18% Gray. How do you make it on screen?

wcoxe1 wrote on 8/27/2003, 6:20 PM

Does anyone know what RGB setting to use to generate a solid color which will very closely approximate a photographer's 18% Gray Card?

Comments

Chienworks wrote on 8/27/2003, 7:09 PM

You can't really do this since the grey card is a reflective medium and the screen is a self-illuminated medium. The screen simply can't be used as a grey card this way. Also, the appearance of the grey card will vary depending on what angle the light hits it and what angle the camera sees it.

That being said, the idea of the grey card is to be exactly in the middle of both color and brightness. The RGB value that would correspond to this idea is 127.5, 127.5, 127.5 (halfway between 0 and 255). You can only use integers so you'll have to either round it down to 127,127,127 or up to 128,128,128.

wcoxe1 wrote on 8/29/2003, 2:47 PM

I understand the limitations. I wanted a neutral gray, 18%, background, so that I wouldn't have such stark, ugly black bars. Gray seems to be ignored by people easier than BLACK!

BD wrote on 8/31/2003, 12:26 PM

As Chienworks indicated above: the brightness of a card varies according to the ambient light level, unlike a video screen image.

Set your RGB levels to get a brightness of gray that you like, while keeping them equal (e.g. 127,127,127).

Brandon's Dad

musicvid10 wrote on 9/1/2003, 8:19 AM

wcoxel1,
You might be confused by the two different forms of math being used here. The 0-255 scale used in to create RGB/Hex colors is logarithmic, so 127-127-127 is 50% neutral gray.

The equivalent in old-school photography is 18% gray, which is a linear math statement. All other factors being equal, they are exactly the same shade of gray.

As Chien pointed out, there are many real-world variables in the outcome, the biggest having to do with reflected light vs. screen output, but 50% RGB and 18% photographic are the same theoretical starting point.

HTH

Jay Gladwell wrote on 9/1/2003, 8:42 AM

Just in case anyone is confused by the above, the "18%" in 18% gray card is referring to the amount of light being reflected off the surface of the gray card. A white card reflects 90% of the light.

The percent is not a reference to the amount of gray. Therefore, referring to a 50% gray card would be saying that the card is reflecting 50% of the light falling on the card.

In the above example that mentions a "50% gray card" it is actully referring to the "value" of the gray or the amount of white to black--equal amounts of white and black--50/50.