Have I got a lens issue here???

[hauff]

The coyote was about 45 yards away and I was using centre point focus on him but the brown grass on the lower right seems to be in focus instead. That grass was about 10 yards in front of the subject. I have done no processing other than to crop it. The lens was my 70-200mm f/2.8 IS L. I had noticed this problem yesterday for the first time. Any ideas?

http://www.telusplanet.net/public/jhauff/coyoteoof.jpg

[tim]

If it's only happened once it's more likely to be user error, such as a strand of grass getting in the way and AF locking on that instead of the doggy. If you're worried about focus accuracy do this focus test (http://www.canon-dslr.com/Canon_Jan05/Canon_SLR_Focus_Test.htm) with the proper target, on a tripod, with mirror lockup and a cable release - or just on a tripod with the timer.

[ed2day]

I think occasionally camera/lenses just "misfire" a bit on the focus. It obviously gets exaggerated at f/2.8. If this is the only time I would suspect that.

[slin100]

The image does look soft but I can't really tell where the plane of focus is. The low shutterspeed 1/250 at 200m on a 1.6x crop factor camera also leaves open the possibility of camera shake. Was IS turned on? What was the AF mode? One-shot or AI Servo?

[AXENA]

As much as I love Canon's.... I too run into the same issue.... I can be "dead on target" with a center focus lock on a subject/object, and for unknown reasons, the 20D will want to focus on something rather unobtrusive like a blade of grass that was never even in the frame..... for the most part, unless shooting sports, I have gone back to old-school shooting.... MANUAL. I know it defeats the purpose of all the fancy equipment, but I just got tired of blurry shots when I can just as well do it manually and get it right almost every time.... (see the 50mm f/1.4 thread).

[AXENA]

[QUOTE=slin100]The image does look soft but I can't really tell where the plane of focus is. QUOTE]

I think it is pretty clear.... looks like the plane of focus is on the tall grass about 6-10 feet in front of the "dog?" I noticed the same thing consistently with my 20D..... it just LOVES blades of grass to focus on rather than a center-focused BIG object.... any ideas? I'd love to hear them because I am really at the end of my rope with the AF issues and 20D..... G5 - Perfect. 20D - Not so perfect... just better attachments! :-)

I'd love to hear some advice on this one as well.....

[slin100]

[QUOTE=slin100]The image does look soft but I can't really tell where the plane of focus is. QUOTE]

I think it is pretty clear.... looks like the plane of focus is on the tall grass about 6-10 feet in front of the "dog?"

Ah, ok, I see now. Well, I would chalk this up to the fact that the AF sensors on the 20D are quite a bit larger than indicated in the viewfinder. Plus, Canon's AF technology works on the principle of contrast detection. Blades of grass have, likely, much more contrast than a fuzzy fur coat.

As to the tendency of the 20D to focus on something not even in the frame, I doubt the 20D is that prescient! :)

[AXENA]

[QUOTE=AXENA]

Ah, ok, I see now. Well, I would chalk this up to the fact that the AF sensors on the 20D are quite a bit larger than indicated in the viewfinder. Plus, Canon's AF technology works on the principle of contrast detection. Blades of grass have, likely, much more contrast than a fuzzy fur coat.
:)

Yes... this is certainly true... none the less... very annoying. Contrast detection is great, however, I would expect that an AF system would also be able to detect a much larger object in the center of the viewfinder and focus on that, over a wavy blade of grass that is technically "out of view." However, not being a Canon Research and Development expert... I suppose I will need to resort to double-checking my shots before firing.... sort of a waste of time, but better than the "N-word...." with Nikor lenses....

[Poco]

Ya, unfortunately the camera isn't quite smart enough to know what it is you are shooting at. It is just pixels. If you were trying to get the grass in focus and got the coyote instead then you might also have reason to complain. The processor is just looking at pixel values around the focus point for contrast (the contrast is, I believe, at its maximum when the image is in focus).

Some have suggested that the camera tries to find the closest contrasty thing (particularly if you use multi-point AF). It assumes that this is, in most cases, what you are taking the picture of. This is how most people take pictures (people in front of stuff).

I often have trouble autofocusing through branches when hiding in the bushes (that came out wrong ;-).