Erratic Auto Focus With Ultra Wide Angle Lens

[GrahamS]

My thoughts are that with a focal length of 10mm,

The focus ring has to be moved a fair bit for the image to go significantly out of focus. This means that the sensitivity of the focus adjustment is low and as a result the loop gain of the AF phase comparison control circuit is also low (causing inaccuracy)

The detail in the focus area is very compressed due to the low focal length and hence the AF system has difficulty in detecting contrast changes.

The AF focus area (I’m using centre point focus) covers a relatively large part of the image in which different objects are at different distances and it is difficult to know which object will be chosen for focus adjustment

But I’m only speculating !!!

If anyone can shed some light I’d appreciate it.

Dave

Dave, I would say that your "thoughts" are spot on and that you have answered your own questions. Personally, I don't think thaat auto-focus is necessary with a lens of 10mm focal length anyway. Thye DOF is too great, even wide open, for the camera's AF system to be able to determine the actual plane of focus.

[dje]

Thanks for your comments Grahame, L Paul, Tom and Graham. I'm starting to feel more comfortable with this business now.

Dave

[Black Pearl]

I pre ordered and got a 10-20mm from the very first batch of the original 4-5.6 versions to be available in the UK back in 2005. I can't recall any pictures where the AF has been 'out' nor do I recall any time where I've looked at the focus indicator while taking a picture......actually I don't think I've ever paid attention to the focus indicator.

I have found the lens is pretty rough until its stopped down a bit so use the larger apertures if I absolutely must (wandering around inside without a tripod) meaning virtually every shot has such a gigantic DoF I would never see a variation in AF accuracy.
If as you say you are going to use a tripod then your lens is always going to be stopped down so you don't need to use manual focus - frankly you don't need to focus at all.

[dje]

Robin I understand that the large DOF you get with 10mm is very forgiving focus wise. I'm just a bit of a perfectionist !

I'm actually quite happy with the performance of the lens so far - it seems to give nice sharp images. See my other thread.

Dave

[pnodrog]

Robin I understand that the large DOF you get with 10mm is very forgiving focus wise. I'm just a bit of a perfectionist !

I'm actually quite happy with the performance of the lens so far - it seems to give nice sharp images. See my other thread.

Dave

Surely a perfectionist could not be just a bit of a perfectionist? You have me slightly confused.

[dje]

Surely a perfectionist could not be just a bit of a perfectionist? You have me slightly confused.

Well it's like being " a little bit pregnant" !

[pnodrog]

Dave I am going to have to take your word for that.

[dje]

Dave I am going to have to take your word for that.

OK L.Paul, I asked for that didn't I ! But I like that expression.

A couple of others I find quirky :

Extra virgin olive oil

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