This was bothering me for a while and I just spent some time to confirm. I have the three light primes, EF 50 1.4, 28 1.8 and 85 1.8. They give me more out of focus pictures than other lenses when using the outer AF points on my 5d2.
- It's not the lenses. They are sharp, including the corners.
- They autofocus perfectly fine in center.
- Doing manual focus in the same place that the outer AF points sit has a perfectly sharp picture.
- Even the outer AF points come out sharp randomly.
- It's not the camera. The outer AF points are sharp with other lenses such as the 28-70 f/2.8 and 135L. Besides, the 7D shows the same behavior.
- It's not horizontal versus vertical AF points, I tried with diagonal and cross-shaped targets, and obviously the 7D has cross-type out AF anyway.
So then I was noodling around more and I noticed that when I use servo AF then the lenses never stay still when trying to focus on the outer AF points. They constantly go a little bit back and forth. Both on 7D and 5d2. Same thing doesn't happen in the middle AF point. Same thing doesn't happen with 28-70 f/2.8.
So I think what is happening here is that the greater maximum aperture, and hence the shallower DOF while focusing, make the AF unsure - somehow. In theory the greater amount of light should help, and it does in the center. But on the outer AF points the "mini-hunting" starts. Then when you shoot you get it at a random time in the mini-movements.
Somehow the focus measuring doesn't react well to the shallow DOF when using the outer AF points.
- Is this a known phenomenon?
- Anything I can do about it?
- I plan to replace the 50mm with a Sigma anyway. Any hope for improvements?
- L primes?