Light primes versus outer AF points. Is there a problem here?

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.

However:

• 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.
  

• 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?
  

This is where 5dmk3 helps.

bobbyz wrote in post #14797878
This is where 5dmk3 helps.

Even if the 7D does not?

Well, the outer points are not high precision (f/2.8) points like the center. I think the vignetting on the edges also hinders accurate AF. Did you test this in good light and with a high contrast target?

EDIT: I'm pretty sure that the AF assist beam from the 580EX will give you better results

bobbyz wrote in post #14797878
This is where 5dmk3 helps.

Bobby, I was wondering how much better the 5D3 is for this. Some of the outer points (a lot actually) are cross type, but they are only f/4 sensitive. Whereas the 1D/Ds series has f/2.8 line sensors on the outside points. Although I don't think that is the only factor in AF accuracy, so the 5D3 may indeed beat out 1Ds3 for example. I'd be interested in seeing a comparison.

no side by side tests but so far 5dmk3 outer points arw working out much better than 5dmk2/5d. This is when using 35l, 85mm f1.8 and 50mm f1.4.

frugivore wrote in post #14797975
Well, the outer points are not high precision (f/2.8) points like the center. I think the vignetting on the edges also hinders accurate AF. Did you test this in good light and with a high contrast target?

EDIT: I'm pretty sure that the AF assist beam from the 580EX will give you better results

High contrast target but pretty low light (the point about me using these lenses is indoors no flash).

I was actually thinking that CA is maybe what is doing the trouble there, although there isn't any visible in the pictures.

Another thing I can try is use aperture preview mode for a quick test while focusing. If the large aperture is causing the problem somehow the mini-hunting might go away when doing that.

uOpt wrote in post #14799216
High contrast target but pretty low light (the point about me using these lenses is indoors no flash).

I was actually thinking that CA is maybe what is doing the trouble there, although there isn't any visible in the pictures.

Another thing I can try is use aperture preview mode for a quick test while focusing. If the large aperture is causing the problem somehow the mini-hunting might go away when doing that.

I don't understand this last part. Do you mean the depth of field preview?

Not sure if you have read this article, but I has some great data about Canon AF sensors:

http://www.the-digital-picture.com …-Autofocus-Explained.aspx

In any case, I'm going to do a test tonight and see if my findings match yours.

frugivore wrote in post #14801132
I don't understand this last part. Do you mean the depth of field preview?

Left side lens mount button, press to preview with aperture applied.

In any case, using the button doesn't help since AF stops doing anything while the preview is in effect.

This is beginning to annoy me. Considering the cash Canon wants for 5d2, which essentially ships with a 30D autofocus, with only 9 AF points, I expect that I get more than one AF point that is usable.

And that is with lenses that are all faster than f/2.0. What the heck? The stupid thing can hardly complain about lack of light.

can you post some problem shots? Maybe it is something else.

I know a number of 5DIII owners that shoot the gamut of lenses with it. Flat out the 5DIII rocks! It even kicks ass on the 1DIV in low light.

uOpt wrote in post #14797751
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.

I have noticed crappy outer AF point performance with the 5D, 5D2 and 1Ds3 in combination with fast glass used wide open. Just like you describe. I notice it more with fast wide angle lenses, like the 35L and much less with fast tele lenses. I also have noticed that stopping down reduces the "issue". Strange, because a lens focuses wide open. Still, stopping down helps. So, that got me thinking... And the only 2 things I can come up for an explanation are:

- sharpness/contrast rapidly reduces on fast lenses when you move away from the center (albeit this doesn't explain why it gets better when stopping down).
- Most fast wide angle lenses suffer from heavy field curvature. I suspects this has a negative impact on the AF when you move away from the center.

I don't think it has something to do with shallow DOF. Otherwise the center AF point would give problems too.

To me it is. For quite some time as well. I invested quite some time to figure it out as well. Talked to Canon techs and other photogs who reported the same "issues" about it... and the above reasoning is what I came up with.

Stopping down. In One Shot use an AF assist light (helps to increase contrast for AF). Use only the center AF point with wide open apertures.

bobbyz wrote in post #14797878
This is where 5dmk3 helps.

Can you back that up?

This is an interesting read too regarding the 5D3 outer AF points and the effects of field curvature: https://photography-on-the.net …/showthread.php​?t=1172486

Daan37 wrote in post #14803445
Can you back that up?

All I am saying is that I am getting lot more sharper shots when using outer points on 5dmk3 compared to 5d and 5dmk2. I wish OP posted some problems shots. Shooting f1.2, f1.4 is hard, not denying that. But it can be anything, camera, lens, technique.