AF in Rebels (450d) not at low light

[stanislaw32]

Hi
I am third time here talking about Canon EOS 450d.

First time it was about crazy colors and contrast when shooting in JPG, I got your help here:
http://www.thephotoforum.com/forum/showthread.php?t=125661

Second time it was about 17-85mm lens being possibly too heavy and a necessity to buy a better tripod, I got nice advice here:
http://www.thephotoforum.com/forum/showthread.php?t=143968

Now I would like to come back to the same issue I started in the second link above.
I have noticed my tripod pictures became not sharp, a little unsharp when switching from 350d to 450d and even more unsharp when switching to heavier lens (18-55 to 17-85).

I have performed many tests and currently I believe that not the tripod was a troublemaker here. I think it is a combination of 450d and a slow lens (17-85).
I have read somewhere (I cannot find the source anymore) that Canon rebels were not design to autofocus above certain aperture value (i.e. at low light).
I have never experienced this kind of trouble with EOS300 (not digital), and not even 350d (however I used 18-55 lens which is low quality but is faster than 17-85.

17-85mm IS USM being better quality than average kit lenses is surprisingly slow (4-5.6).
It gives me amazing images with 450d when shooting in daylight but I do have trouble when the light is scarce.

Yesterday I performed some blue hour tests and all the
shots which were taken with the manual focus are ok and all autofocused are not sharp.

I know some people rely mainly on manual focusing anyway but I wear glasses so AF was a convenient thing for me.

Now live-view mode in 450d is of some help because you may manually focus much easier, however when it gets darker it is not really working well.

And low light shots were main reason for me to buy slr and not a compact.

I do consider now purchasing much faster lens Canon EF-S 17-55 F2.8 IS USM, which is very expensive and have smaller zoom range than 17-85 but is very fast. But I am not really sure if it really helps me. If I knew that it will make my pictures look like the ones I took with predigital EOS300, I would not hesitate, but if it does not help... may be it makes more sens then to switch to full frame already. I am not sure if I am financially ready for that though.
Or I just get used to manual focusing.

I was curious if anybody has similar experiences?

 

[Big Mike]

When shooting on a tripod, are you turning off the Image Stabilization? Leaving it on can result in blurry shots when the camera is very stable.

Can you post some example shots (along with EXIF info)?

 

[stanislaw32]

That is very interesting.
Thanks for this advice.
It is true that my low light trouble started with 450 which has in the kit 18-55 with stabilization and then I have this 17-85 with stabilization and I never switch it off. 10-22 does not have stabilization or 80-200 and they were fine.

I do not have anything here (at work) with me to post.
Link to a shot from yesterday with manual focus though:
http://www.trekearth.com/gallery/photo991327.htm
it is however without exif and just a small version for the web so rather useless in here.

However shots with manual focusing were yesterday always much better even if the IS was still on.
I have to test it again...

From wikipedia:
Most manufacturers suggest that the IS feature of a lens be turned off when the lens is mounted on a tripod as it can cause erratic results and is generally unnecessary. Many modern image stabilization lenses (notably Canon's more recent IS lenses) are able to auto-detect that they are tripod-mounted (as a result of extremely low vibration readings) and disable IS automatically to prevent this and any consequent image quality reduction.http://en.wikipedia.org/wiki/Image_stabilization#cite_note-PopPhoto-6