Analysis of High Speed Sync performance

tdodd

tdodd

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Today I found an interesting analysis of how the power available from a flash gun diminishes as soon as you turn to High Speed Sync, and diminishes further the higher you go with your shutter speed. Results are included for both Canon and Nikon.

It should help explain to those of you battling against bright backlighting or ambient light in general should probably consider using ND filters if you want shallow DOF, rather than simply increasing shutter speed.

http://neilvn.com/tangents/2010/08/02/high-speed-flash-sync/
 
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Thanks Tim, I have been struggling with this for a while now.
 
what an excellent document! :clap:
 
Good link Tim :thumbs:

High speed sync works like continuous light in terms of shutter speed because that's what it actually is - continuous light just long enough to cover the complete shutter cycle.

High speed sync flash is not really flash in the generally understood sense, but continuous light - not just effectively continuous, but actually continuous light. The peaks of the flash pulses are virtually invisible if you see them on an oscolloscope trace. So when you halve the shutter speed, you halve the exposure. It's just like turning on a light bulb very briefly, taking the picture, and then turning it off again.

The new auto-TTL Pocket Wizards are clever in that they can adjust the power of the flash according to shutter speed, or at least partially - the relationship is not linear, or anything like it. Basically, at very high shutter speeds like 1/4000sec the shutter cycle is slighly shorter than it is at say 1/300sec or so, and the the PWs compress the HSS output into this shorter time, with a relatively small but useful increase in brightness. Every little helps with HSS.*

Edit: * sorry, this para probably just adds unnecessary confusion to Tim's link.
 
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thanks for posting that, i am absorbing flash info atm so it was a great read ta :thumbs:
 
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