Stupid question? Focus..lens or camera?

[tmjjk]

I am having a slight focusing issue. Canon 7d and 70-200mm 2.8 IS.

I notice this especially when shooting smaller subjects (insects etc) at the 200mm end. Is this something I should research about the lens or camera or both? I know I probably sound like a tool even asking. Thanks for any help.

 

[tmjjk]

I am spot metering.. hand held for the most part

 

[marc.christoffel]

Can you post an example?

 

[tmjjk]

This is just an idea of what I am referring to
The focus is to the left of the subject.

 

[marc.christoffel]

To me it looks like your camera is front focusing. Focusing in front of the subject, it probably does this at all focal lengths, it's just that the depth of field is much shallower and longer focal lengths. You need to fine tune your focusing, there will be a menu for it.

 

[Overread19 7]

A few thoughts to consider:

1) You mention that you're using spot metering, but what kind of auto focus mode are you using? Spot focusing is going to be superior to area focusing modes when shooting a busy scene like the one you showed above.

2) If you think your lens and camera setup has front/back focusing issues you can check this. You want a ruler or you can find online and print off a focusing chart - from there hold it at 45 degrees to the camera and select spot focusing and focus on a single line. From there you can see how the auto focus performs and if its focusing too far infront or behind the line you chose to focus upon.
Test at multiple apertures since focusing performance can vary over the aperture range. Typically a little front/back at smaller apertures (bigger f numbers) isn't as much of a problem as it is at the other end of the scale where you've wide apertures (smaller f numbers) and thus a much thinner depth of field

2b) If you find that there are AF problems with front/back focusing check the manual as the 7D has AF focus correction that lets you adjust the focus slightly so that any constant back/front focusing can be corrected.

 

[tmjjk]

I did mean spot focusing. Thanks so much!! I have been looking into the different methods of micro adjusting (newly learned lol). I am going to try a couple tests first. I could very well attribute the focus issues to hand shake possibly. I set up a tripod and played around with the moire pattern and didn't really see a problem. I am going to try the slope method you referred to. I think that would help pin point the issue if there is one. Thanks so much for taking the time to help.

 

[Derrel]

On a really complex target, like 259 blades of grass in a 3-square foot area...in contrasty lighting, MANY AF systems are going to have some trouble. Phase detect AF works best on high-contrast targets that have strong, distinct, linear detail, and boy howdy, a field full of grass is a nightmarishly confusing AF target!

Secondly, MOST AF brackets are just "generally accurate guidelines" on many cameras; the area covered by the AF brackets is often larger than the area the actual AF SENSOR is reading. On many cameras, the AF brackets in the viewfinder can have a small amount of "offset" from the actual area the AF sensor is "seeing", and so at times, the user might think he's aiming a laser-sighted sniper rifle...but he's actually aiming with a shotgun...there can be relatively large discrepancies with wide-angle lenses, or even with telephoto lengths.

One of the users here, I think it was Ron Evers, mentioned how he uses MANUAL focus when shooting songbirds in his trees...why? Same scenario as you have... a SMALL target, situated within a very complex, intricate, high-contrast background with loads of detail and strong, linear shapes...similar to the old needle in a haystack...

This might not really be "equipment error" so much as "equipment quirk", or "equipment characteristic under unusual circumstances". A very careful, tripod mounted test of the AF brackets, using something like a pencil jammed into a styrofoam block, can show you if the AF brackets have a slight bit of "offset". There was a good Thom Hogan article about this issue, discrepancy between AF brackets and ACTUAL AF sensor reading area, some years back.

 

[tmjjk]

Thank you, that makes perfect sense. I have been manually focusing when I need to get the shot. This thread has already helped quite a bit. The other time this issue was bothering me quite a bit was shooting my daughter from far away against a brick wall. SAME idea as with the grass. Thanks so much!

 

[marc.christoffel]

Do you "focus and recompose" or manually select your af point in the situation with the brick wall?

 

[tmjjk]

Focus and recompose

 

[weepete]

There is the problem then. When you focus re-compose you can shift the plane of focus behind or in front of your subject. If you are going to focus recompose you need to make sure your dof is large enough to cover the shift in the focal plane.

 

[weepete]

Here's an explanation

The Problem With The Focus-Recompose Method

 

[Gavjenks]

What I do is focus and recompose, and then lean back slightly before hitting the shutter. Works pretty well.

If the recompose was completely from the middle of the frame way out to one edge, lean back several inches.
if the recompose was only across part of the frame, lean back just a couple inches.

Avoid doing this for any very shallow DOF situations (200mm 2.8 headshots at 20 feet, etc.). But for "normal" settings, it's effective. The geometric amount you "should" lean back tends to counteract the effect of DOF. For example, shooting things far away with a wide lens introduces huge distance changes when recomposing (could be several feet), but when using wide angles at far distances, your DOF is massive anyway, so it ends up not mattering. Whereas with longer focal lengths, with shallower DOF, the geometic change is smaller, and the correction actually works.

So in other words, when you actually need it, it's about right. When you don't, it doesn't really do anything, but doesn't hurt either. So just always doing it as a habit works well (unless again you're intentionally setting everything on your camera for razor thin DOF)

Be careful not to let your leaning motion turn into motion blur! Lean, STOP the motion, then snap.

 

[C4n0n.Fan]

Your 7D has 19 cross-type AF points. 18 are jealous.