)

The spotmeter is a tool used to meter a particular area of the scene.  For instance, you may want to meter a particular area of your scene, as a highlight, a midtone, or whatever.  The raw based spotmeter in ML works in EV (Exposure Value (http://en.wikipedia.org/wiki/Exposure_value)) units, and represents the EV from saturation (overexposure).

Here is a teddy bear, showing the midtone patch metered for a midtone, checking the white level at this metering, and then Exposing to the Right.
(https://s1.postimg.cc/77emw8deq7/Spot_Midtone.jpg) (https://s1.postimg.cc/8l1609lw5r/Spot_Highlight.jpg) (https://s1.postimg.cc/32j1k4ilj3/Spot_ETTR.jpg)



Histogram

(https://s1.postimg.cc/59xhqkpsa7/Histogram.png)

The histogram provides a graphical representation of the exposure of the entire scene.

(https://s1.postimg.cc/60vbnmdcwf/Histogram.jpg)

The colors in the histogram, represent the color channel in the camera (Red, Green, Blue).
You will also notice, that ML displays Cyan, Magenta and Yellow.  If you look at the color chart below, you can see that Yellow falls in between Green and Red, and hence, Yellow represents data in both the Green and Red channels.  Cyan being the data from Green and Blue channels, and Magenta being the data from Blue and Red channels.  White indicates data from all color channels.

The white vertical lines at the rear of the histogram display, represent full stop EV.  So the very right hand side of the histogram represents overexposure.  The first line from the right hand side, represents 1 EV (a full stop) from overexposure, the next line is another 1 EV (2 EV total) from overexposure, and so on.

(http://upload.wikimedia.org/wikipedia/commons/3/3b/CIE1931xy_blank.svg)


When the scene contains overexposure, the histogram will provide warnings.

(https://s1.postimg.cc/1e9omxht9r/Histo_Over.jpg)

The colored circles in the histogram, warn of overexposure in the color channel.  Small circles show a very small amount of exposure, with the circles increasing in size.  When at least 1 % of the pixels of a channel are overexposed, the numbers inside these colored circles, describe the percentage of pixels that have been overexposed in that color channel.  In the above example. 1% of red pixels have been overexposed, 10% of green pixels and 2% of Blue pixels.

Note:  This information is derived from the downsized Live View stream.  So the percentages reported, may not be an extremely accurate representation of the actual percentage, but provides useful information nonetheless.



The histogram in ML also contains Exposure to the Right (http://en.wikipedia.org/wiki/Exposing_to_the_right) information.  This feedback show the user how far they can Expose to the Right before overexposing.

(https://s1.postimg.cc/4xiuykuycf/ETTR_hint.png)

Take this scene metered with Canon Av mode.
(https://s1.postimg.cc/5movwricfj/Under.jpg)

Here we are being given feedback that suggests we can push the exposure 1.5 EV (stops) before overexposure.
And in the two images below, on the left hand side, the scene has been Exposed to the Right, and on the right hand side, the scene has been overexposed.

(https://s1.postimg.cc/8z7lr4oeov/ETTR.jpg) (https://s1.postimg.cc/64exlcg2u7/Over.jpg)

Note:  Due to inconsistencies in the original firmware design, determining the correct overexposure point is extremely difficult.  You should be very wary when the histogram reads from -0.4EV through to (slight) overexposure.  There may be some slight overexposure when none is reported, or vice-versa, there may be no overexposure even though some has been reported.  If you know the true saturation value for your sensor at the shot settings, you may wish to use raw_diag (https://builds.magiclantern.fm/modules.html#raw_diag) if you like to nitpick.

Otherwise, to be certain of no overexposure, target a ETTR hint > -0.4EV.

Further discussion. (http://www.magiclantern.fm/forum/index.php?topic=20579.msg192432#msg192432)



The histogram in ML also contains Dynamic Range (http://en.wikipedia.org/wiki/Dynamic_range) feedback.  This describes the ratio between the reported white level, and the Noise Floor (http://en.wikipedia.org/wiki/Noise_floor) of the camera, and is only very accurate when the image contains overexposed pixels.

(https://s1.postimg.cc/5f8wn5utwv/DR_hint.png)

(https://s1.postimg.cc/67ls4wdd33/Histo_DR.jpg)

Note:  The brown area in the histogram represents this noise floor.



Simplified Histobar

For LiveView work, there is also the Simplified Histobar.

(https://s1.postimg.cc/9gxnfpuab3/Histobar.png)

The Histobar is shown here at the top left of the liveview screen.

(https://s1.postimg.cc/8s4dvpcbwv/Histobar1.png)

Red indicates overexposure, blue indicates extreme underexposure, and the yellow bar represents the midpoint.
The blue bar increases in size with increasing underexposure.  So the image above shows a small amount of the scene is extremely underexposed, and a full bar would indicate a large amount of the scene is extremely underexposed.

