AF assist beam (350D)

Hey guy's, i was trying to focus in low light last night and couldn't get a lock. I was shooting in AV and the built in flash was up, but no assist beam!?! Any ideas?

The internal flash doesn't have an assist beam. What it does for AF assist is fire off a burst of pulses to light up the scene, sort of like those disco strobes of the 80's. If that didn't happen for you, check your custom settings to see if you have it turned off.

That's strange, i just tried it again with the light off, and it's working now!?! No settings have been changed, so i can't see any reason it wouldn't work last night!?!

Oh, and the manual calls the burst of flashes the assist beam.

beano wrote:
That's strange, i just tried it again with the light off, and it's working now!?! No settings have been changed, so i can't see any reason it wouldn't work last night!?!

Oh, and the manual calls the burst of flashes the assist beam.

If there's sufficient light for the camera to focus without the assist beam, it won't bother turning it on. So that may explain why it wasn't on yesterday.

There may have been enough light for the assist beam not to fire, but if none of the active focus points are over anything in the frame that has any kind of contrast then you wont get a lock either (try AF with a bare wall with no visible texture sometime and you'll see what I mean). Maybe that was the reason?

Not sure!?! It was too dark to get any focus on a person sitting about 2 metres away. I'll just have to wait and see if it does it again. I was hoping it was something really simple, a common mistake or something hehe!

Thanks for trying guy's.

Well, to have "enough light" doesn´t ensures to achive focus !! The AF systems works also based in contrast difference. You must have a contrast difference in the object you are pointing out to achive a correct focusing of the AF system. For example, if with sunlight you try to focus over the center of the big white cristal ball of the street lamp when you are very near of it (I means the ball covers a good percentage of the screen - center focus), no way. But if you move a little bit and you try to focus in the border of the ball with the background, bingo, you get it.

According the light is going down, the lack of contrast is going up; so more dificult for the AF system to focus on the target. The extreme case is by night, if you want to focus a landscape or a city view by night, one trick is sarch for a lamp in the same plane you want to take and do the AF over there.