Author Topic: RAW based exposure feedback  (Read 46185 times)

Audionut

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Re: RAW based exposure feedback
« Reply #50 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 if you like to nitpick.

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

a1ex

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Re: RAW based exposure feedback
« Reply #51 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):


Experimental raw backend (crop_rec_4k branch, updates discussed here):


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):


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):


Notice something that resembles the focus pixels in the high ISO image - there are traces of them in the above CR2 :)

garry23

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Re: RAW based exposure feedback
« Reply #52 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 ;-)


a1ex

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Re: RAW based exposure feedback
« Reply #53 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).

garry23

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Re: RAW based exposure feedback
« Reply #54 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

a1ex

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Re: RAW based exposure feedback
« Reply #55 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?

garry23

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Re: RAW based exposure feedback
« Reply #56 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.

a1ex

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Re: RAW based exposure feedback
« Reply #57 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)

garry23

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Re: RAW based exposure feedback
« Reply #58 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.

a1ex

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Re: RAW based exposure feedback
« Reply #59 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?

garry23

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Re: RAW based exposure feedback
« Reply #60 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