Proper Blind Exposure with a Gray Card

Started Nov 12, 2012 | Discussions thread
panos_m
Senior MemberPosts: 1,217
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Re: The Crux of the Matter
In reply to Jack Hogan, Nov 12, 2012

I believe that is very simple. No matter what reflectance you put in front of it (10%, 60%, 75%, 100%) it will always read it the same that is it will always suggest the exposure that will produce the same "middle gray" raw value (the same distance in stops from saturation). What else do you need to know to make an exposure decision and place the tonal range of the scene where you want?

Jack Hogan wrote:

panos_m wrote:

Hmm! For a camera meter to be calibrated at 18% it must read 2.47 stops lower than saturation for the green channel. I meter my D3 to 3.3 stops and my GF1 to 3 stops. So they are not calibrated to 18%

Thank you Panos! And can you tell us what the reference to which a meter is calibrated has to do with what raw value a Middle Gray tone gets recorded at?

Calibration of a reflected light meter is all about making sure that when the meter thinks it is seeing 100% diffuse light it actually is. It doesn't matter what the standard used to ensure that that is indeed the case. Let's say for the sake of argument that camera meters are calibrated to a 75% reflectance standard. That means that to verify that the meter is reading correctly at the factory they present it with a 75% reflectance standard and it thinks that's 75%. So that when we present it in the field with a middle gray card, it reads it accurately at 18%.

Now you spot meter and take a picture with default settings of your D3 and GF1 of a sheet of paper reflecting 60% of the available light. They record it at -3.3 and 3 stops of saturation, that is at about raw values 1663 and 512, or 10.2% and 12.5% of full scale respectively.

What do 75%, 60% and 10.2% or 12.5% have to do with each other? And would you say in this case that you would need to use a 12% gray card to get the correct exposure?

Jack

PS Nein. With your cameras you would need a Middle Gray card to get 'proper' 'blind' exposure (pull it Thom!)

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Panagiotis

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