Autofocus

As i'm new to DSLR, i am 99% of the time, using Av or Tv mode...but often times, at night time, the autofocus NEVER locks in on shots, even when i switch to AUTO,(figuring maybe it was my settings)...But how do i take pics if the focus won't lock in???

sorry if it sounds silly, but i spend MINUTES trying to take 1 shot, and the AF is zooming all over the place trying to autofocus and NO DICE...even on AUTO...Please help

What body and what lenses?

All Canon cameras have a lower limit to the light they will AF in. The 1D's are the best, the Rebels are the worst and none of them can AF at all in pitch black.

The lens also matters. Cheaper lenses with slow apertures let less light in for the AF system to look at, and so this makes AF difficult in low light.

Finally, remember that the AF needs some contrast under the sensor to work. Make sure in tough light that you are placing the AF sensor on something contrasty in the scene to help it. If that fails then you have three other choices:

1. Manual focus. This is really hard with the small and dark viewfinder of the Rebels and it can be really tough with slow lenses in dark situations.
2. Enable AF assist and raise the flash. You can still turn the actual use of flash off. This will emit annoying blasts of light during focussing, leaving the subjects looking dazed for the picture.
3. Mount a Speedlight or ST-E2 to get the red crosshatch focus assist light.

JeffreyG wrote in post #5950054
What body and what lenses?

All Canon cameras have a lower limit to the light they will AF in. The 1D's are the best, the Rebels are the worst and none of them can AF at all in pitch black.

The lens also matters. Cheaper lenses with slow apertures let less light in for the AF system to look at, and so this makes AF difficult in low light.

Finally, remember that the AF needs some contrast under the sensor to work. Make sure in tough light that you are placing the AF sensor on something contrasty in the scene to help it. If that fails then you have three other choices:

1. Manual focus. This is really hard with the small and dark viewfinder of the Rebels and it can be really tough with slow lenses in dark situations.
2. Enable AF assist and raise the flash. You can still turn the actual use of flash off. This will emit annoying blasts of light during focussing, leaving the subjects looking dazed for the picture.
3. Mount a Speedlight or ST-E2 to get the red crosshatch focus assist light.

Well i have the xti and the kit lens, 18-55..I also have the 70-200 f4...And i've done choice 2 so many times, that it can be flickering those annoying flashes and NEVER actually focus..

SO those are my options huh? that stinks!! i'm in my living room and have dim lighting, but totally NOT black, and i can't even get a shot off...SO FRUSTRATING!!! so i buy a $600 camera and can't use it at night, how WONDERFUL!!

Auto Focus and Auto Exposure (A) are different , like apples and pigs.
You need to switch your lens to manual and try that out.
These cameras are tough to M focus because they don't come with a focus aid on the focusing screen. You can get split-screen finders for the newer models, which will help a lot.

izzy35 wrote in post #5950023
As i'm new to DSLR, i am 99% of the time, using Av or Tv mode...Please help

Welcome aboard, photography is just like raising kids, as soon as you think you know what you are doing something changes.

I suggest you do a little reading about aperture and then some about shutter speeds and then some about ISO.

After that find the M on your dial next to the Av and Tv settings.

You will then find your way to shooting in low light.

Get an external flash with AF assist. Lock on guaranteed most of the time.

+1....

sadatk wrote in post #5950186
Get an external flash with AF assist. Lock on guaranteed most of the time.

Not to sound snotty, but $600 really isn't a whole lot of money in the scheme of (photography) things.

Your lenses aren't that fast (aperture wise), so that limits the amount of light that hits the AF sensor. I have the 70-200mm f/4 IS, and I find that the AF Assist light of my 580EXII comes on most of the time when I'm indoors (f/4 is too slow without a flash in low-ish light anyway). My two fast primes don't seem to need the AF assist light as much as they let alot more light hit the AF sensors.

buy a $75 50mm f/1.8

+1
Assist light of my 580EXII comes on most of the time when I'm indoors (f/4 is too slow without a flash in low-ish light anyway). My two fast primes don't seem to need the AF assist light as much as they let alot more light hit the AF sensors.

if you're at home just try using manual focus. you'll get used to it with time and it works better than AF.

Re. manual focus, my 40D using the center point with my fast primes is plenty fast and accurate, that I can't image it being any better using manual focus (especially if I can barely even see the scene). The 580EXII helps as well.

egordon99 wrote in post #5953418
Re. manual focus, my 40D using the center point with my fast primes is plenty fast and accurate, that I can't image it being any better using manual focus (especially if I can barely even see the scene). The 580EXII helps as well.

the OP doesn't have fast primes though...

If you want the convenience of P&S photography when taking photos indoors in low light conditions, use your lens at the wide angle end, set the aperture as small as you can for the light you are using, commensurate with having a fast enough shutter speed to avoid camera shake (bump up the ISO if necessary), and manually set your focus to a hyperfocal distance that will suit the majority of your subjects.

Bingo - you can now use your dSLR like a P&S.