The midtone point is the median of the scene.  For further information, see this post (http://www.magiclantern.fm/forum/index.php?topic=8539.msg80044#msg80044) by a1ex.



Zebras

(https://s1.postimg.cc/6ggsz6eh5r/Zebras.png)

Zebras provide a graphical overlay of the areas in the scene which are overexposed, and underexposed.

(https://s1.postimg.cc/1htaknr6en/Zebra1.png) (https://s1.postimg.cc/2dd0lxt9zz/97pwi.gif)

And a scene showing zebras/histogram, with overexposure, and underexposure.

(https://s1.postimg.cc/96atmkc24v/Histo_Zebras.jpg)

As with the histogram, the color displayed by the zebras, represent the color channel in the camera that is overexposed.  Again, a Cyan reading is describing overexposure in both the Green, and the Blue channels, Yellow describing both Green and Red channels, and Magenta describing both Red and Blue channels.

Unlike the histogram, black describes overexposure in all color channels, and white describes underexposure.  This underexposure point describes any data that is within 1 EV of the camera noise floor.


Update:  dmilligan added some underexposure adjustment to the zebras.
(https://s1.postimg.cc/9b9h4ytg7j/Zebra_under.png)

This allows you to choose at what exposure the underexposure zebras will be displayed.  0 EV means that the zebras will be displayed where the amount of noise in the image, is equal to the amount of signal in the image (the noise floor). 
A general consensus amongst photographers, is that the limit for acceptable amount of noise in images, is 2 EV above this noise floor.



Dual ISO Zebras

When using dual ISO (http://www.magiclantern.fm/forum/index.php?topic=7139.0), there are dual exposures and the zebras reflect this.  With dual ISO there are two types of zebras. 
Full/solid zebras which work exactly like normal zebras, and describe overexposure in both ISOs
And half/stripe zebras which represent overexposure only in the higher ISO.

Looking at this image below from DeafEyeJedi, we can see there are solid zebras (black/yellow/green) in the lamp shade, and solid zebras (green/cyan) on the laptop screen.

(https://s1.postimg.cc/3uvx1v44q7/Full_Zebras.png)

So both ISOs being used in dual ISO have been overexposed in these areas.  The other stripe zebras are showing where only the higher ISO has been overexposed, in this case, blue, yellow and green on the wall behind.

The image below shows the same scene with only the higher ISO being overexposed, and only half/stripe zebras being displayed.

(https://s1.postimg.cc/4clyqfzpz3/Half_Zebras.png)



RAW vs JPG

So, what's the big deal with raw based feedback.  JPG processing is designed simply to produce good looking images directly from the camera.  Since all of the Canon exposure feedback is based from this JPG processing (even when shooting raw), it suffers from inaccuracy.  Raw data is not affected by picture style, white balance, or any other JPG based processing.  So when using raw data for exposure feedback, we have a very accurate representation of the actual raw data.

As an example, here is a scene showing the accuracy differences. 

The image on the left is showing Canon exposure feedback, and the image on the right is the raw histogram from Magic Lantern.
(https://s1.postimg.cc/7zridyrnkv/JPG2.jpg) (https://s1.postimg.cc/3nsp6f4h6n/JPG1.jpg)

Notice how the Canon histogram is showing a large amount of overexposure in the red channel.  However, the actual raw data still contains some slight headroom.
Title: Re: RAW based exposure feedback
Post by: jose_ugs on August 15, 2014, 11:37:13 PM
Cool find! Good one Audionut!
Title: Re: RAW based exposure feedback
Post by: 88oak88 on August 23, 2014, 04:03:29 AM
loved this post'm kinda newbie in this area, and some post are very technical and does not reach the layman User, much of ML tools are unused for lack of an explanation like this too simply, thank you.
Title: Re: RAW based exposure feedback
Post by: RTLdan on September 04, 2014, 05:08:53 AM
Great info!
 Some of these tools are very common for experienced users, but this is the kind of info that would have helped a lot when I was first learning how to read the meters!

Is it safe to assume that since it is not listed as a RAW metering tool in this post, that the waveform monitor is jpeg based?

Thanks!
-Daniel
Title: Re: RAW based exposure feedback
Post by: Audionut on September 04, 2014, 05:50:13 AM
Correct.  Only the spotmeter, histogram and zebras work with raw data.
Title: Re: RAW based exposure feedback
Post by: a1ex on September 04, 2014, 02:29:15 PM
For this reason, Stevefal proposed to hide the jpeg-based exposure tools (like false colors, waveform, vectorscope) when shooting RAW. It should create less confusion, though some people may wonder where did these features go.
Title: Re: RAW based exposure feedback
Post by: Audionut on September 04, 2014, 03:24:31 PM
And less clutter.  Yamlmo removed  :P
Title: Re: RAW based exposure feedback
Post by: Kharak on September 04, 2014, 08:48:01 PM
Correct.  Only the spotmeter, histogram and zebras work with raw data.

