Filters: Software vs Glass

What Lens Filters to Use?

A couple of decades ago, professional camera folks would buy into a vast library of specialized glass filters, few of which got any particular use on a regular basis. Two or three shots might call for some red enhancer or a starburst effect.

Thankfully, for both the pocketbook and the closet, this is not really necessary anymore. All of these glass filters can be simulated in the computer later, right? Well, almost all of them. There remain only a few categories of glass lens filters you may still want to keep in your gearbag:

polarizers
More about these below, but suffice it to say: you can try to fake it in the computer, but not very well.
neutral density filters
These are like sunglasses: they cut the light before it gets into your camera. When you can't adjust your aperture and shutter time values, a neutral density filter (or two) saves the day.
infrared passive filters
This trick isn't for everyone, but there's no way to fake the effects of infrared wavelengths if you allow normal visible light into your camera.

Just about anything else can be simulated effectively in software post-processing, offering greater flexibility and chances for experimentation.

How does a Polarizer Work?

Grab this virtual polarizer and spin it slowly left and right to see its subtle effects.

Note the parts of the scene which do not change at all: wood, flower petals, paint; these materials are generally matte-surfaced. Note the parts of the scene which change a great deal: blue sky, reflections, grass; these materials are glossy or otherwise align the light energy.

In all cases, the light is reduced by at least a stop, usually two full stops of light lost.

This page contains a Java applet that lets you spin a polarizer.


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