The Canon EOS and Photography Forums

Earlier in the year I photographed hedgehogs using natural light (I don't think using flash with nocturnal animals is ethical) and found focusing was extremely difficult. I expect that the dedicated AF assist light found on the better Speedlites should solve that problem - but can the 430EX II and 580EX II be used for that purpose without flash?

You didn't say what camera model you own as it is a camera menu option.  My wife has a Rebel xti and it has no such ability. My 40D, and 7D can be set to disable the flash under the menu item "Flash Control" while providing the camera  the focus assist beam with both the 430 and 580. My 5DMk2 calls the menu item "External Flash Control" and it requires one of the flashes to be attached and powered on before the feature can be set.

Its a 350D (Rebel XT) so that's why I've not come across it as a camera option. From the manual for the 580EX II it looks like the master controller can be set not to have its flash fire so that might do what I want but I was hoping to not spend that much. Otherwise I guess the 430EX II could be turned off once focus has been set?

According to page 149 in the 350D manual custom function 5 will control the external AF assist beam.

The Rebel xt will not do what you want with flash. You could as you said turn an external flash off after achieving assist focus. You would need to maintain shutter pressure and shoot in aperture preffered or manual exposure . Using a mode like program will set exposure based on flash being used. Also  something not mentioned in the manuals that I recall is you need to be set for single shot shooting and not continuous.

The custom 5 function mentioned has a different purpose. It allows or disallows assist light usage and has no ability to stop the flash from firing. As for turning the flash off in the 580's menu options,  that won't work either. The assist light fickers at intervals to let you know it's ready (multi-flash set up) but the 580 will fire a pre-flash as that's what it uses to communicate in multi-flash set up.

 

With the default setup you're right about needing to maintain constant pressure on the shutter button. I usually have custom function 3 set so that the * button controls autofocus. I find this more convenient when using lenses with full-time manual focus. It should also let me play my tricks with turning off an external flash, irrespective of camera mode selected (though for long exposures in the dark I usually use full manual mode (M) anyway).

Thanks for you help. Very illuminating!   ;)

You might be able to make your own assist light.  Get a flashlight and put some red cellophane over it.  I don't think there is anything special about the wavelength of the on-board assist light.

One item I didn't mention is the pricey Canon ST-E2 transmitter. This is what I use for low light flashless use when contrast is too poor for the camera's auto focus. I have tried it on my wifes Rebel xti with no issues so would think it would work on the Rebel xt.  Just seems like a poor use of money just for that ability.

http://www.adorama.com/CASTE2U.html?searchinfo=ste2+transmitter