I was under the impression that the waveform was RAW too.. Now looking at the menu again I see it doesn't mention anything about RAW.

This changes everything, I've been basing all my exposure on the Waveform
Title: Re: RAW based exposure feedback
Post by: RTLdan on September 05, 2014, 02:00:08 AM
Coming from the video side of things, I too had been using the waveform monitor almost exclusively while shooting h.264... and the habit carried over into taking photos with RAW. I guess I need to retrain myself.

I've been experimenting lately with using the spotmeter to expose a subjects skin (caucasian skin) at between 50% and 70%. This seems to provide a decently exposed image for me -- although it only takes into account the brightest spot of the subject, of course.
Title: Re: RAW based exposure feedback
Post by: Audionut on September 05, 2014, 03:02:12 AM
If the spotmeter is reporting in percentages, it's working in jpeg mode.  Look at the options in the OP, you need to set it to raw mode.  Here, you probably want evenly lit Caucasian skin around -2 EV.  Of course, this only exposes for that spot, and you may want to use zebras to get an idea of the exposure for the entire scene.  If the zebra overlay isn't an issue while shooting, I'd probably just stick with zebras.

If you're shooting with controlled lighting conditions, then I would absolutely use zebras for exposure, then disable the overlay, and shoot away.
Title: Re: RAW based exposure feedback
Post by: Kharak on September 05, 2014, 10:35:30 AM
So is there no chance for the waveform to be developed in to RAW?

And I was laughing at the fs700 waveform :)
Title: Re: RAW based exposure feedback
Post by: a1ex on September 05, 2014, 11:03:33 AM
The hardest part is finding a place for it in the menu :P
Title: Re: RAW based exposure feedback
Post by: Kharak on January 25, 2015, 11:01:04 PM



The histogram in ML also contains Dynamic Range (http://en.wikipedia.org/wiki/Dynamic_range) feedback.  This describes the ratio between the reported white level, and the Noise Floor (http://en.wikipedia.org/wiki/Noise_floor) of the camera, and is only very accurate when the image contains overexposed pixels.

(https://dl.dropboxusercontent.com/u/34113196/ML/Tutorial/RAW_Exposure_Feedback/Histo_DR.jpg)

Note:  The brown area in the histogram represents this noise floor.





I don't really understand the part about Dynamic Range Feedback..

I don't see or don't know how to see the noise floor in my histogram, could you be more specific Audionut? I am sure I am seeing it, but not sure what I am looking at.
Title: Re: RAW based exposure feedback
Post by: Audionut on January 26, 2015, 03:29:41 AM
Two images shot @ ISO 100.  One has overexposed pixels, the other does not.

(https://dl.dropboxusercontent.com/u/34113196/ML/Tutorial/RAW_Exposure_Feedback/ISO100_noise.png) (https://dl.dropboxusercontent.com/u/34113196/ML/Tutorial/RAW_Exposure_Feedback/DynamicRange.png)



Notice the brown shaded area to the left of the histogram.  This is the noise floor.  Here is another shot @ ISO 6400, showing differences.
(https://dl.dropboxusercontent.com/u/34113196/ML/Tutorial/RAW_Exposure_Feedback/ISO6400noise.png)


The noise floor doesn't actually raise per se as might be concluded by looking at the histogram.  Instead, the range between overexposure (clipping, saturation (the right hand side of the histogram)) and the noise floor decreases.

Personally, rather then using the histogram to judge image details in the noise floor, I use zebras.  As the image details become closer to the noise floor of the camera, it contain more noise.  dmilligan added adjustment to the zebras, so that you can control your own personal noise preferences.

Update:  dmilligan added some underexposure adjustment to the zebras.
(https://dl.dropboxusercontent.com/u/34113196/ML/Tutorial/flicker_free/Zebra_under.png)

2 EV is an general recommendation for acceptable noise in the image.  0 EV means only show zebras where the noise in the image equals the detail in the image (low quality, very noisy), and higher numbers mean less noise.  Generally I shoot with underexposure zebras at 3 EV.  This means, if there are no zebras in the image, I know I have high quality pixels with very little noise.  The other advantage to zebras, is that you have visual feedback on the display of where in the actual image, overexposure, or noise is occurring.
Title: Re: RAW based exposure feedback
Post by: Kharak on January 27, 2015, 02:05:09 PM
Thank you for clarifying.

I always use RAW zebras for exposure feedback, but want to understand the other features as well. How does the Histogram measure the noise floor?

is the noise floor absolute useless underexposure?

