The type of contrast filter your choose will depend on the state of your negative, dense or thin, overexposed or underexposed. Different contrast filters are specified by numbers, from 0 to 5, zero being the lowest contrast and five being the most contrast.

When you begin printing an image, it’s a good idea to start out with a 2 or 3 contrast filter and work from there. With most images, you will quickly learn through test strips that you might have too much or too little contrast. If you have too much contrast, your whites will be very hot and blown out, while your blacks will appear very dark; a lot of the detail will be lost. If you have too little contrast, your image will appear too gray and muddy; again, much of the detail will be lost. To print a technically accurate photograph, you should have a broad range of grays while still attaining some true white and black. Another thing to keep in mind is that the edges of the photograph should have some gray in them, especially if the sky is in the image. Your sky should always have a little tone to it.

If you find that you have too much contrast, go back and do another test strip using a contrast filter that has a lower number. If you find that you have too little contrast, choose a contrast filter that has a higher number. It can be helpful to choose one that is only one number away from the one that you are using, or a half number. That way, you don’t get too far off track with each test.

Changing the contrast filter will also affect the exposure time of your print, as the color of the filter affects the amount of light passing through the lens, so make sure to do another test strip for time while you are evaluating the new filter.

Refer to this page on contrast filters from my section on Darkroom Equipment.