Paul C. Buff, Inc. Technical Forum

Shooting in a dark studio with no ambient light. Auto focusing has been difficult lens stopped down (F4). Can I turn up the modeling lights without effecting overall exposure? ie do the modeling lights go off when the flash flashes?

Jeff

Generally speaking, no, model lamps will not affect exposure. Perhaps if you have a very low flash output, and a very high model lamp output, there may be cause for concern, as you have a very high model lamp power, and camera settings to accomodate a low flash power.

If you have the model lamps to indicate when the flash units recycle, the power will be cut to the model lamp prior to the flash firing. However, since it is an incadescent lamp, the filimient is glowing hot, yeilding your light output. Even when the power is cut, it still takes a finite amount of time for the light to be completely extinguished (think about the stove eye that still glows red even after it is switched off).

Even still, at full power under most circumstances, it will be too low to affect anything. For example, if you have a 250W model lamp and a shutter speed of 1/250 of a second, you are only getting about 1Ws of power. Add to that, the efficiency of the model lamp bulb is about 40% of xenon flash, so it would be an equivilent to about .6Ws of flash. Unlike flash, this will follow the rules of reciprocity with shutter speed as well as ISO and aperture.

The best way to see how it will affect your exposure is to take a photo at your current camera settings without firing the flash, and see if you get anything but a black frame.

Great idea. I'll do that. Thanks,