Title: Re: RAW based exposure feedback
Post by: Audionut on January 27, 2015, 02:54:45 PM
It's been a while since I looked at the code, but iirc, based on the noise in the optical black area of the image.

http://en.wikipedia.org/wiki/Noise_floor
Quote
In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where noise is defined as any signal other than the one being monitored.

There are some factors that change the noise floor of the camera.  Temperature is an external component that changes the noise levels.  The hotter the sensor becomes, the higher the noise level and vice-versa.

On Canon cameras, ISO also adjusts the noise floor.  With increased ISO (increased sensitivity), the camera is better able to distinguish between noise signal, and wanted signal, compared to lower ISO.  In essence, the noise floor is lowered.

Underexposure isn't really related to the noise floor, though you can underexpose the image to the point where the wanted signal becomes berried in noise.  Think of the noise floor as the point of no return.  If the histogram shows detail in the brown shaded area, the image detail is gone.  There is simply to much noise and not enough (wanted) detail.  In fact as mentioned previously, 2 EV (stops) above the noise floor is considered the lower limit of noise acceptability.  That is to say, any detail in the noise floor, and through to 2 EV above the noise floor contains to much noise.  So in essence, if your camera is rated as having 11 stops of dynamic range, only 9 stops is useable, the rest is noise.

If you want to know more about noise levels in Canon cameras, I suggest to spend some time reading this thread (http://www.magiclantern.fm/forum/index.php?topic=10111.0).  Check out the Any recommended reading? section in the opening post of that thread to help understand what is being researched in that thread.
Title: Re: RAW based exposure feedback
Post by: a1ex on January 27, 2015, 04:49:10 PM
is the noise floor absolute useless underexposure?

This image should make things clear (click to zoom). The noise floor is at the first tick. The scale is linear - first tick has SNR = 0 EV, second has 1 EV, middle has 2 EV and far end has 3 EV.

(http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/grad-DOI-1d3noise-8bit.gif)

from http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p2.html#SNR-DR
Title: Re: RAW based exposure feedback
Post by: Kharak on January 27, 2015, 06:05:33 PM
Ok, Thank you Alex. First and 2nd ticks are useless to work with, judging from your example.

I think I could use the 4th tick with that noise in a shadow in a scene, possibly the 3rd with some well adjusted noise reduction. Depending on artifacts and other lively noise. Guess it really depends on the scene aswell, how much information I really want to show the viewer in that area. But definitely useless if I did a shot where the viewer has to read the Declaration.

So in the latest example you gave me Audionut, where the noise floor covers about 30% of the histogram, with that information give or take, I can say that 30% of the shot is useless information or too noisy to retrieve anything usable? Not sure if I am reading it right.

Title: Re: RAW based exposure feedback
Post by: a1ex on January 27, 2015, 06:45:58 PM
So in the latest example you gave me Audionut, where the noise floor covers about 30% of the histogram, with that information give or take, I can say that 30% of the shot is useless information or too noisy to retrieve anything usable? Not sure if I am reading it right.

If you set the histogram scaling to linear, yes. By default, it's logarithmic (this makes it easier to see whether a small area of the image is clipped or underexposed, but you can't tell how much by looking at areas on the histogram).
Title: Re: RAW based exposure feedback
Post by: Kharak on January 27, 2015, 08:28:24 PM
Thanks for clarifying A1ex and Audionut.

After using ML for years now and after ML Raw came out, I still stand in awe of how much you guys have managed to pull out of these cameras. I thought of a good comparison to tell "normal people" when I try to explain what great feature Magic Lantern is, it is like a Nokia 3310 turned in to a Smartphone with all the features.

It is an incredibly powerful tool you have created, absolutely mindblowing!

Thank you.

Title: Re: RAW based exposure feedback
Post by: Andy600 on March 25, 2015, 11:27:01 AM
@a1ex - is there a technical reason why raw metering is not available unless you are shooting raw?

One scenario I can think of where it would be useful is when shooting H.264 with Cinestyle. I know you can use a workaround i.e. enable MLV, expose with raw metering, disable MLV and record but if it's possible for raw metering anytime it would be good to have the option.
Title: Re: RAW based exposure feedback
Post by: vertigopix on March 25, 2015, 12:18:59 PM
Quote
One scenario I can think of where it would be useful is when shooting H.264 with Cinestyle. I know you can use a workaround i.e. enable MLV, expose with raw metering, disable MLV and record but if it's possible for raw metering anytime it would be good to have the option.

Great idea !
Title: Re: RAW based exposure feedback
Post by: a1ex on March 25, 2015, 01:39:03 PM
I don't see the point. You can easily find an image that is not overexposed in RAW, but is overexposed by about 1 stop (sometimes more) in JPEG/H.264.

