Author Topic: Question about Spot Metering with ML  (Read 5525 times)

RTLdan

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Question about Spot Metering with ML
« on: April 04, 2014, 01:15:01 AM »
Hello everyone!
 I've just done a quick, quick test using my grey card. Previously I had only been using it to white balance, but I wanted to give it a try using the spot meter to set exposure as well.

Recently I've been just using the histogram and waveform to try and get my videos exposed as far ETTR  as possible without clipping anything important. But it struck me that using the spot meter and a grey card might be very useful for getting consistent exposures and not having to guess so much.

So, I borrowed a minute of someone's time to be my subject, and I had her hold up the grey card  near her face to measure roughly the same light. I then white balanced using ML's auto white balance feature. Then I used the spot meter and took a measurement of the grey card to get it right on the money - 18%.  I then removed the grey card, and took a spot meter reading on her right cheek, which showed about 40-50%.

So here is the first photo. Grey card reading 18%, cheek approx 50%:


I unfortunately did not get a screen grab of the camera to show my waveform and histogram monitors, but they seemed to be showing a lower brightness than I would think would be wise, from what I understand about ETTR techniques. I realize she is not evenly lit, and the scene itself is not well lit, but there was a lot of room left around her face on the waveform.

Anyhow, I recalled reading something indicating that well exposed Caucasian skin usually sits around 50-70%. So I took a measurement from her right cheek again and adjusted exposure until it read approx. 70%. That produced this photo:


Still, it did not seem to use up all of the available room for exposure in the waveform/histogram.
In fact, now that I think about it, the two pictures look remarkably similar. I hope I didn't botch my comparison somewhere along the way.

Anyhow, I guess what I'm really trying to understand is how the spot meter should be used in conjunction with a grey card, if when I get an 18% reading from the card, it's not utilizing all of the available DR I have for an ETTR philosophy.

I am a very amature photographer/videographer, so hopefully this is not a ridiculous question, or answered a hundred times before. I searched the forums but mostly found people asking about HOW to use the spot meter, not about the significance of the measurements taken.

All help is immensely appreciated.
Thanks again for this great community!!
-Daniel

Audionut

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Re: Question about Spot Metering with ML
« Reply #1 on: April 04, 2014, 02:57:20 AM »
There's raw based, and JPG/H.264 based photo/videography.

Raw based in non-destructive, so here, you want to ETTR for minimum noise. 
JPG/H.264 is destructive.  By that I mean, any editing will further reduce quality.  It may not be a perceptual reduction in quality, but the reduction is there.  These formats also contain a small bit depth (8 bits), so exposure adjustments in post are likely to lead to banding.  In other words, you probably want to just get the exposure correct, in camera.

The sport meter, histogram, and zebras in ML, work in both raw based mode, and JPG/H.264 mode.  The waveform, is limited to JPG based mode.

In raw based mode, where you want to ETTR, you want to place the spot meter (in RAW (EV) mode) over the brightest object in the scene, that you still want to retain detail.  In your examples above, this would (probably) be her right knuckles.  You may want to check her left hand also.

In raw, the midtones are around -4 EV.  And the spotmeter (in raw mode) will reflect exposure in negative units.  So a correct exposure in camera for Caucasian skin, will be around -3 EV to -2.5 EV.  And -0 EV represents over exposure.

If you are working with raw based capture, you probably want to work with raw based exposure feedback, since JPG based exposure feedback is subject to the JPG processing in camera (picture style etc), and will not accurately reflect the data being captured.

With JPG based, 18% represents the midtones (-4 EV), 50% is close to -3 EV in raw, and 70% is close to -2.5 EV in raw.  So you can see, that even though you increased exposure by 20% (a large number otherwise), you only increased exposure by around 0.5 EV (half a stop).  Judging by your samples above, the result looks pretty close.

Also note, that camera exposure feedback uses reflective metering.  In the examples you posted, the angle of her cheek has changed, and the reflective nature of the skin has changed.  With the defuse light in your scene, the change is small, but it is something to consider when spot metering.

Anyhow, I guess what I'm really trying to understand is how the spot meter should be used in conjunction with a grey card, if when I get an 18% reading from the card, it's not utilizing all of the available DR I have for an ETTR philosophy.

That's correct.  Here, you are exposing technically correct, in camera, not to ETTR.

If you are working in raw based mode, you may want to try (raw based) zebras instead. 

Note:  I explained some basics that you may otherwise know, this is for the benefit of other readers.

RTLdan

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Re: Question about Spot Metering with ML
« Reply #2 on: April 04, 2014, 08:38:33 AM »
Wow!
 Thanks Audionut!
You cleared up a whole lot. Thinking about it now, I understand how I was confusing two separate workflows, but
your explanation was very helpful and thorough. Interesting too, about using the spot meter for the brightest tones that should retain detail in a RAW workflow.

Now I've just got to go put this knowledge to use and shoot some stuff ;)

Thanks again to all involved with ML!
-Daniel

Audionut

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Re: Question about Spot Metering with ML
« Reply #3 on: April 04, 2014, 10:58:53 AM »
Interesting too, about using the spot meter for the brightest tones that should retain detail in a RAW workflow.

Where you want to ETTR, absolutely.  :)  If you prefer to shoot through the viewfinder, you can also use the camera exposure meter to accomplish the same task.
Set the camera to spot meter metering, and spot meter the brightest part of the scene at +3 EV on the camera meter.  ;)
This should leave you a little highlight headroom, before overexposure.

I'll leave this here, Zone system, as I found it very useful myself.

Now I've just got to go put this knowledge to use and shoot some stuff ;)

Heh, I like to think I have the technical stuff under control, of course, applying that in practice, and generating good results, is an entirely different matter.  ;)