Originally posted by mikemike If you are bouncing the flash or shooting through a gel or diffuser, how does the flash know this and compensate for the lost effectiveness?
Others have told you about the magic of TTL or p-TTL. I'm telling you about the lowly light sensor on the flash used in flash "auto" mode. It's not as high tech, but is just as effective. In the case of K10D and K20D, it's even more accurate than p-TTL.
Many flash units, the ones with "auto" mode setting, have a light sensor to tell the flash's electronics when there has been enough light and turn off the light output.
There are some drawbacks:
1. You have to make sure the light sensor is facing the scene being photograph. If the flash unit has head that can bounce and swivel, this is pretty simple to do (you as the operator still have to pay attention though). If the flash can't bounce or can't swivel (e.g. Vivitar 285, 283, or Pentax 360), this may be difficult or impossible. Some flashes have "remote sensor" to handle this situation. Here's one with such remote sensor:
2. The light sensor has a fixed field of view, which may not match the field of view of the lens. In most cases in which the scene is "average," this is not a problem. Some remove sensors have two settings, one "normal" (e.g. 35mm FL in 35mm format) and a "tele" (70mm FL). It's not obvious in the photo below, but with the Metz remote sensor Mecamat, I call pull the lens of the sensor out for the "tele" setting:
3. The light sensor on the flash doesn't know if there is any filter attached to the lens to compensate for the filter. But with DSLR, how often do you use lens filter for flash photo?
"Auto" mode does have some advantages over p-TTL:
1. No preflash: some people and animals react to the preflash strobe. For this reason, I don't use p-TTL when taking photo of a group of people - someone may blink at the preflash and appears to close his/her eyes when the photo is actually taken.
2. Works with any lens. p-TTL requires at least an A lens to work properly. This is a reason why I don't recommend Metz 48. It does not have "auto" mode so has to be in manual mode when used with a manual lens. It's not the end of the world, just not as convenient.