Did some more experimenting.
The problem I am battling with (one of them at least) is that using the AF module out of the Canon 550EX flash uses way too much current to be used almost continuously. In my last post (AF Assist tool v3.0 – more info) I measured 400 mA current draw. That will kill a set of 2500mAh AA rechargeable batteries in 5hrs ish. Not to mention the beefier power electronics that are needed to handle nearly 1/2 amp continuously. (though as I write this, I’m beginning to think that maybe 5hrs may not be too bad. Assuming that the tool will be used 50% of the time, that puts the usage time at about 10hrs. That’s a day’s worth of shooting. Not outrageous.)
What if I take the AF assist module from the 540EZ flash, and shoot a defocused laser beam through it? I found a Ø 4mm laser module from DealExtreme and stuck it where the LED would go on the 540EZ AF assist module. The laser runs on 3v, and draws 25mA.
The pattern of the bare unfocused laser is just a splotch of light. When the AF assist module is added to the mix, a clean sharp pattern emerges.
The images shown above are the actual frame coverage at 50mm on a crop body (x1.6) Canon 40D. The pattern on the right has no problem getting the camera to focus a f2.8 lens in pitch darkness at 40ft away even on the external focus points (that aren’t cross type like the center one). A f2.0 or f1.4 lens should be able to focus with a much less intense laser power.
The pattern of just the defocused laser (on the left) will focus with the center point only, and it’s hit and miss, and takes longer to focus.
I may be onto something…