Can you help me solve the exposure problem with my new digital SLR camera? It’s fine when I use the fully automatic modes. But I tried taking photos in a church during a wedding (without flash) and all my photos were underexposed. I was using ISO 400 and set the shutter speed to 1/500 sec. in Shutter Priority mode to make sure the photos would be sharp. But they are all too dark. D.M.
This is also a common problem among students in my BetterPhoto courses when they first try night photography. The answer is simple, but you need to fully appreciate how this semi-automatic mode works. In theory you can set any shutter speed and the camera will set a suitable aperture for a good exposure. If you change the shutter speed, the camera will change the aperture to maintain the same exposure.
That often works well, especially in outdoor photography in daylight. However, in a very dark location, you need to use a much higher ISO – and a much longer shutter speed – for a good exposure. If you set 1/500 sec. at ISO 400 in low light, the camera cannot find an adequately wide aperture to provide a good exposure. (The same would apply if you set a small aperture in low light when using a low ISO level.) When your settings cannot provide a good exposure, most cameras will provide a warning in advance: blinking numerals in the viewfinder.
Note: Instead of providing a warning, some cameras will actually change inappropriate settings when that’s necessary to avoid a grossly incorrect exposure. For example, certain EOS DSLRs can provide this feature when the Safety Shift AV/TV or the Safety Shift ISO custom function is set to On. Do note, however, that neither custom function is available with all EOS cameras.
When you get a warning in low light, start by setting a higher ISO, such as 1600. If the blinking still continues, you must also set a longer shutter speed, such as 1/125 sec. If it’s really dark, you may need to use an even longer shutter speed, such as 1/15 sec. When the blinking stops you should be able to get a well-exposed photo. Granted, at the long shutter speed, the photo may be blurred: from camera shake or from movement of your subjects. If your camera offers even higher ISO options, try those. They will allow you to shoot at a faster shutter speed but your images will be more “grainy” due to digital noise.