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This month we are putting ourselves in front of the lens for a project on the self-portrait with guest instructor Jackie DiBenedetto. Please add your photo to the thread and enjoy the conversation....

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Photoshop 18 Percent Grey

Jim Vanson , Jul 20, 2003; 02:58 a.m.

Does anyone know what the PhotoShop settings for 18 percent Grey are?

Responses

Leonard Evens , Jul 20, 2003; 09:29 a.m.

It depends on what you mean by 18 percent gray, If you take that to be middle gray and make plausible assumptions about the gamma of the monitor, then in principle you should take the middle values for RGB, i.e, R=G=B=127 fo a scale of 0 to 255. But that is unlikely to look on the screen like a gray card you hold up next to it under normal lightling. The card is likely to look darker. Also, if you print normally, you are not going to get something that looks like the gray card.

The math goes as follows. The native gamma of a typical CRT is 2.5. The light intensity on the screen is roughly the relative value raised to the gamma power. 127/255 ~ .5, and .5 raised to the 2.5 power is about .18 or 18 percent. However, the human visual system tends to exaggerate responses based on viewing conditions. For viewing a TV screen or monitor, there is an adjustment for gamma to compensate for this, so one is usually advised to set a PC monitor's gamma to 2.2. Since .5 to the 2.2 is about 22 percent, that is brighter than the expected 18 percent, but if you are looking at a step wedge on the screen and want the .5 value to appear to be in the middle, you are better off with the smaller gamma.

At least this is what I've managed to glean from reading Poynton and other technical notes on the subject.

Jim Vanson , Jul 20, 2003; 02:29 p.m.

Excellent reply...tkx Leonard...jim

Kevin Connery , Jul 20, 2003; 05:32 p.m.

The gamma of the color space used ("working space") will have a large effect. 127R, 127G, 127B in sRGB is significantly different than 127/127/127 in ColorMatch or ProPhoto RGB, and a tiny bit different than in Adobe RGB (1998).

The gamma of the display itself should be corrected out by Photoshop's internal color management, and shouldn't have any visible effect, if it's working properly and the display is calibrated.

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