Wednesday, April 2, 2008

Obtain perfect auto-focus in the pitch dark

You guys all probably know this trick, but for those who don't: One of the side-benefits of an external flash is the 'flash assist' feature. On most (if not all) of the Canon external flashes, there is a 'flash-assist' feature where the flash assist paints a grid of red lines on your subject so the camera's auto-focus engine has something to focus on. This allows the camera to obtain focus in the pitch black - pretty cool. On most Canon DSLR bodies you also have the ability to enable/disable flash and also enable/disable 'flash assist'. So, you can even use flash-assist without firing the flash (if for example it's too dark to focus, but you don't want to fire the flash). If you aren't using this technique - it's a good one :)

The external flash 'flash assist' also does not perform the very annoying 3 'pre-flashes' the Camera's built-in flash uses for flash-assist (which makes all the subjects close their eyes and/or grimace.) No one even notices the red lines from the external flash, flash-assist.

This 'flash assist' technique was required to make the following photos:

This shot was made in the pitch black. Canon 5D, 50mm f1.4 lens, manual exposure of 2.5 second @ f1.8, ISO 400, external flash is on, flash-assist is on (this is how the camera was able to focus on the subject.)
(see the post here for complete details on how to make photos like this)

This shot was made in the pitch black. Canon 10D Camera, Canon 17-40 f4 Lens, Focal length 17mm, Shooting mode of 'program', 1/60th of a second shutter-speed, f4 aperture, ISO 400, exposure compensation -1/3, external flash is on, flash assist is on (this is how the camera obtained focus on the subject.)

If you're going to buy an external Canon flash, I'd recommend the Canon EX430 at minimum, this flash not only gives you TONS more lighting power than the 'built-in' flash on the XT, XTi, XSi, 30D, 30D, etc, but the ability to tilt/swivel the flash head and 'flash assist'. With the excellent ISO 800/1600 performance of the modern Canon DSLR, the external flash gets uses less and less, but I would never remove mine from my bag -- it's a required tool for me. The Canon EX580 is the 'Cadillac' of Canon flashes, but it's huge and costs $400 $US.

Here's the 430EX on for about $250 $US.


Clint Bogard said...

I received an email from a subscriber asking 'how do I enable flash assist on the 5D + 430EX'. Below is my reply:

1) go into 'custom function'
2) there is a custom function for 'flash' (flash fires, flash does not fire)
3) there is a custom function for 'flash assist' (on, off) [you want to turn this on]

AF must be set to one-shot. flash assist does not work with AI-servo ;)

Obviously the flash must be 'on' for the flash assist to work :)

If flash assist is working, it will emit a grid of red lines on your subject (you can test on a wall)


ecopix said...

Hi Clint,
Have you noticed a cut-off focal length for focus assist with the Canon gear? I photo nocturnal wildlife with Nikon's SB900, which is limited to about 180mm, irrespective of lens speed. I like the red grid because it doesn't spook animals, as shining a bright light does. I have a Canon 5D2 and 100-400, but don't want to buy another flash if it won't give me at least 300mm.
Thanks for this discussion,