To get the best experience when viewing photos online, your computer's display should be calibrated. The best way to calibrate your display is with some dedicated hardware, like the
ColorVision Spyder2. If you don't want to spend the money for dedicated calibration hardware, or can't borrow any from a photo-geek friend, you can do a reasonable calibration by eye.
The patterns below will help you manually calibrate (or verify the calibration of) your display.
Below is a greyscale strip. With a calibrated display under good viewing conditions, you should see transitions between each of the grey values.
If you can't make out each of the transitions, your display's brightness or contrast may need adjustment. In certain circumstances, the display's gamma may have been set to a wacky value.
Now, take a look at the color pattern below.
The image above contains 21 steps for each color from left to right, from midtone to black. The saturated colors on the left are steps around the colorwheel with a matching grey scale at the bottom. If your monitor is properly calibrated, you should see distinct steps between all 21 steps.
The image above contains 21 steps for each color from left to right, from white to midtone. The saturated colors on the right are steps around the colorwheel with a matching grey scale at the bottom. Most displays will not show separation between colors at the far left of the scale. Beyond the two or three colors on the far left, you should see sparation between the steps in the scale, however.