Living in a relatively touristy city, I see streams of people around me taking pictures using just about any camera and the most common mistake people do is using the flash regardless of subject distance.
This occurs by a huge margin above any other mistake. People do it with P&S and DSLRs equally. It seems less forgivable when done with a more expensive camera though! After all, those people payed that much for something so capable.
Using the flash constantly is not simply unnecessary, it often makes matters worse:
- Flash has a limited range which depends on ISO and aperture. This is usually a few meters, rarely more than 5 with a built-in flash. Anything further gets too little light to make a difference. The biggest offense in this category is flashing a sunset! The chances your flash is powerful enough to brighten up the sun is less than zero.
- Cameras have several flash modes and that influences the chosen exposure. When a camera which does not use TTL-flash is set to Auto Flash or Forced On, as opposed to Slow-Sync or Fill, it expects the flash to light up the scene. When it does not reach the subject, the resulting image is under-exposed. A number of DSLRs and a few other cameras use TTL-flash which avoids this problem by testing the flash before firing it for real. Small cameras rarely use this because they would run out of power too fast, which brings us to the next point:
- Flash uses power, a considerable amount of power. Battery-life is measured with 50% flash use according to CIPA. For 100% flash use, it will be a lot shorter!
- Flash reflects easily. Some people shoot scenery from a moving vehicle with the windows closed. The results? A picture of a bright flash being reflected. The same happens in museums for artwork behind glass. To use the flash through glass, one must shoot at an oblique angle so that light is reflected away from the camera.
- The background is a part of the image too. As mentioned earlier, flash illumination depends on subject distance. So, unless your subject contains nothing but a wall, your shot will have elements at different distances which get different amounts of illumination. This is why most photographs with a single flash as primary illumination looks like it was taken in a cave.