Which flash to use?

Started 3 months ago | Discussions thread
peripheralfocus Veteran Member • Posts: 4,691
auto flash exposure works fine within its limits
1

brandon77 wrote:

Oh Thankyou. I didn’t even know that existed. I wonder with that kind of range, what the chances are that you’ll get the right exposure. Have you worked with this?

There's a sensor on the flash that, when you're in auto mode, reads the light reflecting back from the subject in real-time (i.e. at the speed of light) and stops the flash, in the middle of its firing process, at the proper output level. It's perfectly capable of controlling the flash output to make proper exposures from 1.2m to 9m, consistently and accurately. It stops the flash quickly for lower output when your subject is close, and it lets the flash fire longer for higher light output when your subject is further away.

I've used a dozen flashes like that (known as 'auto thyristor') including the Vivitar 283, and they work fine within their specified auto distance ranges.

The one gotcha is that they are just like reflected light meters (that's what the flash's sensor is, in fact), so they will be fooled by very dark or very light subjects. If your subject is dressed all in black, the auto thyristor will overexpose the picture, and if your subject is all white, it will underexpose the picture.

In those situations, you can correct for the error, by setting a different f-stop on the camera than the one the flash is telling you to use. You mismatch the f-stop on the camera and the f-stop indicated on the flash. To correct for overexposure, set a higher f-stop (narrower aperture) on the camera. To correct for underexposure, set a lower f-stop (wider aperture).

Also be aware that flash exposure controls (the auto modes or the guide number) are calculated based on the idea that you are shooting in the dark and the flash is the only light source. If you're mixing flash with ambient light, you may want to use a lower flash output than the normal exposure standard. Again, you can do that by setting a different f-stop than the one that the flash is telling you to use.

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