flash question

[Skyeg]

when I use a non-dedicated auto flash like a vivitar 283 or 285. i set my camera to the flash sync speed, my vivitar is 1/125 my nikon is 1/250 and my other nikon is 1/90, set the iso on the flash, set the aperature and put the flash to the coresponding setting, lets say F5.6 and the yellow one on a vivitar 283. why do the same settings all work at differnt shutter speeds? that confuses me

 

[ksmattfish]

A camera's flash sync speed is the fastest speed at which the focal plane shutter actually opens 100%. Speeds faster than the flash sync speed are created by moving an open slit across the film frame. So as long as you set your shutter at the sync speed or slower you'll get the 100% open shutter that the flash needs. If you set the shutter higher only part of the image will be exposed to the flash. You can find examples of this blooper in most how to books and sites.

The burst of light from an electric flash is extremely brief. You could think of it sort of like a shutter set at 1/100,000th+ sec. as far as the subject matter lit by the flash is concerned; because it is the only source of light it overrides the camera body shutter. Of course in some compositions there will be subject matter not lit by the flash. The shutter speed set on the camera dictates how the part of the photo not lit by the flash will expose.

For instance, if you are taking a pic at night of a couple on a downtown street, set the aperture for the flash, then set the shutter for the background lights and stuff beyond the flash range. A shot taken at 1/250th would have a much darker background with less detail than a shot taken at 1/30th, while the couple, exposed by the flash, would look the same in each image.

This same idea works for daylight fill flash also; you can control the flash exposure of your subject in relation to the ambient exposure of the background. You can manipulate the look of the portrait by setting the subject to expose the same, or over or under compared to the background.