Zoom lens repair advice needed.

[Bruce Oudekerk]

When I bought my KM 7D 4.5 years ago, it came with a KM 28-75 f2.8. I liked the lens but didn’t especially like the effective 43-112 mm range. My 50mm f1.7 was used for indoor natural light, the 28-75 was relegated to far less use, and a Sony 18-250 or Beercan or 70-300G for the lion’s share of my shots. When I bought my a850 the 28-75 went back into my everyday camera bag along with the 50mm but the majority of shots were on the back of my trusty old Minolta 28-135. When I do use the 28-75, I normally got excellent results but occasionally the shots were soft but I wrote it off as anomalies…it’s always low(ish) light and it could be camera shake etc. In one session of similar shots every thing was slightly soft…and since I’ve admittedly gotten spoiled by sharp images with this a850, I’ve done a lot of testing. I have a serious problem.

The lens is fine at 75mm down to somewhere significantly below 50mm. I'm not sure exactly where the transition takes place but at and around 28mm at f2.8 it... significantly... back-focuses. Just at the smaller focal lengths. Obviously depth of field has been masking the back focus problems at the shorter focal lengths. FWIW it seems as though this lens is tack sharp (somewhere) at all focal lengths and is equally sharp across the entire field. Before I was positive what was happening, I micro focus adjusted the a850 and moved the focus forward and its fine now at all f stops at 28mm but at the long end its front focusing now…which is especially noticeable at f2.8 of course. I’m assuming the lens was always like this and for a variety of reasons never noticed it because so many images are tack sharp and I used it infrequently.

My quandary is this; is this likely a fixable-repairable issue or not? Or do I just have to eat a screwed up lens? That sounds like a strange question even to me, but this doesn’t ring true of something like a de-centered lens element or sticking ‘something’. On the other hand this might be a common repair. I don’t mind a substantial repair bill if I have a realistic chance of this getting repaired to perform like this lens is capable of…which should be very good.

I really like this lens for indoor work with the a850 but I have absolutely no frame of reference for a realistic solution to this problem. I would appreciate any helpful input short of “buy a 24-70 ZA.”

Bruce

[David Kilpatrick]

It unlikely that the lens needs repair. First, zero your micro focus adjustments. Then test the lens using only the centre sensor - be sure to turn off wide area focus, and do not use ANY sensor other than the central one for this lens. Do the micro adjustment based on a subject at 3m distance, 28mm, and the centre sensor, in daylight.

Hopefully this will give you an optimum which will also prove OK at longer focal lengths. Micro adjustment should always be done at short focal lengths, as at 75mm you have too much depth of focus at the sensor plane (less depth of field at the subject plane, but this is NOT what micro adjustment tackles). For the same basic reason, subjects at 3m or further away should be used for micro adjustment, not closer targets.

The 28-75mm will always focus more accurately (roughly 4X more accurately) using the centre sensor only. That is the benefit of owning an f/2.8 constant aperture lens and using the A850.

David

[Bruce Oudekerk]

Thank you very much for the reply. It was not the response I expected but I believe it is the correct one and it forced me to rethink and re-perform my testing and the results.

In the last iteration of tests I did do them outside and all of the testing has been around 10 feet away and was using center focus. The difference this time was that I was more critical and more systematic, I believe. Before I had arbitrarily jumped from 0 to -7 and that seemed to be OK at some focal lengths. But this time I went back to 0…reshot a complete sequence (28mm, 50mm and 75 at various f-stops ) but made my further decisions based on 28mm only. I arbitrarily tried -4 micro adjust and everything was much better…across the board. I retweaked it to -5 but eventually jumped back to -4 as a nice compromise. I always have excuses why I’m a bonehead and this time I think it’s because 28mm is just plain soft on this lens compared to the stellar sharpness of the rest of the focal lengths. That combined with the lack of detail due to everything being so small at the wide angle and the f2.8 makes the decision making process difficult. No depth of field and too tiny detail at a soft focal length is a prescription for disaster to judge things by. You can’t get blood from a stone. 50mm and 75mm are OK at f2.8 and are becoming really sharp by f4 even. I’ve been spoiled by my a850…the 50mm f1.7…the 28-135 and my 70-300G. I just expect everything I have to look like it went through a $2000 lens, I guess.

At any rate, I now don’t think anything is wrong with the lens…its just of very modest quality at 28mm…especially in the center where I expect it to be really sharp. I now need to shoot some real world shots at greater distances to insure everything is really OK, but I think it will be. Ironically my cheap old Canon 28mm FD prime was absolutely amazing and the sharpest lens I’ve ever owned…so sharp only PanatomicX could start to stress it.

Thanks again, David.

Bruce

[agorabasta]

I've tested a few copies of a Tamron version of that lens. They were more or less equally sharp at 28mm as they were at 75mm. There were some variations, most likely due to some element misalignment which is a very common issue with Tamrons. But they all lose some contrast above 70mm.

[Bruce Oudekerk]

Just for kicks I decided to actually go out and take a couple shots with the newly adjusted lens. One set was a duplication of a set of indoor images I had taken a couple weeks ago that were WAY out of focus and I had excellent success in the reshoot. This was at 8 feet 60mm and f5.6. No surprises:)

Then I went outside and took a few shots of my house from almost 100 feet away at 28mm and f2.8. I expected them to be somewhat soft…similar to my tests at 10 feet. Instead they were quite sharp! I also took some at 28mm and f8 just to compare and it was a bit sharper ...but just. Of course there was depth of field sharpness variance as the lawn stretched from me to the house and the roof line sloped back with trees in the distant back. But generally I’m VERY pleased now. However I’m unable to explain my discrepancy of variance in apparent OPTIMAL sharpness at 10 ft vs. 100 ft. I also took some shots at 75 mm f2.8–f8 and got expected results.

It appears as though the lens is very sharp everywhere across the field at any focal length and even quite good at f2.8 and naturally getting better at higher f-stop. The only exception is at wide angle at short(ish) distances where it is softer than I can account for. My testing at 10 feet was deceiving me to a certain extent. It is what it is, and now with a proper micro adjust has become a very usable and hopefully reliable lens.

I haven’t noticed the lack of contrast above 70mm as agorabast has stated. It might be there…I haven’t looked. To a certain extent I can handle this in PP if needed but I was doomed with the excessively soft images I had previously seen.

Again thanks. I was almost ready to send the lens in, which would have been an expensive exercise in futility.

Bruce

[David Kilpatrick]

I can only tell you Bruce that I try to stop down to f/4 with my 28-75mm, I find it makes a very big difference in cleaning up the image, but it is very sharp and I have felt no need to buy anything more expensive for the A900.

Glad to hear that you have got it working to satisfaction. Hopefully most shots will be around f/8 anyway and at that setting, it's stunningly good.

David