The image above shows eight blocks of grey tints, with a pure white stripe running across the middle. The numbers on each block show the pixel value that block contains. (That is, the block labeled 251 has red, green, and blue pixel values of 251, 251, 251.)

On a perfectly calibrated monitor, you'd be able to distinguish (if only just barely) the difference between the white central row and the block labeled 254. More typically, a "good" monitor would let you see the boundary between the center row and the 250 or 251 block. How many blocks can you see? (Many photo-oriented web sites start their "Is your monitor OK?" greyscale wedges at 245 or so. That's an awfully loose standard, your monitor could be pretty messed up and you'd still be able to see the difference between 245 and pure white.)

The chart above is similar to the earlier white-point checker, only this time for deep shadows. The center of the image is pure black (pixel values of zero), with the numbers on each of the 8 blocks surrounding it corresponding to the pixel values of the dark grey tints they contain. You'll have a hard time seeing shadow detail if the light's too bright in your working area, so make sure that the room is somewhat dim before performing the check. (The whole issue of proper viewing environment really begs for a whole separate article, one we haven't written yet.)

How's your contrast?

There's more to accurate tonal rendition than just highlight and shadow handling though; contrast is also very important. Computer displays behave differently than you might expect: Being digital, you'd expect each unit of pixel value would produce the same change in brightness. While it would be easy to build computer monitors that did this, it turns out they wouldn't work well for editing photos.

The problem is human eyes aren't very linear. It turns out that our eyes respond much more to changes in brightness than absolute brightness levels. So the same brightness change that appears small in a strong highlight would look enormous in a shadow area.

If we want equal increments in pixel value to produce equal increases in perceived brightness, we need to tweak the way the monitor translates pixel numbers into brightness onscreen. Engineers call the input/output curve that does this a gamma curve, but you and I can just think of it as contrast, since that's what changes in gamma tend to look like to our eyes. Higher gamma numbers mean higher contrast, lower numbers mean less contrast. The sRGB standard used in most computer displays calls for a gamma setting of 2.2.

So how do you tell whether your monitor is set to the correct gamma level of 2.2? It's actually pretty easy. If your monitor is adjusted properly, the pattern below will appear as all the same shade of grey when you view it at some distance from the screen, or if you just throw your eyes out of focus at a closer viewing distance.

The central portion of this pattern is just an alternating series of black and white lines. Half white and half black is by definition 50% as bright as the brightest white your monitor can display. The surrounding gray area is set to a brightness of 186, since that's the pixel value for 50% brightness at a gamma of 2.2. Simple.

So, if you don't see a uniform grey when you look at the image above, your display's gamma setting is off. Fixing this really requires a display calibrator, the "contrast" control on LCDs doesn't actually control the gamma.