# Understanding "The Zone System" & Why 18% Gray?

### #1 manigandan srinivasan

manigandan srinivasan
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Posted 21 October 2011 - 06:14 AM

Hi, i got myself Introduced to the concept "18% or Mid Gray" & to Ansel Adams "Zone System". I understood what zone system is why it was found by Ansel Adams. i also understood the use of a gray card & Meter callibrations to favour middle Gray etc. But, i have few doubts about this concept. Ill ask them in this post kindly clear my doubts

1. How much luminace Black has & how much luminace white has & how the no 18 came ? How a middle gray (ie) Zone 5 exactly Reflects 18 % of light hits it ?

2. What is "sensitometry" & "Densitometry" (Kindly explain in simple terms) & what way will it help me to know these 2 to understand this concept better ?

Note:I definetly have put efforts before posting Here so i am just making sure am not wasting any ones time for my laziness

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### #2 Philip Ulanowsky

Philip Ulanowsky
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Posted 27 October 2011 - 11:47 AM

The 18% is partly a result of a geometric, not arithmetical increase in light intensity; this is something you can look up in in-depth Zone System explanations or the first of Adams' series of books on photography. As for the intensity of illumination of black and white, these depend in part on the medium and its processing. however, all other things being equal, the revised, 10-Zone System will tell you that black is Zone O (zone one being an untextured value just appreciably above this), hence, 5 stops less than Zone 5. If Zone 5 is reflecting, say, 1000 lux, 500 will yield Zone IV, 250 Zone III, etc.., so that about 32 lux will yield Zone 0, though anything much below 60 will, too.
Sensitometry involves measurement of the effect of light; densitometry involves measurement of the density, or relative opacity, of, in this case, the developed film negative.
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