This is a warning to the people thinking of getting the 18-55mm lens that comes with the 20D and other Canon digital cameras, DON'T GET IT. I knew it wasn't going to be a good lens but I had no idea it would be this bad. Tonight I was atempting to get some pictures of my cat in light that is far from dim and the lens had a hard time finding a focus point. The lens works fine outside but in anything under normal interior lights without a flash you may run into issues like I did. If I turned on the on camera flash the lens worked ok but I don't like using a flash when taking pics of pets.

This will be true for any slow aperture lens, not just the kit lens, on low-contrast subjects.

Yeah, next on my list after the remote shutter release is the Canon 50mm f/1.4 or 1.8 prime lens.

No need to go to such extremes. I take REAL low-light shots with my f/4 all the time. You just have to know your equipment. The center sensor is usually the most sensitive, as well as being a cross sensor. Point it at something with a lot of contrast (eye, stark lines, etc.) and it will focus.

Last night my f/4 was focusing in light that was giving me 1/6 shutter speeds at ISO 1600. I didn't pull out my 1.8 once, no need to.

It may not be the lens either, some 20D's have low light issues w/AF, although you'd think Canon would have fixed the issue before shipping new units, it's nothing new though. The 20D has had this since it first came out.

JS

I have had problems focusing with the 18-55 on the 20D with a 580EX when taking pics at events with very low light. I had thought the focus assist on the 580 would help out, but quite a few of my shots came out quite unfocused.

However, I wouldn't underestimate the 18-55 for outdoor shots in bright daylight. Stopped down to f/8 or f/11, it can be surprisingly good for the money. Its not the sharpest thing in the world compared to a 24-70 at f/8 obviously, but it holds its own in doing landscape or building shots.

I have had problems focusing with the 18-55 on the 20D with a 580EX when taking pics at events with very low light. I had thought the focus assist on the 580 would help out, but quite a few of my shots came out quite unfocused.

However, I wouldn't underestimate the 18-55 for outdoor shots in bright daylight. Stopped down to f/8 or f/11, it can be surprisingly good for the money. Its not the sharpest thing in the world compared to a 24-70 at f/8 obviously, but it holds its own in doing landscape or building shots.
I understand the lens works well outside given its price, I was just disapointed on how it worked inside without a flash.

The AF sensors work by sensing contrast differences, it there is dim light, the contrast is lower, and thay won't focus, BUT if you aim the sensor at a point where there is a big contrast difference, it will focus even with low light (unless it's REALLY dim). All the AF sensors are like a thin line, but the center one is like a cross, and can sense a contrast differences horizontally and vertically.

As already told, use the center focus point over a part of the scene with a great change in contrast, and it will be fine.

Focusing in low light is an issue with all lenses, to a greater or lesser degree. Surprisingly, the 50 1.4 is a poor low light focusing lens. Not sure why, exactly, but it hunts on my 10D in low-light. One solution, if you want to have focus assist but no flash, is to set one of the Custom Function to "emits/doesn't fire." This will let you shoot available light. The flash is deactivated except for the focus assist beam, and the camera sets its shutter speed and aperture based on the available light. This is VERY handy in dim light.

Also, use the center focus point in dim light. The center is a "cross sensor" (as others have mentioned) so it is the most sensitive focus point.

Here's one I shot using the center focusing point, and the flash focus assist light, but no flash, at ISO 3200 with a 28 1.8 lens at (probably) f/2.2. It was shot in a really dark Goth club, of two vampire goth chick friends of mine (johnna, on the right is a lawyer. :))