Autofocus issue with Sigma 10-20mm? Help!

Started 11 months ago | Discussions thread
mosswings
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Re: Autofocus issue with Sigma 10-20mm? Help!
In reply to banflu, 11 months ago

banflu wrote:

Hi everyone,

Been lurking for a while but finally registered here as I ran into a problem!

I bought a Nikon D5100 a few months ago (my first DSLR) and after a while I noticed that I really started to like wide angle photos so I also bought a Sigma 10-20mm f3.5 lens. From the start I had a feeling that it's autofocus wasn't 100%, but at first I thought nothing of it - I am only a beginner after all.

But recently I was on holiday so I used the camera (and this Sigma lens) quite a lot .. and I've had quite a number of out-of-focus photos. I later realised that they are never out-of-focus if I use live view. Of course, live view is much slower so it is quite an issue.

Anyway, after making extensive testing I came to the conclusion that the Sigma lens is to blame. Please take a look at this video: http://www.youtube.com/watch?v=zOeoLUQrp_A

In this test I placed the camera on a small tripod (gorilla pod) and attached a focus test chart on the wall. I focused on the chart multiple times using live view. It always focussed perfectly - any photo i take in live view is 100% in focus. Later I put it out of live view, and the focus setting is visibly different in the focus window on the lens. Also, the focussing is inconsistent when not in live view. Put it back to live view and it becomes 100% consistent and perfect.

I later tried this same test with a Nikkor prime 50mm lens, and it worked perfect - the focus is always exactly the same, both in live view and in normal view.

Any thoughts? Has anyone else had a similar issue? The lens is still under warranty ... should I try to get a replacement, or should I fork out more $$$ and get a Nikkor 10-24mm?

Many thanks in advance for your help!

Talk to Sigma about your focusing issues. Provide them with sample pictures demonstrating the problem. It is possible that your lens is out of calibration, in which case Sigma should accept it for warranty service. The focusing inconsistency, though, is another issue. I have the older version of the 10-20, the f4-5.6, and it shows no such inconsistency in focusing. Usually, UWA lenses have relatively few focusing issues because of their extreme depth of field and relatively slow maximum aperture. There is a natural spread to PDAF focusing results, but usually the spread is very tight. Double check as well that you are not at or near the minimum focusing distance of your lens. It varies with FL.

The Nikon 10-24 is a fine lens, but more expensive than the 10-20. It might work better than the Sigma does, but there's no reason why your 10-20 should perform as it does.

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