If you shoot with a picture profile that does not reach 255, simply adjust the overexposure threshold.
Title: Re: RAW based exposure feedback
Post by: Andy600 on March 25, 2015, 01:54:06 PM
That is the point (the 'sometimes more' bit).

What I'm asking is not useful at all for shooting H.264/Jpeg with ProLost Settings or default Pic Styles but Cinestyle would benefit from more accurate metering and raw ettr because there is literally no wiggle room with 8Bit log recording if you want to capture the max DR.

You can monitor RGB meters and stop down to guesstimate the clipping point but I prefer not to guess. The workaround I mentioned does give good results considering the limitations of H.264.
Title: Re: RAW based exposure feedback
Post by: Audionut on March 26, 2015, 12:40:47 AM
Adjust the overexposure threshold.  235 is probably a good starting point.
Title: Re: RAW based exposure feedback
Post by: mothaibaphoto on July 26, 2015, 07:42:50 AM
@Audionut: Please, make this thread "Sticky" !!!
Title: Re: RAW based exposure feedback
Post by: osterizerble on August 08, 2015, 03:32:20 AM
Hi,

Thanks for the hard you guys have put in the development.

I cannot see my raw histogram. When I am in live view it doesn`t seem to show up on screen. It does not show in image review either. When I press info, it only shows me canon histogram. I have 650D and running latest nightly from July, 02, 2015.

Global draw on > all modes
Zebras > Raw RGB
Spotmeter > Raw, AFBOX
Histogram > RGB, Log, ON, Full Histogram, ETTR Hint
Auto ETTR > Always on

How do I activate Raw Histogram to show up on live screen and also in Image review mode.

Thankyou
Title: Re: RAW based exposure feedback
Post by: Walter Schulz on August 10, 2015, 01:42:15 PM
RAW histogram not showing up in liveview is user error. Press Info button several times to bring up ML overlay screen.
RAW histogram not showing in Image review is indeed a bug introduced in Build #367 (24. June 2015):
https://bitbucket.org/hudson/magic-lantern/issues/2339/build-367-2015-june-24-raw-histogram-not

Thanks for testing!
Title: Re: RAW based exposure feedback
Post by: Guillermo Luijk on November 06, 2015, 12:27:35 AM
Would it be possible according to the way Liveview works, to have information in the RAW histogram not only about clipping taking place, but about how many stops of highlight info are currently getting clipped? i.e. an estimation of the histogram of clipped RAW data.

E.g. the orange bars represent info that would get lost if we click:

(http://www.guillermoluijk.com/misc/histogram.gif)

With Liveview capture working 2 stops below user settings (either with lower capture ISO or exposure), ML could plot a RAW histogram including those 2 (or any other number) extra stops of clipped RAW data with present user settings. This could visually and intuitively help in achieving accurate ETTR in M mode.

I actually prefer zebras over the histogram, but this would provide an enhanced histogram anyway.

Regards
Title: Re: RAW based exposure feedback
Post by: a1ex on November 06, 2015, 12:54:55 AM
I've been thinking about this lately as well. Currently, ETTR does some guesswork on this based on past histogram data (past frames), but there's no reason why this estimation couldn't be plotted in the regular histogram.
Title: Re: RAW based exposure feedback
Post by: Walter Schulz on November 06, 2015, 07:48:41 AM
How do I activate Raw Histogram to show up on live screen and also in Image review mode.

Fixed in recent nightly build (06-Nov-2015)
https://bitbucket.org/hudson/magic-lantern/issues/2339/build-367-2015-june-24-raw-histogram-not#comment-23106904
Title: Re: RAW based image feedback
Post by: AOK on October 11, 2016, 12:01:45 AM
Hi everyone.

The best article/post on the subject so far. Thank you very much for taking the time posting it. :-)

Question: I understand that the analyzed data e.g. DR, histogram, ETTR goal, etc. are displayed based on RAW. However the image itself when shot - is its jpg preview as in the regular firmware or it's the RAW file?

I am asking because as bad as the camera display could be, still one could better judge the results if it's (trying to represent at its best) the raw actual image and NOT the discrete contrast and compressed jpg version. The feedback would be great.

So is ML displaying the RAW image or (its closest representation) or it's also dealing with bad jpg version of the image?

Thank you in advance for your answer and excuse my limited knowledge on the subject.

Title: Re: RAW based exposure feedback
Post by: dmilligan on October 11, 2016, 03:04:50 AM
The display itself is low resolution, 8 bit/channel, YUV 422. There's no doubt it would be impossible to tell the difference. All the more reason you should never judge anything about exposure based on the way the image looks on the screen. Always rely on exposure aids like histogram and zebra.
Title: Re: RAW based exposure feedback
Post by: AOK on December 23, 2016, 01:06:00 AM
True. :-) But why would be there such huge fallback in all firmwares? I mean since ML does it, then it's possible to view histogram and zebra highlights based on RAW and not on JPG.

