Re: Using a 113 gray standard in digital photo
Re: Using a 113 gray standard in digital photo
- Subject: Re: Using a 113 gray standard in digital photo
- From: "Bruce J. Lindbloom" <email@hidden>
- Date: Thu, 21 Dec 2000 14:42:34 -0600
Terence L. Wyse wrote:
>
Just my $.02 worth but what I think they're going for is the 18% gray card
>
(gray patch 3 roughly equals a Kodak Gray Card density of .74).
>
>
From what I understand, a Lab value of 47-48 equals the Kodak 18% Gray Card.
>
I did some poking around and in Adobe98RGB, a 113 is equal to this Lab
>
value. In ProPhotoRGB, this same Lab value works out to 92-94. I intend to
>
get my Spectrolino out soon and measure a few gray cards I have plus the
>
Macbeth Color Checker just to see for myself.
A middle visual gray (L* = 50) has these equivalents:
Reflectance = 18.419%
Density = 0.73474
Pixel value for gamma 2.2 (e.g. Adobe RGB) = 118
Pixel value for gamma 1.8 (e.g. ProPhoto) = 100
A true 18% gray card (18% reflectance) has these equivalents:
L* = 49.496
Density = 0.74473
Pixel value for gamma 2.2 (e.g. Adobe RGB) = 117
Pixel value for gamma 1.8 (e.g. ProPhoto) = 98
Here are the ideal pixel values for each of the six Macbeth grays with
respect to different gamma values of RGB working spaces. (I have posted
this table to this list before):
Gamma 96 81 67 52 36 20 <- L* of Macbeth grayscale
----- --- --- --- --- --- ---
1.0 230 149 93 51 23 8 <- pixel values for each step at this gamma
1.1 232 157 102 59 29 10
1.2 234 163 110 67 34 14
1.3 235 169 118 74 40 17
1.4 237 174 124 81 46 21
1.5 238 178 131 88 51 25
1.6 239 182 136 94 57 28
1.7 240 186 141 99 62 32
1.8 241 189 146 105 67 36
1.9 241 192 150 110 72 40
2.0 242 195 154 114 77 44
2.1 243 197 158 119 81 48
2.2 243 200 162 123 85 52
2.3 244 202 165 127 90 55
2.4 244 204 168 131 94 59
2.5 244 206 171 134 97 63
2.6 245 207 173 138 101 66
2.7 245 209 176 141 105 69
2.8 246 211 178 144 108 73
2.9 246 212 180 147 111 76
3.0 246 213 182 149 114 79
I think any sort of "calibration" information is useful, whether just one
patch (e.g. 18% gray card) or many (e.g. a full ColorChecker DC chart). You
may always choose to ignore such information, in which case you are no worse
off than you started. But I also agree with those who have stated that you
must be careful in _how_ you use it. There's no substitute for experience
and common sense.
--
Bruce J. Lindbloom, Pictographics Intl. Corp.