flash sync ^ need some help

[cgennoe]

Hi there,

i am trying to get my head around flash sync speed.

The book i am reading is fantastic - 'Digital SLR Photography for Dummies'.

Basically the book ascertains that 'Most electronic flashes built into SLR's have a max duration of about 1/1000 of a sec. And that if you are shooting at 1/250 at f8, that the exposure will be provided by the electronic flash at 1/1000 of a sec irregardless. That essentially the camera's shutter speed is irrelevant when ambient light is insufficient and exposure is provided by flash.

The book goes on to define flash sync speed as the shortest shutter speed that can be used to take a picture with the electronic flash. This can vary from 1/500 sec in some Nikon cameras to 1/200 in the Rebel XT.

I hope that you can help me understand in theory the actual duration of flash speeds in SLR's and also coordinating flash and shutter. It would appear that the longest they will provide exposure initially is 1/1000 of a sec and then that they cannot provide a shorter exposure than 1/500 of sec when defining flash sync.

I appreciate this! :thumbup:

 

[Big Mike]

First of all, the flash duration isn't really important. It has to do with the power of the flash and how quickly it can release it's stored power as light. It might come into play if you were doing some high speed photography...but that's a whole other issue.

Long story short...don't worry about the flash duration.

The max sync speed however, is a limitation of the camera and it's shutter. Most modern SLR cameras use a focal plane shutter that is made up of curtains. There will be two sets of curtains that open in either direction to expose the film or sensor. The curtains may not be able to open at exactly the same time...although they should open for exactly the same length of time. So during an exposure, there may be a very short period of time when one curtain is open and the other is closed or not fully open.
Now, we know that the flash duration is very fast, so if the flash fires when one of the shutter curtains isn't fully open...the reflected light won't be able to reach the whole film or sensor...leaving a dark or black section in the photo.
The max sync speed is the fastest shutter speed at which the shutter is guaranteed to be full open when the shutter fires.

 

[Garbz]

That sums it up. Also flash duration number that is quoted is usually for a full high-powered blast. The Nikon SB-800 at full blast fires for 1/1030th of a second. At it's smallest power it lasts only 1/40000th of a second.