Lens Filter/Autofocus question

[chriznatch]

I'm not sure if this is a lens issue, a camera issue, a lens filter issue, or just user error. On a recent trip, I took a number of pictures of my wife standing in front of various landmarks, buildings, etc. I was using an a700 and a Tamron 17-50. I would center the lens on her, autofocus, recompose to move her slightly off center so I could get more of the background, and then shoot. I usually used an aperture ranging from f/9 to f/11. This technique has worked well in the past using my old a100 and a Sony 18-200 -- I could always get her in perfect focus, even if the background game out slightly soft. so with a better camera and a better lens, I figured these pictures would come out better as well.

Sometimes I got decent shots, but there were some situations where I took a whole series of pictures in front of a given background but just could not get her in focus, no matter how hard I tried. I could see in the LCD that she looked a little blurry, but now matter how many times I tried, I could not get a good shot using autofocus. (I also tried manual focus, but that's a different issue.)

Recently a friend of mine using a Nikon told me he had the same problem using a telephoto lens. He said that he was using an expensive new lens filter, but he ultimately ditched it because when he took the filter off, the focusing error went away.

It just so happens that I bought a new lens filter before this trip (I think it was a B + W). Could the filter cause a focusing problem? Has that ever happened to any of you?

[David Kilpatrick]

It's very unlikely, especially with a B+W. I do have one polarising filter which has got problems, it causes blurred patches in the image and I must throw it away and get a new one. It is actually a recent Minolta one which cost a fair amount. It took me a while to realise what was happening.

So it is not impossible, but unlikely. It's more likely you had the camera set to continuous AF, and it was just refocusing every time you re-centered the picture. This technique only works when set to AF-S. It is likely to fail if you use any of the Scene modes or Green Auto as well, as these can switch the AF to continuous even if you have AF-S set as your default.

David

[motor]

I have nothing but B+W filters and have never had a problem. In fact I have 2 sets and one set is specifically for the A700 and tamron 17-50 2.8 @ 67mm. I have their ND's, Graduated, Circular polarizer,linear polarizer and clear. I would agree with David. This was my first synopsis. My second........I use them all the time and found something out this summer when I tried to create a variable ND filter. I used the 77mm B+W Kears..... circular filter on first and the 77mm B+W MRC linear on the outside. One day I took them apart and put the linear on instead of the Circular polarizer by accident. I could never get the image to focus correctly. Everything looked like junk. By chance are you using a linear filter instead of a circular filter? There is a huge difference when it comes to Digital. Just a second thing to check.

Motor

[chriznatch]

good question ... I actually don't know if it's a linear filter. It's a B+W 67 mm UV Haze 1x MRC -- at least that's what it says on the filter ring. I bought it mostly to protect the lens in case I knocked into something. Any idea if that's a linear filter?

I usually set my camera to single shot focus. But I've used Auto occasionally. Maybe I had it on auto and the camera switched into continuous when I was swinging the lens around to catch more of the background? It never ceases to amaze me how much there is to learn about using a camera!

[David Kilpatrick]

Linear only applies to polarisers, not UV. That issue is irrelevant. Linear polarisers cause focus errors because they effectively block out the focusing image at one orientation (all beam splitters and semi silvered mirrors, used in AF lightpaths, polarise light and can be opposed to the linear filter).

David

[twm47099]

I have had the problem twice, but not with that brand of filter.

The first was with a Cokin resin warming (1B) filter. When I investigated the problem, I found that even moving the filter in and out of my field of view caused image shifts. Examining under a loupe, I saw doubling of the image. The filter surfaces were most likely not parallel to each other.

The other case was with a Hoya HMC UV filter. I had bought a used lens that had a very good reputation for sharpness. I put a brand new filter on the lens and ran some sharpness tests (mounted section of newspaper with lots of different type fonts). When I got the film developed, I was surprised to see how soft the images were -- worse than a cheap lens I was replacing. Even in 4x6 prints they were soft. I was using a manual focus camera, so don't know if I would have had problems with AF. It turned out that the new filter had a very thin, transparent coating of an oily substance on both surfaces of the filter.

The residue was very difficult to clean completely (Kodak lens cleaner just beaded up). I finally used spit followed by Eclipse, a microfiber cloth, and a lot of time (30 minutes). I retested the lens without a filter and with the cleaned filter. The improvement in quality was amazing (lens lived up to its reputation), and I could not see any difference between the no-filter and cleaned-filter tests when examining the negatives with an 8x loupe. Make sure the filter is clean and correctly mounted in the holder.

tom

[chriznatch]

I'll try cleaning the filters. Most of my shots actually seem to be in focus though, and I used the lens filter for all of them. There were just a couple of sequences where I could not get my wife in focus. On a couple of pictures, the camera seemed to focus on an object slightly behind her. In others, I can't tell what (if anything) it was focusing on. I believe I had the camera in single-shot focus mode, but I can't tell from the metadata. It's possible I had it in auto.

[twm47099]

If you are getting some shots sharp with the filter, then it is most likely not a dirty or defective filter.
Another time I have had problems with AF was when harsh lighting caused flare from the filter. In that case removing the filter helped, because the front element of the lens was recessed. Were you using a lens hood? Or did you see any flare in the photos?

tom

[chriznatch]

I don't think I was using a lens hood, but it was overcast and I don't see any flare in the shots. Maybe I have a back focusing issue with this lens. I think I'm just going to have to experiment more with this camera/lens combo and see if I can replicate the focusing problem to see what is causing it. Thanks everyone for all the helpful tips.