Filters decrease the amount of light that reaches your camera’s lens. When your camera cannot meter properly through the filter, you need to manually adjust the exposure settings in order to take a properly exposed picture – that is, a picture with adequate detail preserved in both the shadow areas and the highlight areas. Otherwise, you’ll end up with a digital photo or film negative that is too light or too dark, and probably unusable.
To understand exposure compensation, you need to understand exposure. It is assumed that you understand the basics of exposure – but just in case, here’s a brief recap:
Exposure is made of up three elements (dubbed the “exposure triangle” by Bryan Peterson in his book, “Understanding Exposure”).
- Aperture – size of the lens opening, measured in f/stops. Smaller numbers are wider apertures, and vice versa.
- Shutter Speed – amount of time that the shutter is open, measured in seconds or fractions of a second.
- ISO – measure of how sensitive your camera’s sensor or film is to light.
All three elements need to be considered when determining a proper exposure: changing one setting affects the others.
If this sounds like Greek to you (assuming you don’t speak Greek), you might want to pick up Peterson’s book. Short of that, you can still achieve great results with your filters and a modern camera using the exposure compensation feature.
How Filters Affect Exposure
Filters are often rated by the number of stops of light that they block. Polarizer filters block 1 to 2 stops of light. The R25 and #87 filters, often used for infrared photography, block 3 to 5 stops of light, respectively.
In order to shoot a scene correctly, you need to tell your camera to allow in as much light as is being blocked by the filter. You need to increase the exposure settings.
Using Exposure Compensation
To increase exposure, you can adjust one or more of the settings in the exposure triangle. Or, you can use the exposure compensation setting built into most modern cameras to automatically calculate an adjustment for you, based on the mode that you’re shooting. If you’re set to Aperture Priority mode, the exposure compensation feature will adjust the shutter speed to increase exposure. If you’re in Shutter Priority mode, it will widen your aperture to increase exposure.
Here’s how: Arrive at the scene and compose your shot. Meter the scene with your camera’s light meter, without the filter attached. Note the settings. Then, attach the filter and meter the scene again. Has your camera properly adjusted to compensate for the filter?
Let’s say you’ve got a Red 25 filter on the lens, in Aperture Priority mode. With the filter installed, your camera should be metering an exposure close to 3 stops longer than without the filter. If not, add exposure for as many stops are necessary to make up the difference. If your camera cannot meter through the lens, adjust for the full number of stops for your filter.
With these tips, you can enjoy the benefits and creative results of shooting with filters, without the frustration of improperly exposed images. Happy shooting!