70-200 f/4 L Lens questions (focus points)

[Pb2Au]

Although everyone absolutely raves about the portability and sharpness of this lens, I have a very hard time getting it to AF properly on smaller objects (mainly wildlife.. birds, lizards, etc). ..Even things that are absolutely motionless. The AF point (usually using center only) likes to lock on the subject, but when I look at them on my PC, the closest BIG object is in focus, and usually not my subject. I've heard that it could be a problem somehow related to the small AF point size on a 1.6 crop factor camera (20D), but I know many others have great success with this lens. Could this be a problem with the lens? I never have this happen with my 100-400L or any of my other (6) Canon lenses. Just the 70-200L.

Sample (since I know people will ask) :)
http://img.photobucket.com/albums/v683/pb2au/_MG_4349.jpg
This SHOULD be focused on the lizard, but obviously the lizard is a little OOF. The texture of the rock looks just well-focused in places though. (these are crops)
http://img.photobucket.com/albums/v683/pb2au/_MG_4350.jpg
This is a "larger object" that doesn't seem to suffer from the same problem (ie, it looks in focus). These shots were taken in the same place, with the same camera, right after one another.

Anyone in the Glendale, AZ area have a 70-200L f/4 or f/2.8 that I could try. You can try my 100-400L ;)

[Chazs]

I'm no expert on this lens since I don't have one,...yet. But on the lizard the DOF looks to be less than half an inch (his front left foot is sharp, but not much else.) The big rock looks like to have a few feet DOF. A 10 percent error on being bang on is certainly going to be more noticble on the lizard. The rock could have a 30% error in focusing, but still look decent. It looks like the problem might be related to the aperture setting more than anything else.

[Pb2Au]

I'm no expert on this lens since I don't have one,...yet. But on the lizard the DOF looks to be less than half an inch (his front left foot is sharp, but not much else.) The big rock looks like to have a few feet DOF. A 10 percent error on being bang on is certainly going to be more noticble on the lizard. The rock could have a 30% error in focusing, but still look decent. It looks like the problem might be related to the aperture setting more than anything else.
Thanks for your quick reply,

Very true, and I meant to post two images with identical aperture information (EXIF is intact on both).... but they are in fact, different apertures. f4 is the lizard and f7.1 for the rock. However, even at longer distances (i.e. 15 feet with birds), I get the same effect. (Ie: bird not sharp, rock it's on is tack-sharp.) At 15 feet, even with f/4, the DOF should be acceptable for a bird, and even at f/1.2 (which my lens cannot do..), the FOCUS POINT (i.e. bird) should be IN FOCUS, right?

I can shoot my 100-400L at f4.5-5.6 with outstanding results in very similar situations (see the nature threads for some of the shots). But for some reason, the 70-200L eludes me... perhaps the IS helps keep the focus point centered on the subject?

[Skip Souza]

I believe it is a DOF problem. In the lizard photo I see three 'in focus" points.
1. White spot at 4 0'clock from lizard's chin.
2. Lizards left foot.
3. white spot behind lizards tail.

I is possible that the actual focus point is one of the white spots because of point of greater contrast.

If you are shoot at maximum focal length at a short distance, like 15 feet, you will get a very limited DOF. Any error in focus point will screw everything up.
Since I assume you are hand holding it could be the IS is making the difference, especially at open aperatures. I believe that f/8 or better yet f/11 is necessary to get the kind of results you seek. When the light gets weak you need IS to hold the damn thing still enough with the slow shutter speed needed to acheive the exposure at the tight apreature needed for the necessary DOF.
Viscious circle, isin't it?

[JKD]

I feel like an idiot but I fail to see a lizard in this picture. Could you point it out for us slow folks?

[JustJerk]

Have the same lens; it does hunt a bit if the image is not in the center. Usually have to slightly move the subject over, get it in focus (which red focus point lights up), lock that point in and reposition the shot. If I have time, I usually focus in manual mode, once you get use to it, you can focus just as fast.

[Mike H]

There are a few things that you can check into here.

1. Make sure that you don't have the auto focus mode set to servo for stationary objects, as that can mess you up for sure.

2. Are you using automatic AF point selection? If you are the camera may be locking onto the nearest object, and that may not be your subject.

3. Are you using the "focus lock and recompose" method? That can definitely do you in when the depth of field is shallow. See the link below.

http://visual-vacations.com/Photography/focus-recompose_sucks.htm

Mike H

[mr.photoguy]

This is something you should be able to see through the viewfinder.
You also have AF fulltime override, so you can make these corrections for DOF.