Canon 50mm 1.4 focus issues

ClaudiaC wrote:

Hello Bill! Thank you for all those questions, I do understand the need of those and I am sorry for not providing more information. So my answers are the following:

- What camera body do you use? Canon 5D MKII

- What mode do you shoot in? AV

- What AF mode do you use? Auto-Focus

- Do you select a focus point, or do you let the camera choose for you? I always shoot with the central focus point and recompose with the use of the AF-ON back button.

- Do you put the focus point on a high-contrast edge? Or do you just point the camera in the general direction of what you are trying to photograph? I'm not sure I understand this topic here... Can you be more specific?

- Do you focus first, then shoot? Or do you just press the button? I use the halfway shutter to focus, sometimes what I actually want in focus is in the middle, so I don't recompose using the back-button, sometimes I do, so I use the back-button to lock focus and recompose.

And I think that is all, let me know if I need to answer something else other than this, please!

Thank you!

Thanks, that is good info. You use back-button AF, so you are not a complete novice.

Are you using one-shot AF? I assume so, but just checking.

When you focus using the center point, is it always just the center point that lights up when you achieve focus, or do other focus points sometimes turn red? In other words, do you have AF point selection set to "Automatic selection" or is it set to "Manual Selection", and you have selected the center point?

Turn on the option to display the AF point. This will put a red cross where the AF point is, so you can review in the camera if you are focusing where you think you are. The red cross doesn't actually end up on your final image, so don't worry about that. Make sure the red cross is always in the center, and not some other random point. The camera will not know that you recomposed, so it won't leave the red dot on what you actually focused on, though.

There is a geometry problem with the focus-recompose method. When you focus on something, and then move the camera to recompose, you are tilting the focus plane, and the object you want to focus on can move out of focus. This is more of a problem with wide apertures and short distances. If the subject is sufficiently far away, and you shoot stopped down enough, the focus-recompose trick works fine. But shooting close objects with wide-apertures (and thus thin depth of field) can be a problem if you focus-recompose.

Also, if you are shooting one-shot focus-recompose, there is a time lag between when you focus and when you release the shutter. Things can move in that time period - the subject, yourself, and you obviously just moved the camera. That time lag can allow things to drift out of focus, especially with thin DOF.

If you watched the video I referenced, you saw that the AF sensors are looking for high-contrast edges to focus on. If you put the focus point on something with little contrast, you will often have focus problems. Make sure you are always putting the focus point on a high contrast edge, preferably, or part of the subject that has good texture and contrast.

Here is a good article on focusing with wide aperture lenses: http://www.lensrentals.com/blog/2010/10/how-to-shoot-with-wide-aperture-lenses.  Although the picture you show is at f/4, not f/1.4, that is still pretty shallow DOF with a full frame camera at that distance.  One suggestion I like from that article is to shoot using the center point, but don't recompose.  Just shoot with enough extra room around the subject so that you can compose it by cropping in post.  With 20 MP, you have a lot of flexibility to crop with the 5D2, and you will never notice unless you print huge.

Check how well your camera/lens combo is focusing by taking as many variables out of the equation as possible.  Put the camera on a tripod, use a remote release, point the camera at something with a high contrast edge and take a picture.  Then zoom in with 10X mag to check the focus (or shoot tethered to a computer and check focus there).  You might try shooting at a row of batteries angled so some are at various distances.  You can then see if the camera is front-focusing or back-focusing.  If you see a consistent pattern at various subject distances, you may need to use the MFA feature of your camera.