Lens Focus Issue, what to try next?

mtngal · 07-13-2008, 08:57 PM

I have a DA*50-135 that I think I might have damaged in some way. It never focused right with a K10 I used to have, but would focus all right with the K100. It seemed to be right on when I first bought the K20 and it became my most used lens for a while, but not so much recently (been doing other things). Tuesday I got a 77 Limited and have been shooting some comparison shots this week. The first thing I noticed was that the DA*50-135 was back-focusing compared to the 77 Ltd, which was right on. The first series (indoors with flash) I took I wasn't that careful with my focusing and thought it was my error, rather than the lens. The second series was outside and I was very careful about the focus, and it definitely seemed to be backfocusing.

Yesterday I took out the K100 with the 50-135 and the K20 with the 77 Ltd, taking pictures while hiking. The 50-135 seemed to be backfocusing, or at least not focusing on what was in the center (I was using center focus with both cameras). I thought that was odd because it had been reliable with the K100 in the past (why I thought my problem was the K10, not the lens). I just figured I'd shoot some focus charts and adjust the K20 to focus correctly with the lens and not use it on the K100, which doesn't get used much any more.

Unfortunately, the charts, especially when I shoot when the camera is close to the minimum focus point, look well within specs, perhaps a bit of front focus, but the focus line was always sharp, and I was shooting at f2.8. When I tried to back away a bit, the chart pictures became in-conclusive (I don't have a tripod at the moment).

That doesn't match my real-world shooting at all. The lens did fall out of my camera bag a while ago but it didn't even have a scratch. So it might have some damage I can't see. Is there something I can use beside the focus chart to test the focusing? While I think the lens may still be under warranty, I accidentally washed the receipt and I'm not sure I can replace it.

Does anyone have suggestions?

ricardobeat · 07-13-2008, 09:54 PM

The best test is "real life" use. Maybe it's psychologycal, you need a lot of back-focusing to have a noticeable effect on anything more than a meter away at most apertures. Try shooting some portraits wide open and focus on the eyes, AF and MF, then check at 100%. That should do it.

mtngal · 07-13-2008, 11:03 PM

Here's an example - the link is to the full sized file (K20, large file). The picture is an outdoor light fixture that's well above my head - definitely more than a meter (3 feet) above my head. I carefully focused on where the frame top meet the part of the frame that drops down, near the center.

The link to the 77 Limited (f2.8) shows the correct focus: http://mtngal.zenfolio.com/img/v3/p664017269.jpg .

The link to the 50-135 also f2.8. At first I thought I might have some camera shake or it was much softer than the 77, but if you compare the center support (the main mount for the light) sharpness and the various chains, you can see that the shot is actually very sharp, but the focus is further back, I think: http://mtngal.zenfolio.com/img/v3/p976540462.jpg . The night before I had similar results, though the distances were much smaller but I wasn't as careful with my focus point.

Do you think I have a problem that I can solve by AF adjustment or should I work at getting a duplicate receipt and send it in?

mtngal · 07-14-2008, 06:21 PM

Update - went out this morning and shot some general shots. No sign of a problem, no focus problems at all. Oh well - I'm going to give up trying to figure it out. Maybe the lens just doesn't like not being used every day or something.

NicholasN · 07-14-2008, 06:34 PM

Could be an intermittant problem which would be very hard to diagnose.

ricardobeat · 07-14-2008, 07:19 PM

Originally Posted by mtngal

Here's an example - the link is to the full sized file (K20, large file). The picture is an outdoor light fixture that's well above my head - definitely more than a meter (3 feet) above my head. I carefully focused on where the frame top meet the part of the frame that drops down, near the center.

The link to the 77 Limited (f2.8) shows the correct focus: http://mtngal.zenfolio.com/img/v3/p664017269.jpg .

The link to the 50-135 also f2.8. At first I thought I might have some camera shake or it was much softer than the 77, but if you compare the center support (the main mount for the light) sharpness and the various chains, you can see that the shot is actually very sharp, but the focus is further back, I think: http://mtngal.zenfolio.com/img/v3/p976540462.jpg . The night before I had similar results, though the distances were much smaller but I wasn't as careful with my focus point.

Do you think I have a problem that I can solve by AF adjustment or should I work at getting a duplicate receipt and send it in?

You have to leave a bit of room to account for the "Limited magic", but there is definitely a difference. The 50-135 has a lot less contrast, is the glass dirty?

Maybe the focus plane is tilted (try shooting a flat object at 90º with some regular feature you can compare across a horizontal line), or maybe the center point (you're using spot focus right?) is not exactly in the center, or maybe you have very shaky hands

Usually my pictures don't require much precision, and I don't have any macro lens. I bet someone could find lots of faults with most of my lenses, but I never notice them. If over time you find that it's not good enough, sell it and try another one!

ll_coffee_lP · 07-14-2008, 08:37 PM

Originally Posted by NicholasN

Could be an intermittant problem which would be very hard to diagnose.

Ya....like my last car. Glad that sucker is gone.

c[_]