Why is 18% gray mid-tone?


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ckhaos

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Dec 28, 2002
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#1
Dear experts,

Why is 18% gray the min-tone? If I use the standard assumption that films see a 5-stop range of tones, i.e., ranging from 0, x, 2x, 4x, 8x, 16x, 1,where x is then 1/32, then the mid-tone is at 4x which gives 12.5%?? :dunno:
 

ckhaos

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Thanks mpenza, I've read the page you listed. But it still does not explain satisfactorily why 18% is the mid-tone. The geometric mean is 12.5% by the way (see above), not 18%.

The only acceptable answer seems to be that 18% is an empirical value dictated by Kodak. But how accurate is this?

Are really all our cameras calibrated based on an empirical value, where a more accurate 12.5% should be used instead? :dunno:
 

GitS

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er, 18% refers to the reflectance, regardless of color. can't remember why 18% but in the zone system, zone 5 is 18% grey, right in the middle and the zones run from 1 (white with no detail) to 10 (black with no detail)..hope this helps...:)
doesn't matter why or how 18% was chosen but key thing is to decide where to place the reading you have taken on the scale and compensate accordingly....:)
 

sriram

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You are right. It is around 12%. I believe Ansel Adams had a big say in convincing Kodak to make their gray cards 18% because of his zone system. This is why the instructions on the Kodak gray cards tell you to meter the card and open up... not just shoot at the metered reading which will underexpose your photo. The 18% is just a reference. It is not "middle gray".
 

GitS

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actually its an issue of what the camera meters are calibrated to, whether its 12% or 18%. i have checked many times for myself that canon (the EOS 50, 30 and 3) calibrate to 18% (double checked with grey card and sekonic lightmeter). this is of course referring to spot or partial readings...god knows what kind of algorithim takes over when you use ETTL on high contrast subjects....i never take that risk.

yah, i remember the instructions for using the kodak grey card, by the time u set up the card facing in between the subject, light source and the camera blah blah....zzzzzzzzz
key i think is to angle the card towards the camera, much like how you would use a light meter....

shooting at grey card suggested readings sometimes gives me a slight overexposure, but that fine as i shoot film most of the time!

;p
 

denizenx

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iirc some urban legend says that it's typical untanned angmoh flesh tone? LOL that's how it went I think... ;p
 

GitS

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hahahaha! :) but seriously, my untanned pasty sickly chinese skin is 1 stop brighter than 18% grey...grass or asphalt is still the best approximation.....
 

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