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Why are different methods of finding focus in low light used by cameras and flash guns?
If my flashgun can use a discrete infrared AF assist beam, why can't the camera? I am really peed off by the strobe assist of the pop-up flash - it irritates people and sometimes they find it hard to know when the picture is being taken. I know the pop-up flash is not exactly the be-all-and-end-all, but sometimes it's useful.
As far as I know, all Canon DSLRs with built-in flash all use this strobing light, but I wonder why. Is it a matter of power or technology or both? Is it actually supposed to be better, in theory? What do other cameras do?
I'd need to look this up again, but off the top of my head:
The IR system on a flash doesn't have to be too accurate, within a centimetre or so. So the IR beam just bounces back off a surface and gives the flash an estimated flash to subject distance. But the camera has to focus accurately, if it was out by a centimetre you'd be a bit peed off. To focus it uses contrast, and in low light it cannot see contrasts so the flash fires a rapid set of pulses to provide light with minimal battery drain allowing the camera to see the contrast and focus correctly.
I think it was done to save cost, i.e an extra IR source costs money, while the on-board flash is already there. Rather sadly, I have to confess to having once used the flash gun to provide the IR light source with flash suppressed. My compact has a small focus assist light, it is a bit more discrete than the flash gun.
My understanding was the IR source sends a controlled light beam in a grid pattern and the AF sensors happen to be sensitive to visible and IR light so predict focus to give a sharp pattern. as the camera knows it has a flash gun attached it knows not to strobe.
But I could be wrong as it is all a guess.
Oh good evening John 
That makes sense - if the strobing flash works, why bother with the additional expense.
Shame, though.
I have been looking at the smallest hotshoe flashguns to see what they use - Canon's 270ex strobes, whilst the Metz 24 has Infrared. If the IR one is pocketable, has a bounce head and some extra power, I am inclined to give one a go.
What do the other camera brands use?
Quote: All of Canon’s Speedlite flash units for EOS cameras have patterned red AF assist lights - sometimes called AF auxiliary lights in older Canon manuals - built in. These are clear red panels on the front which use one or two high-brightness LEDs to project red circles of light striped with dark lines, in order to give the camera a high-contrast pattern to focus on. Red is chosen in part because high-output red LEDs are readily available, but also because red light does not cause the pupils of the eye to dilate as much as does white light. The red light is sometimes described as being “near infrared,” though it is in fact visible.
Yup its a red visible light.
Reading this may help AF Assist paragraphs
the Nikon SB-900 and SB-800 use a red grid pattern that is visiable to the eye... so not infa red, but the Nikon D80 and D300s camera uses a small white led to iluminate the subject and get focus etc.... which i turn off, as i dont like them,,,, and with the lens hood attached, the light doesnt make it past it anyway.
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