Canon Autofocus issue?

[taschrief]

Hi,
My primary body is a T1i. I have absolutely no focus issues with that body. About a year ago I picked up a second hand XSi. It had erratic focus issues with my fast lenses. So you know, I am talking about: 85mm f1.8, 100mm f2.8 macro, 135mm f2, 200mm f2.8 - all Canon. Between about 5 feet and 20 feet, when shot wide open (or close to it) the XSi would miss focus about half the time with all these lenses. I did not have any of these problems when shooting the T1i. OK. So I sell the XSi and use the funds to buy a brand-spanking-new 60D. It is better than the XSi, but still missed focus A LOT when shooting any of these lenses wide open. By f4, everything is sharp, with all lenses. Since I can't make my T1i have this problem, I know it is not the lenses. It seems odd to me that 2 Canon bodies had similar problems. FYI: I also have an Elan 7NE (film body for you youngsters), and while I did not take hundreds of pictures to test it, it seems to handle these lenses wide open just fine. I took the 60D back for a refund while I still could. Does anyone have any idea what is going on here? I liked the 60d. I don't like the idea of paying for fast glass I can't use wide open. I considered the 5DII. Does it have these problems?
Thanks,
Tom

[inkista]

Would suggest you read up on Autofocus calibration. And the 5DII is likely to have the same issues, but can help you fix it on your own with AF microadjust.

It may not be that your T1i is great, and all the other bodies are crap. It could be that your lenses are off, and the T1i is off enough to match up, while the other bodies were fine. The usual way around this is to send everything into Canon for calibration. But this is why the higher-end bodies have autofocus microadjust, so you can do the calibration yourself.

It's obviously going to show up with superfast lenses wide open, because the DoF is so thin that the margin of error in mms will show up in the shot. The other bodies (aside from the 1 series) that offer AF microadjust are the 50D and 7D.

[taschrief]

Thanks for the quick reply. I considered MFA. If the focus is only an issue between 5 and 20 feet, wouldn't MFA mess up really close shots and distances longer than 20 feet? As I said above, it is not an issue from f/4 on down.

[inkista]

Originally Posted by taschrief

Thanks for the quick reply. I considered MFA. If the focus is only an issue between 5 and 20 feet, wouldn't MFA mess up really close shots and distances longer than 20 feet? As I said above, it is not an issue from f/4 on down.

Remember that DoF relies on subject distance as well as aperture setting. The smaller your aperture, the greater the DoF. The farther away your subject is, the greater your DoF. Close distances and wide open are where your accuracy is more critical. Farther away and with smaller apertures, you have more wiggle room. You're probably still off by the 0.7mm (or whatever), you just don't care at 20 feet or f/8, because your DoF more than covers that margin of error.