BetterPhoto Q&A
Category: New Questions

Sharpning Filters

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March 02, 2002

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(1) If you're asking about optical filters for camera lenses:

Filters can increase contrast with some types of scenes. Usually this is done with B/W by using certain filters to provide more separation between the shades of gray that some colors create grayscale. A classic B/W example is the yellow (K2) filter which darkens the gray blue skies produce in B/W; this separates sky from the light gray clouds more. This effect, and similar ones can increase apparent sharpness through increasing contrast, but it cannot solve a focusing problem; contrast edges must be sharp to begin with in order to be enhanced by the additional contrast.

Every Fall, Tiffen's marketing goes into high gear touting their Didymium "enhancing" filter which tends to saturate certain reds, oranges and browns. Shouldn't be much wonder that their advertising campaing is timed just ahead of when leaves turn color. I've never found any reason to use one. Choosing the proper weather conditions and time of day have been all I've ever needed to enhance Fall color saturation. While it can make certain colors bolder, it won't "sharpen" an image to fix focus problems.

I've mentioned these (the B/W filters and the Tiffen "enhancing" one) because they're sometimes confused with being able to sharpen an image somehow. No filter made is a substitute for a high contrast, high resolution lens and accurate placement critical focus to put the depth of field where it's desired. Contrast and resolution are a trade-off in lens design and must be balanced. Too much of one will start to degrade the other. Specific lens designs sometimes lean a little one way or the other, but not by very much.
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(2) If you're asking about manipulation of a digital image:
Most digital imaging software for working with digital camera or film scan images have a "sharpening filter" feature. Just like the optical lens filters, it cannot fix a photograph that's out of focus. All the different "sharpening" algorithms increase contrast at contrast edges and nothing more than this. The "unsharp mask" alogorithm has its roots in darkroom work and produces nearly the same subtle effect digitally that creating and using an unsharp mask to print as a composite with the negative or slide when printing them in a darkroom has. Again, it won't fix a poorly focused, soft image. Contrast edges must be sharp to begin with if making a contrast increase along them is to work well.
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-- John

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March 03, 2002