Setting White Point and Black Point
If you use a scanner this is esential knowledge. It's easy.It's important. But not many people know how to do it.

Your scanner is designed to see the world in absolute terms. Black is the absolute absence of light and white is the brightest tone that the sensors can represent. The things that you scan, however, will rarely take this into account.

When was the last time you saw a photograph where the blackest black was really totally black? Would the term 'blackest black' even be necessary if truly black blacks were the norm?

How about a photograph with true white? Not a light yellow, pink, or blue?

Here's the dilema - How do you reconcile the real world of things you want to scan, with the ideal world that your scanner expects?

It's fairly easy actually - you have to tell your scanner which pixels contain the blackest black and the whitest white. This is usually done in the scanner control window with a tool that looks like an eyedropper. There are actually two (sometimes three) eyedroppers. One is black, for setting the black point, the other is white, you guessed it , for setting the white point (the third is grey and is used for setting neutral grey which is called gamma in the digital world).

Here's what you do. Select the black eyedropper tool and pass it over what you beleive to be the darkest pixels in your preview scan. There will be an info window somewhere which will give you readout which contains numeric data about the pixels under the dropper. Look for the very darkest pixel you can find. Try to find one that has very close readings in all three colors - red, green, and blue (if you are scanning film the clear part is your best bet). For example 1, 4, 1 or 5, 7, 6 not 1, 3, 17. When you have found your pixel click on it. Your image will change. The pixel you chose will now be absolute black and all of the pixels in the image will adjust accordingly.

Now select the white dropper and pass it over what you beleive to be the whitest pixels in your preview scan. Look at the info window and choose the brightest pixel you can find. Make sure that you pick a white pixel that you want to use for referance white for the color balance of your entire image. The pixel you choose will become white in your image (this can be a way to help eliminate an overall color cast in your original).

That is all there is to it. Very easy but very important. What you have done is take what would have been a lifeless scan and make it into one that takes advantage of the widest possible dynamic range.

If your scanner controls do not offer this kind of adjustment you can do pretty much the same thing in PhotoShop and other image manipulation programs. Usually under the heading of 'level controls'. If you have the choice it is better to make your adjustment before you scan, in the scanner controls, rather than after you scan, in your imaging program.