automatic focusing on Rebel 2000 (EOS 300)

ckshen · Apr 26, 2004

when i use the automatic focusing feature on the camera, it focuses. but if i zoom in some more i notice that the image is still a bit blurry so i focus again and then zooms out to my original composition.

is this the right technique? or should i just stick to the original 'focused' image? does this happen because its supposed to happen or is it because of problems with the lens/body? i am using the cheap 28-80mm that comes with the body.

photobug · Apr 26, 2004

Some zoom lenses have the ability to stay focused throughout the zoom range. I believe the term is "parfocal."

I wouldn't expect a less expensive lens to have this ability. Try it both ways. If it focuses ok on its own, do it that way. Otherwise the zoom in, focus, zoom out thing, while a PITA, does seem to work.

markc · Apr 27, 2004

Thanks Jim. That's something I had wondered myself in the past but never looked up.

It's probably not parfocal. I just looked up some lenses, and even with the Canons, it's only the fixed aperture L series pro lenses and those with non-rotating front elements. If you focus while telephoto and then put it back to wide-angle, the focus will be off once you move it back. You are best of to focus at the focal length you are going to shoot at.

fotoadam · Apr 27, 2004

when it comes to zoom lens, be prepared to spend a lot. I bought a Phoenix 35-300m lens and im EXTREMELY dissapointed. You have to be atleast 6 ft from the subject in low light to get even remotely focused.. i still love my canon 35-80mm lens that came with my Rebel..

adam

Galaxy_Stranger · Apr 27, 2004

With a Telephoto, if you zoom in all the way and properly focus on your subject you can zoom strait out and be fine - as long as you're telephoto. Just be careful you don't turn the ring any.

Besides, your depth of field changes anyway - in fact, you should be able to do the math. Stick it on a tripod and take a couple test shots both ways.

Am I rambling??

markc · Apr 27, 2004

Galaxy_Stranger said:
With a Telephoto, if you zoom in all the way and properly focus on your subject you can zoom strait out and be fine - as long as you're telephoto. Just be careful you don't turn the ring any.

I don't think it works that way. It's not like the focus goes out of whack as soon as you hit 50mm. It's a gradual shift.