Autofocus Stopped Down.

Hello.

Can anyone tell me if it's possible for the 6D to autofocus and take a picture with the lens stopped down using the DOF button?
I understand there are limits on how small the aperture can be for autofocus to work but I'm thinking of around F1.8 to F4.0.

No, it's not possible. AF is disabled when you stop down. For a good reason.

Thanks.
Just as a point of interest, what is the reason for af being disabled under these conditions?

With the lens stopped down there would be much less light available for the autofocus system.

fordmondeo wrote in post #17534140
Thanks.
Just as a point of interest, what is the reason for af being disabled under these conditions?

Honey Monster wrote in post #17534206
With the lens stopped down there would be much less light available for the autofocus system.

If you take an f/1.4 or even f/2.8 lens and stop it down to f/4, there is still more light available than a 400mm f/5.6 which auto focuses just fine.

I am still wondering why you would want to do that.

I know you can't, but one reason would be to beat focus shift.

2ndviolinman wrote in post #17534290
I know you can't, but one reason would be to beat focus shift.

Bingo!

I just bought the 50mm 1.2 and the focus shift is horrendous.
It doesn't really matter anymore as, having done some more tests about ten minutes ago, I now realise I can't live with this lens and it's going back to the store.

Big thanks for everyone's help.

Focus shift is of course an issue, but otherwise the problem is not so much less light as a narrower path for the light. To get good focus accuracy the light rays need as wide a path for the light as possible.

By no means an expert in anything digital. One other reason for not being able to focus in stopped down mode is that I would think that the AF, and you, would want a very defined point of interest to focus on. The greater the DOF the less defined the focusing area. At least that has been my experience when shooting macro, I will stop down to check DOF but open up to check focusing.

The challenge of stopped-down AF is that AF has inherent error, Canon states it as defined by an acceptable range within one DOF zone. When the lens is stopped down, the DOF zone grows and the range of tolerable error widens, reducing the inherent accuracy of the focus achieved!

This illustration shows the principle. Any focus within the zone of AF ambiguity (pink zone) could be 'acceptable' to the camera's AF sensor, yet it is not necessarily focus on the main point of interest where the AF point is aiumed. (note: The DOF 'in focus' should be labelled f/8 DOF 'in focus'.)

Couldn't you use live view for shooting with that lens? Using live view contrast detect AF or manually focusing would give you the creativity of the lens in question without sacrificing where the focus actually ends up. Or is it that what you are shooting isn't conducive for Live View contrast AF or Live View manual AF?

fordmondeo wrote in post #17534304
Bingo!

I just bought the 50mm 1.2 and the focus shift is horrendous.
It doesn't really matter anymore as, having done some more tests about ten minutes ago, I now realise I can't live with this lens and it's going back to the store.

Big thanks for everyone's help.

You can have the AF system account for focus shift by AF tuning the lens stopped down.