Filters for Digital Photography

There are a lot of different kinds of filters out there, but not a lot of information as to whether they are actually useful. I've found the UV, Polarizer, ND, and Macro filters to be the most useful of the filters. While other filters might seem tempting, you'll often find that they just don't deliver on their promises. In the digital world it is often easier and better to achieve an effect digitally than with a filter.

What follows are my experiences with a number of different kinds of filters and my digicam.

UV/Skylight (1A)

The effect of UV and the various skylight filters is pretty subtle. UV filters are intended to remove the blue cast in photographs taken at high altitudes, I'm not sure how a digital camera's CCD responds to them. Most popularly, UV and Skylight filters are used simply to protect a camera's lens from physical damage.

Skylight filters give a very slight red tint to photographs to make up for the color of blue skylight. It isn't really enough to convert open shade to daylight balance, however.

Polarizers

Commonly, polarizers are used to deepen blue skies in outdoor photographs. They also do a great job reducing reflections when shooting through glass or water at an angle.

There are two kind of Polarizers, Linear and Circular. Circular polarizers are needed with cameras that have phase-based autofocus systems (many 35mm SLRs do). All other cameras will work fine with a Linear Polarizer. The effect is the same with each, so it is probably better to get the Circular just in case you need it one day on a camera with phase-based autofocus.

Neutral Density

Neutral Density (ND) filters are sunglasses for your camera. They reduce the amount of light coming in without changing the color. Typically ND filters are used when shooting in bright light to allow for a larger aperture and consequently a smaller depth of field. With a smaller DOF, you can make the background less distracting by letting it blur.

ND filters come in many strengths. A special numbering system is used to identify each strength. Basically, .1 = 1/3 f-stop in the ND filter numbering scheme. So, a .1 ND filter cuts 1/3 stop, and a .3 ND filter cuts 1 stop.

Macro Filters

Macro filters are magnifying glasses for your camera. They work surprisingly well, and can turn a non-macro camera into a competent macro camera. If your camera already does macro, the macro filters will let you get even closer. These are very useful filters to have around.

Macro filters usually come in sets with strengths of +1, +2 and +4. You can mix the filters to get more macro power. Always keep the strongest filter closest to the camera's lens.

Color Balance

Digital cameras have some really great white balance algorithms that can work wonders when dealing with various light sources. The drawback is a loss in dynamic range of the image and the dreaded purple fringing (sometimes erroneously referred to as "Chromatic Aberrations") of overexposed portions of the image. These white balance algorithms work on the digital data after it has been captured, so some information will be lost. You can get better results using external filters to match the light you are shooting by to daylight, and leaving the camera in daylight white balance mode.

Here's an example using my Canon G1. This scene was lit by incandescent light (GE 60w krypton flood). I shot it first with the camera's incandescent white balance setting, then with an 80a filter and the camera's daylight white balance setting.

Incandescent White Balance ISO100 1/20s f/2.0

80a filter ISO100 1/5s f/2.0

At first glance, the images appear identical, but upon close examination it becomes clear that the 80a image shows no purple fringing around the overexposed desk lamp, and a little bit more shadow detail. If you're familiar with the Maryland state flag, you'll also note more accurate color in the 80a image. The main drawback to using the 80a filter is the loss of two stops worth of light as evidenced by the exposure values.

Decrease in Purple Fringing
Incandescent White Balance	80a filter

Slight Increase in Shadow Detail
Incandescent White Balance	80a filter

I also shot this image using daylight white balance and no 80a filter. This combination made it difficult to judge proper exposure. The exposure that looked right was actually overexposed by 1 stop and lost detail in the red channel.

To summarize, if you must shoot in strange lighting situations, use the camera's white balance features. Otherwise if you can swing it, use an external filter. Finally, if motion blur is a problem in a dimly lit room, use the camera's white balance feature, and try underexposing by one stop. This will give you a better shutter speed, and you should be able to brighten the image up when you get it into the computer.

Soft Focus

You can always do a Soft Focus effect in Photoshop, but if you want to retain the CCD noise (digital grain) as part of the image, using a Soft Focus filter will let you do that. Bear in mind that the soft focus effect with these filters varies based on the aperture and focal length. Long focal lengths and wide apertures give maximum effect.

I've tried the Tiffen Soft/FX #1 filter and found it to be far too little effect for my Canon G1. The #3 would probably be best to start with, although a #5 might be better for a digicam.

Tiffen Ultra Contrast

The Tiffen Ultra Contrast filters are intended to reduce the contrast of an image. They are made of milky white glass (like a dirty window) that will bring up the value of the shadows in an image. The effect varies based on how much light is hitting the milky glass, even from outside the frame. Using a lens hood can reduce the effect significantly.

Unfortunately, shadow detail is lost, instead of gained with these filters, so they don't really live up to expectations. In fact, it's just like shooting through a dirty window. A much more impressive effect can be had by using a tripod, and taking two shots at different exposure settings to capture the light areas and dark areas. Then layer these together in Photoshop and pick the parts you want.

Infrared

Infrared filters give interesting effects with digital cameras by only passing the infrared portion of the spectrum. My Infrared Digital Photography page has more info and some examples.

Hot Mirror

Hot mirror filters remove infrared light. Because digital cameras are very sensitive to infrared, most digital cameras have a hot mirror filter already built-in. Adding another can sometimes help when the built-in hot mirror isn't good enough. Unfortunately, hot mirror filters cost a lot of money, so experimenting with them is a costly proposition.

Color Correction Math

If you hate math, I'm sorry, skip this. The effect of Color Temperature conversion filters on color temperature is measured in units called "reciprocal megakelvins" (MK^-1). Given the filter's shift value in MK^-1, you can figure out how the filter will change a given temperature of light. The equation is as follows:

T2 = 1 / (1/T1 + F)

T1 is the source light color temperature in megakelvins (MK), F is the Filter shift value in MK^-1, and T2 is the resulting color temperature. Kodak's charts provide the necessary filter shift values.

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