Why with firmwares it's always JPG analysis and representation? Battery drain, image processor (most probably not)? Why do they keep using the embedded JPG?
Title: Re: RAW based exposure feedback
Post by: Roberto Mena on January 12, 2017, 01:52:28 AM
I need clarification on how to use the ETTR tool with the RAW Histogram on ML when shooting video. When the ETTR tool reads EV 0.0 that's a perfect exposure being read by the ETTR tool, correct? But if I really want to do ETTR I should then open the camera iris for it to read a negative value correct, like EV -1.0 that way in post I can lower the exposure, etc. to get a better image with more data, correct?
Title: Re: RAW based exposure feedback
Post by: dmilligan on January 12, 2017, 03:29:01 AM
If you expose too far to the right then you will clip parts of the image you probably don't want to and actually loose information. Exposure is like "The Price is Right", you must get as close as you can without going over.
Title: Re: RAW based exposure feedback
Post by: Audionut on January 12, 2017, 07:53:57 AM
Use zebras.  Then you can keep pushing the exposure until wanted detail is ETTR.

The histogram will only tell you that some part of the image is being over-exposed, not specifically which part of the image.
Title: Re: RAW based exposure feedback
Post by: garry23 on January 12, 2017, 08:37:17 AM
@Roberta Mena

From my experience, as a stills photographer and not a videographer, I believe some get 'confused' by the additional functionality provided by the mid and shadow SNR.

The SNRs will/may push your image over into the highlights being overexposed, because these setting are saying to ETTR, 'please set a highlight ETTR, BUT, if my shadow (or mid) SNR criterion is not met, then 'sacrifice' the highlights.

You cant' have your cake and eat it  ;)

For this reason, personally, I set the SNRs to zero, and 'just' use the shutter speed, exposure target and highlight ignore, plus RAW Zebras. ETTR+RAW-Zebras+RAW-Histo-Ev-Hint is a very powerful combination, giving me the control, rather than the camera (if you rely on the SNRs).

Cheers

Garry
Title: Re: RAW based exposure feedback
Post by: Roberto Mena on January 12, 2017, 07:42:39 PM
Thanks for all the responses but I pretty much knew most of the things I was told on here. FYI, I have been using the ETTR tool for shooting video and I do usually over expose by -1.0 EV and so far the images I've gotten are good. I might starting knocking it down to -0.5 EV to be more of the safe side. My confusion comes from the values of 0.0 EV on the ETTR tool. In other words, when it does read 0.0 EV that means it is already overexposed to the right and now its a matter of how many more stops to push it? Or does 0.0 EV read as an already properly exposed image and now I need to start opening the iris more to do ETTR?
Title: Re: RAW based exposure feedback
Post by: garry23 on January 13, 2017, 12:16:26 AM
0Ev means no highlight clipping. Full stop.

Thus any speculars will control/dominate your exposure.
Title: Re: RAW based exposure feedback
Post by: a1ex on January 13, 2017, 12:25:13 AM
0Ev means no highlight clipping. Full stop.

Wrong answer. This is only valid when highlight ignore is set to 0 and SNR limits are disabled.

The exact meaning of 0.0 is given by the options you have set in ETTR submenu. It can range from very underexposed to severely clipped, depending on your settings and your subject.

http://wiki.magiclantern.fm/ettr
https://www.magiclantern.fm/forum/index.php?topic=5693.0
https://www.magiclantern.fm/forum/index.php?topic=17994
https://www.magiclantern.fm/forum/index.php?topic=18024
https://www.magiclantern.fm/forum/index.php?topic=5200.msg114522#msg114522
Title: Re: RAW based exposure feedback
Post by: Roberto Mena on January 13, 2017, 01:36:14 AM
a1ex @ "The exact meaning of 0.0 is given by the options you have set in ETTR submenu. It can range from very underexposed to severely clipped, depending on your settings and your subject." Cool but does that mean that the default settings in ML's ETTR the value of 0.0 EV is already ETTR with a little wiggle room for safety? This according to the last URL you posted which has four photos of a white flower, first two using Canon's meter and the last two using ML's ETTR. As mentioned, I think this might be true because I have been shooting my scenes at -1.0 EV and the images did not look overexposed. I also imagine since my 7D's dynamic range is only 11.7 stops it would make sense that I could only over expose one stop anyways unlike the BMPPC that has 13 stops of DR and probably could get away with even overexposing 2 stops to do ETTR... not that I would do that, just saying if that happened by accident the image would still be recoverable in post with curves. Right?
Title: Re: RAW based exposure feedback
Post by: Audionut on January 13, 2017, 02:23:05 AM
I do usually over expose by -1.0 EV and so far the images I've gotten are good. I might starting knocking it down to -0.5 EV to be more of the safe side.

