Should auto-focus need adjusting?

Question

I own a Canon XTI, and it always seems my shots are slightly out of focus. I usual have my camera set to the center focus point, and usually half press the shutter button to establish the focus on the item I want to focus on before actually taking the shot. Yet I still seem to have a problem with a lot of my shots being out of focus.

I was searching around and found out that higher end Canon DSLRs actually have an option in their menu to adjust the auto-focus, yet my XTI does not have this option. I searched around some more and found this page on adjusting the autofocus on a 350D with an Allen wrench. I assuming there will be a similar screw in my 400D?

Can someone tell me what this screw adjusts exactly? Is this something that typically needs adjustment? Should I even mess with this, or should I just send my camera/lens into Canon? I'm no stranger to opening up small electronic devices, so I'd rather not pay $100+ to have someone turn an easily adjustable Allen screw.

Answer 1

I have sometimes suspected that my Canon needed some focus fine-tuning, but when I use a tripod and take a picture of a ruler (like this) the focus always turns out to be dead-on; it's my shaking hands or a very shallow depth of field that's making my pictures un-sharp, not the camera.

Answer 2

If your lens has anything like a flat field of focus (that is, if you can shoot a brick wall and the bricks in the corner are almost† as sharp as the bricks in the centre), then focus-and-recompose except at telephoto-class angles of view is going to result in some amount of focus error. You aren't focusing according to the distance between the camera and the subject, but on the distance between the film/sensor plane and the subject. When you focus then recompose, you are changing the angle of the sensor plane, and so you are changing the distance between the sensor plane and the subject.

In this exaggerated example, you move the camera to focus on the subject at the centre of the frame and everything looks hunky and dory. But when you recompose, your focus point lies on the red line, which is now behind the subject. The wider your lens and the further from centre your subject is in the final composition, the worse this gets. With very long lenses and subjects that are not too very close, you'd hardly notice, but on a standard lens or wider it really gets to be a problem.

Somewhat counterintuitively, "badly corrected" lenses are going to be better-behaved in a focus-and-recompose scenario than "high quality" lenses. Their spherical field of focus means that you are more likely to be closer to being in focus after recomposing -- even if the brick wall shot looks horrible at the corners.

If you have other autofocus sensors available that are closer to where the subject is going to be in the final image, use them instead. The less you can move the camera between focusing and shooting, the better.

† I say almost because very few lenses, even those with very flat fields, can focus obliquely as well as they can focus normally, so some amount of corner softening is the usual case.

Answer 3

Are you sure you aren't moving a little after holding down the shutter halfway? I noticed that I really had to watch myself because I was doing:

1. Hold down shutter halfway
2. Now that it's in focus, shift a little forward or backward to recompose
3. Take out of focus shot :(

It took some conscious work to break this bad habit. If you are doing this it's very easy to be slightly out of focus, especially at larger apertures.

Note, I am not talking about focus / recompose, where the movement is left-right, but more front back, which is more likely to cause focus issues.

Answer 4

small electronics are one thing, a digital camera is another. If you are concerned about focus problems, I suggest you send it to one of canon's authorized repair facilities. Yes for $100 they may only turn an allen-wrench to fix the issue, but they will also service your body, clean it out, and give it a good bill of health.

Answer 5

The menu option to adjust the AF is only meant to work as a temporary fix until you can get the camera and/or the lens to a workshop to get it fixed properly. It is there to allow a (pro) photographer to get the job done even if the equipment is a little off, but is not meant as a permanent solution to AF misalignment. It is a band-aid.

Cameras and lenses are manufactured to certain tolerances. Even if both are within tolerances as such, if the camera is at one end of its tolerance range and the lens is at the opposite end, the sum can be suboptimal autofocus accuracy. If either is outside of tolerance, so much the worse. For this reason, it is usual to send in both camera and lens(es) for adjustment so that they can be matched to each other.

Answer 6

you haven't said how out of focus, or what lenses or indeed focus mode you are using. Are the shots still out of focus when you manually focus?

What it could be is that all lenses and cameras are slightly different due to build tolerances, usually this wouldn't be a problem, say your camera was front focussing by +5 and your lens -5 (back focussing) the focus should be spot on, however, if both are front focusing this could take you out of the acceptable tolerance.

The fix for this is to get both your camera AND lens calibrated together.

Answer 7

It's important to test that your camera and lens is actually setting the focus correctly.

However, it's quite straightforward (if a little involved) to test this. What you need to do is the following:

1. Set up your camera on a heavy tripod on a solid floor in a brightly-lit room
2. Create a high-contrast detailed subject (perhaps a newspaper) directly in front of the camera, a short distance away (depending on the focal length of the lens).
3. Set your camera to auto-focus
4. Disable any image stabilisation (as this tends to make the lens or camera move slightly when on a tripod)
5. Use the mirror lock-up mode, if available
6. Set the aperture to be mid-range (e.g. f8)
7. Use the viewfinder or live view to confirm the subject is correctly positioned in the frame
8. Use a remote trigger of some sort (or a timer delay) to trigger the camera to take the shot, so that pressing the shutter button doesn't move the camera when it's taking the photograph

You can then examine the photo at 100% (i.e. at a pixel level) in the center of the short to see the sharpness.

If you are still seeing blurry images, try using live-view (if available) in a zoomed mode with manual focus, to adjust the focus yourself. If you can get a sharper shot using this method, it's a problem with the autofocus. If not, it's a problem with the lens or camera that will need to be fixed or replaced.

Answer 8

I have noticed that this question is over 2 years old, so I was wondering if you still are experiencing issues. If so, I may be able to help. If so:

1.Please tell me whether auto or manual focus is posing a problem.

2.Try taking otherwise identical photos in both manual and auto focus. You should use a tripod for this.

If I see those photos, I should be able to help.