Flash overexposure - it's the mirror!

Started Oct 18, 2012 | Discussions thread
ser dpreview
Forum MemberPosts: 99
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Re: Flash overexposure - it's the mirror!
In reply to philbot, Oct 18, 2012

philbot wrote:

ser dpreview wrote:

is the camera that controls the flash when to stop the emission of light.
The firmware of the flash does not affect this.

The camera doesn't tell the flash when to stop anything, it just tells the flash when to fire, and what power level.. How the flash generates that power is up to the flash electronics..

Flash does not illuminate for the entire shutter period, it's usually an incredibly short duration of intense light.. the more power usually means a longer period of intense light, but it's still way shorter then the typical shutter speeds..

For a Nikon SB910, the durations are

Flash Duration1/880 sec. at M 1/1 (full) output
1/1100 sec. at M 1/2 output
1/2550 sec. at M 1/4 output
1/5000 sec. at M 1/8 output
1/10000 sec. at M 1/16 output
1/20000 sec. at M 1/32 output
1/35700 sec. at M 1/64 output
1/38500 sec. at M 1/128 output

If a flash was the only source of light illuminating the subject, you can change the shutter speed from 1/30th to 1/200th and the image would be identically exposed, within the realms of the flash duration, it is the aperture that determines the exposure for flash..

The only difference the shutter makes is in how much ambient light contributes..

 

 

http://en.wikipedia.org/wiki/Through-the-lens_metering#Through_the_lens_flash_metering

Digital TTL works as follows: Before the actual exposure one or more small flashes, called "preflashes", are emitted. The light returning through the lens is measured and this value is used to calculate the amount of light necessary for the actual exposure. Multiple pre-flashes can be used to improve the flash output.

how  do you consider this ?



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