Are you talking about the "exposure target"?

Quote
Where to place the highlights with respect to overexposure.

By setting this value to -2.0 EV, you are telling ETTR to place the highlights 2.0 EV (two stops) away from overexposure.  By setting this value to -0.5 EV, you are telling ETTR to place the highlights 0.5 EV (half a stop) away from overexposure.

If you want to maximize the signal to noise ratio of your images, you want to place the highlights as close as possible to 0EV (saturation / clipping / overexposure)

The exact meaning of 0.0

In the histogram?

Title: Re: RAW based exposure feedback
Post by: garry23 on January 13, 2017, 08:23:40 AM
A1ex

Sorry my 0ev answer went with my earlier answer saying I set SNRs to zero.

Garry
Title: Re: RAW based exposure feedback
Post by: a1ex on January 13, 2017, 09:25:31 AM
Got it.

Even so, with a nonzero highlight ignore setting, you will get some clipped highlights (the percentage you have selected, as area or number of pixels). So it's still a bit different than "No highlight clipping. Full stop.".

In other words, with a reading of 0.0 and SNR limits off, if Highlight ignore is set to 1%, that means 99% of the pixels will be below the target level (Exposure target), and the remaining 1% will be above. If there are no specular highlights, there won't be anything clipped. If you do have specular highlights and their area is smaller than 1% of the total image, they will be clipped, regardless of how bright they are.

With highlight ignore set to 0, and SNR limits still off, any strong specular highlight (or even some hot pixels, if unlucky) will dominate your exposure, even if most of your image ends up black.

This is where the SNR limits come in: if the shadows and/or midtones become too dark, enabling those limits will sacrifice the highlights to protect the shadows.
Title: Re: RAW based exposure feedback
Post by: garry23 on January 13, 2017, 09:37:25 AM
Understood  ;)

I will not be so lazy with my attempts at helping next time  ;D

Cheers

Garry
Title: Re: RAW based exposure feedback
Post by: Audionut on October 28, 2017, 06:59:42 PM
The OP has pretty images again.
Title: Re: RAW based exposure feedback
Post by: garry23 on October 29, 2017, 08:40:00 AM
@Audionut

Thanks for refreshing things.

One thing I've noticed is that when using dual-ISO with RAW zebras, I don't see the diagonal stripes that you show in the OP.

I'm using an EOSM with lua-fix build from July.
Title: Re: RAW based exposure feedback
Post by: a1ex on October 29, 2017, 10:50:14 AM
If you upload a CR2, I'm able to re-create the overlays in QEMU (http://www.magiclantern.fm/forum/index.php?topic=20579.msg190334#msg190334) (as they were displayed in the camera using any given build).
Title: Re: RAW based exposure feedback
Post by: garry23 on October 29, 2017, 11:24:00 AM
@A1ex

Here is a test image and .xmp.

Taken with latest (sic) experimental Lua fix.

https://www.dropbox.com/sh/1s82dhko4k1jqvr/AADH9Gb2qvoVi-5XL33PmqT3a?dl=0

Cheers

Garry
Title: Re: RAW based exposure feedback
Post by: Audionut on October 29, 2017, 12:47:40 PM
I wanted to add some extra detail based on discussion with heyjoe, and noticed this tut didn't have it's image links fixed. 

Note:  Due to inconsistencies in the original firmware design, determining the correct overexposure point is extremely difficult.  You should be very wary when the histogram reads from -0.4EV through to (slight) overexposure.  There may be some slight overexposure when none is reported, or vice-versa, there may be no overexposure even though some has been reported.  If you know the true saturation value for your sensor at the shot settings, you may wish to use raw_diag (https://builds.magiclantern.fm/modules.html#raw_diag) if you like to nitpick.

Otherwise, to be certain of no overexposure, target a ETTR hint > -0.4EV.
Title: Re: RAW based exposure feedback
Post by: a1ex on October 29, 2017, 02:57:10 PM
https://www.dropbox.com/sh/1s82dhko4k1jqvr/AADH9Gb2qvoVi-5XL33PmqT3a?dl=0

"Classic" raw backend (unified, lua_fix, everything but crop_rec_4k):
(http://a1ex.magiclantern.fm/bleeding-edge/raw/clip/EOSM-dual/uni.png)

Experimental raw backend (crop_rec_4k branch, updates discussed here (http://www.magiclantern.fm/forum/index.php?topic=20579)):
(http://a1ex.magiclantern.fm/bleeding-edge/raw/clip/EOSM-dual/4k.png)

Just for fun - raw overlays on the processed image (as if the captured image were the output of cr2hdr) - with experimental raw backend from crop_rec_4k (this is a hypothetical scenario):
(http://a1ex.magiclantern.fm/bleeding-edge/raw/clip/EOSM-dual/4kpro.png)

I've got these images by emulating the silent picture module on top of the 60D firmware, using the above CR2 as reference image. EOSM/700D/650D/100D were a little stubborn; the 60D has the same resolution, but different offsets, so the screenshot might be off by a few pixels. This method uses our own preview (so that one won't match exactly what you have seen on the screen, as the previews on regular image capture are rendered by Canon's image processor - a black box for us) but the raw overlays are the same as with a regular image capture (not yet emulated). So, I expect the raw overlays to be the same as on the real camera (maybe off by a few pixels because of different offsets).

