I recently bought my first DSLR, a Sony a57, and some accessories, including a polarizing filter. While doing some research about this filter on the Internet, I found some confusing information. A couple of websites suggested “opening up by about two stops to compensate for the loss of light” for a correct exposure. What does that mean and how exactly should I do that?
Actually, a polarizer is much easier to use than those articles may lead you to believe, Karel. While the dark glass in this filter does reduce the amount of light reaching the sensor there is no need to compensate unless you are using an external hand-held light meter. All of today’s cameras employ a built-in light-metering system that considers the amount of light that actually reaches the sensor.
The expression “opening up” means using a wider aperture, or a longer shutter speed—when using the manual exposure mode—than you would use when not using a filter. That step is necessary when using a polarizer only if you are measuring the scene brightness with an external light meter as some professional photographers do. If you are not using such an accessory, simply ignore that advice. Mount the polarizer on a lens and shoot as if you were not using a filter.
Admittedly, no light-metering system always provides a perfect exposure as discussed in this month’s other Q&A. But that’s because no technology is foolproof; it’s not because you are using a polarizing filter. You may find that photos of light-toned scenes will cause the camera to underexpose. In that case you will need to set some plus exposure compensation (+2/3 often works well if you are using the Sony camera’s Evaluative light metering.) That override is available when you are using the Program, Aperture Priority or Shutter Priority mode. Your next photo will be brighter.