Question about the AF on the Olympus 12-40 Pro Lens

Swandy

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Dec 15, 2009
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I just received a refurbished Olympus 12-40/2.8 lens and am very happy with the images, but there is something strange with the AF - at least compared to the other Olympus and Panasonic lenses I have had.

When the lens (currently using it on the Olympus EM10-V2, previously on EM10-V1) when the lens can't acquire AF quickly, there is this weird effect as it tries to get AF Lock. Looks like a (very hard to describe) bubbly or wavy effect. Not sure if this is "normal" for this lens or not.

Also, I was taking some pictures this evening at an honor night at my lodge in a pretty well lit room and I was surprised how many times the lens took what I would consider a while at least for a "pro lens" (watching this wavy effect) to achieve AF. (I use the single center AF point and did have Face Detection on.)

Sorry I can't describe what I am seeing better, but curious if others who have used this lens (as I said I saw it on both my older EM10 and the new V2 body) have seen this and whether you have found instances where the AF was not what you would expect.

Thanks, Steve
 

David A

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The camera is using contrast detection autofocus. If there's little contrast in the area the focus target covers, especially in low light, it can have trouble locking on to focus. I suspect that may be worse at longer focal lengths since the target area is then covering a smaller part of the actual scene than it will at shorter focal lengths. I haven't tested that idea out. My suggestions would be to make sure you're using the larger autofocus target size rather than the small one, and if you're indoors and at closer distances you could also try using the focus assist light function if that isn't going to be a nuisance to your subjects.

DSLRs tend to use phase detection autofocus which seems to work better in situations where contrast detection autofocus often has problems. It's not a lens problem, it's a camera problem. I suspect the bubbly/wavy effect is caused by the lens tracking backwards and forwards through the actual focus point while it's trying to acquire a lock.
 

PakkyT

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I suspect the bubbly/wavy effect is caused by the lens tracking backwards and forwards through the actual focus point while it's trying to acquire a lock.
Agreed. With Phase Detect AF on dSLRs, the sensor not only knows if the scene is out of focus but also which way it is out of focus and how far out of focus it is. So the AF system can move directly to the focus point and stop. Where as with Contrast Detect AF the only way the camera can achieve focus is to move towards focus, go ever so slightly beyond then move back to perfect focus. So you always get a tiny bit of bi-directional movement when locking focus. As you said, if the camera is having trouble locking focus, the bi-directional movement may happen rapidly over and over while it tries.
 

Swandy

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Dec 15, 2009
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362
I've noticed something like that on my ZD 7-14mm... I'm not sure what causes it.

Do you see it more at any focal length in particular?

Barry
Tended to notice it at the longer end of the 12-40, especially if I was using the 2x digital zoom (as far as the lens having difficulty achieving AF lock). But noticed the "wavy" effect whenever it was hunting.
 

Swandy

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Dec 15, 2009
Messages
362
Thanks Dave and Pakky. I realize that it was during the back and forth "hunting", it was just on all my other m4/3 lenses, I was used to just seeing the scene/object to in an out of focus - same as when I would use manual focus and go back and forth. This "wavy" effect seems - at least to me - to only occur with this particular lens. Hence the call to Olympus (and the post here).
 

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