Notice the solid areas in the last two runs (both with crop_rec_4k's raw backend) are overlapping very well - the solid zebras on the unprocessed dual iso image (what you'd get when capturing a dual iso image) correctly predicted the overexposure from the end result (after processing with cr2hdr).

The diagonal zebras are also present - most of image was rendered as half-resolution by cr2hdr (as only the lower ISO had useful data). The exceptions were the dark TV screen and some darker areas on the flower. This matches cr2hdr output for this image:
Code: [Select]
Semi-overexposed: 85.00%

Were the raw overlays different on the camera screen? Can you provide a screenshot?



edit: by playing around in zebra.c, I've got the individual zebras for each sub-image (low and high ISO):
(http://a1ex.magiclantern.fm/bleeding-edge/raw/clip/EOSM-dual/lo.png) (http://a1ex.magiclantern.fm/bleeding-edge/raw/clip/EOSM-dual/hi.png)

Notice something that resembles the focus pixels in the high ISO image - there are traces of them in the above CR2 :)
Title: Re: RAW based exposure feedback
Post by: garry23 on October 29, 2017, 03:24:57 PM
@A1ex

Slightly different composition, but I can't see the diagonal shading.

But it could be my eyes ;-)

(http://thumb.ibb.co/fPphkm/VRAM1.jpg) (http://ibb.co/fPphkm)
Title: Re: RAW based exposure feedback
Post by: a1ex on October 29, 2017, 03:27:55 PM
Press the shutter button ;)

In photo mode, dual iso is only enabled outside LiveView (that is, when reviewing the captured image).
Title: Re: RAW based exposure feedback
Post by: garry23 on October 29, 2017, 03:43:03 PM
@A1ex

Thanks for the clarification.

Reviewed the image and not that informative on an EOSM.

I think I stick with 'normal' exposure control in photo mode rather than zebras ;-)

Cheers

Garry
Title: Re: RAW based exposure feedback
Post by: a1ex on October 29, 2017, 03:46:11 PM
I still don't get it - are the raw zebras on EOS M different from the ones in the above emulation?

If yes, can you provide a CR2 and a screenshot?
Title: Re: RAW based exposure feedback
Post by: garry23 on October 29, 2017, 04:14:31 PM
@A1ex

Here are the files you requested: https://www.dropbox.com/sh/1s82dhko4k1jqvr/AADH9Gb2qvoVi-5XL33PmqT3a?dl=0

I simply don't see the diagonal lines.
Title: Re: RAW based exposure feedback
Post by: a1ex on October 29, 2017, 04:30:38 PM
The screenshot should not be in LiveView; it should show ML overlays right after taking a picture.

Easiest way to take it: set Image Review to Hold in Canon menu, activate the Screenshot function in ML menu (to trigger after 10 seconds), then take a picture.

Is the behavior different from 5D3? (it shouldn't be)
Title: Re: RAW based exposure feedback
Post by: garry23 on October 29, 2017, 04:36:50 PM
@A1ex

I'm on an EOSM.

Not sure how to time taking a non LV review shot.
Title: Re: RAW based exposure feedback
Post by: a1ex on October 29, 2017, 05:26:53 PM
Step 1: Canon menu, Image Review - set to Hold.

Step 2: ML menu, enable Debug -> Screenshot (10s).

Step 3: Press the shutter button to take a picture. The camera should show the image you have just taken (not the LiveView screen). That image should be stationary (should no longer move as you move the camera). Wait until the screenshot is taken; do not press anything to return to LiveView.

Step 4: Press half-shutter (or other key?) to return to LiveView.

IIRC, you have a 5D3 and an EOS M to try. I only have the former. Is the behavior different between the two models?

If that's still not clear - can you record a video of the camera screen?
Title: Re: RAW based exposure feedback
Post by: garry23 on October 29, 2017, 05:51:29 PM
@A1ex

I'm not going to waste anymore of your time, including uploading anymore images.

I reviewed the image, with hold on, and clearly see the diagonals, indicating the higher ISO clipping in dual mode,

As I said before, for me, I can't see much use for the zebras, other than providing extra spacial info regarding clipping, ie where in the scene. Which I can see in LV.

Bottom line: as usual,  ;), I fear my 'reporting' was a reflection of my ignorance.

Cheers

Garry