Front Focusing for 20D... How Picky are you?

[christopher_diao1]

I tried 3 new copies of 20D. With my 135mm/2.8 SF and 50mm/1.8II,

they all front focusing on the testing chart as well as on carefully

conducted real life test. They are front focusing about 10mm at the

closest focusing distance.... more in longer distance.

 

Am I too picky? Have you all tested lenses and yoru dSLR very

carefully?

 

Fortunately, my better L lenses: 200/2.8, 70-200/4, 100/2.8 macro

are all doing right on.

[digitmstr]

There's picky and there's obsessed. I am not suggesting you are either...only you know ;)

 

But, if you feel there is huge difference with those two lenses compared to the others Canon does offer a calibration service. You sent the body and the lenses and they take care of it for you.

[christopher_diao1]

The problem is that if the camera is calibrated for those two lenses... would it throw off the focusing with other lenses?

[steve_dunn2]

<p>Canon says that the AF system should focus to within DOF. The high-precision sensor, when activated by a lens which is f/2.8 or faster (as all but one of yours are), should focus to within 1/3 DOF. Is 10mm within DOF or within 1/3 DOF, as the case may be? If so, you may not be happy about the focusing error, but it's within spec.</p>

 

<p>If it's out of spec, Canon should be able to fix it. There have been lots of reports of front or back focusing here over the last couple of years, including some reports by people who've sent their camera and lenses in to get the problem fixed; you should be able to find them by searching. Either here or on another site, I've read something, which I believe came from Canon and quite possibly from Chuck Westfall, saying what the procedure is for fixing it. They have a lens which they know to work properly (a 50/1.8, IIRC) and they first use this to test your body's AF system. If adjustments are needed to the body, they are made. Once your body is within spec, they use your body to test your lenses, and any necessary adjustments are made to the lenses.</p>

 

<p>Could sending only some of your lenses, along with your body, for service cause the other lenses to stop working properly with it? I suppose it's possible, yes, if the problem is that the body is out of spec and somehow your other lenses are out of spec the other way so that they cancel it out. And I believe some people have sent all of their lenses in, even the ones which work properly, to avoid this situation.</p>

 

<p>I haven't tested carefully to see if my 20D has focus errors with any of my lenses. I have noticed that in real-life use, my body does seem to be able to produce correctly-focused pictures with all of my lenses, including in shots with the lens wide open (such that DOF for the picture itself is the same as DOF for focusing). It's entirely possible that my body with one or more of my lenses does have a focusing error which I could discover by doing a test, but as the equipment works well in real-life use, I'm not inclined to perform a test and give myself something to worry about.</p>

[taner]

On my XT/350 ancient designs like the 50/1.8II, 28/2.8 lenses front focus, and all my (real) USM lenses, like your USM lenses apparently, focus with no problems. Since the results are super consistent, they are neither due to a problem with the camera body, nor due to a problem with the test application.

 

The sad truth is that those ancient designs suck big time when it comes to focus accuracy, and the alternatives are either expensive (135/2L), or less than perfect themselves (50/1.4; not real USM, and is known to have 'durability' issues).

[peza]

20D can not focus PERFECTLY in more than 50% cases under good conditions - this is my experience with 70-200/2.8L in typical distance about 2-3 meters.

 

Try yourselves - tripod mounted, expose by flash, defocus manualy and take pic of newspaper in AF mode repatedly and look for variation in AF accuracy.. It is quite bad.. Can't wait to try EOS 5D for that particullar reason.

 

EOS 3 was much more accurate, under less than ideal conditions several fine corrections of AF have been done by the camera until it felt focus is accurate - 20D NEVER does it: 1-2 corrections, that's it.

 

And I experience this behavior with basically all lenses.