Problem Focusing with Pentax SMC DA Series 50-135mm f/2.8 ED IF SDM

I recently bought this lens after reading terrific reviews. I am having trouble getting the subject of the photo to be crisply in focus, and I have never experienced this before. I am using the lens on a Pentax K-S2. I have both the lens and the camera set to AF mode. Any suggestions as to what I am doing wrong?

Thank you.

Please post an example with either exiif data in place, or provide a full description is settings.

Autofocus on something using live view, if it is sharp then af probably needs fine tuning on the body (if your camera has it).

Hopefully you are not having the same problem as this chap in the DPReview forum

Haven't got that lens but I had similar problems with the PLM 55-300. Firstly, it is really finicky about precise aiming of the af caret. It is phenomenally accurate. Secondly, it was backfocussing something rotten and needed +7 from the camera body to correct it.

Yes, it has AF Adjustment. However, i am having trouble figuring out how to get it to work. The manual is no help and i am only finding online suggestions that i need to do the adjustment, but not how. Just need direction on the mechanics of moving it + or - on the scale. Pretty basic question, I know, but i need to figure this out or return the lens ASAP.

Any technical help out there? Thanks.

If you post a picture with exif intact, or do the Live View comparison, we may have more of an idea what may be the issue. If you start changing the autofocus fine adjustment without establishing what the problem is first, you may just make things worse.

In which case, you'd simply set it back to default and you're back to square one, surely? No better - or worse - off than when you started.

It's not like you're filing bits off of the mount

You can't go too far wrong to be honest.
Take a shot as you normally would and take another with live view. Preferably using a tripod for both as this will eliminate some other possibilities.
Check the focus accuracy of both.
If both are out then you have a problem with the lens. If only the viewfinder shot is out then try the af fine tune.
Live view is not affected by front or back focusing.
When applying focus adjustments, apply to 1 (not all) when the option appears.
Hope you get it sorted

I have this lens, by and large it is very accurate at focusing on both my K-3 and K-5iis, but has required some fine AF adjustment on both. Also, in common with many other lenses (and camera bodies) it's a bit more hit and miss in low light, something I've noticed most when doing gig photography where light levels can be very poor. Consider when adjusting the fine AF you can never get it 100% right because this is a zoom lens and the focus inaccuracies may (will) differ at different focal lengths. My suggestion would be to adjust the AF at a middling focal length, or at the one you are likely to use most often. The technique I use is as follows:

- Place on a tripod and shoot a subject with good contrast at a medium distance, with the subject angled at approx 45 degrees to the lens. A white ruler, or a white page with black writing is a decent subject

- Set to F2.8 aperture and focus using AF and the viewfinder (not Live View), take 3 images and for each image make sure the lens is de-focused and then re-focused using AF prior to pressing the shutter

- Review all three images (easiest to do on a PC monitor) to see if you have a consistent front or back focusing problem. You may find some variation, if necessary repeat until you see a definite pattern emerging

- Once you've established whether you have a front or back focusing issue, adjust using the AF Fine adjustment in the camera menu, ensuring you only it apply the adjustment for this single lens

- Once adjusted, repeat the steps above and re-adjust if necessary until you hit a 'happy medium'

Once you've done this you will be at the closest point possible to reliable focusing, but please bear in mind that you may still experience some issues especially using the lens at its widest apertures. 135mm and F2.8 makes for a wafer-thin depth of field, so errors are not unusual.

Please don't give up on the DA*50-135, it is one superb lump of glass.

Cheers
David

Many helpful suggestions here, thanks. I now know how to figure out whether i need to adjust up or down. The basic question i have is HOW to make that adjustment. I see it on the C4 menu screen (#27 AF fine adjustment) but can't figure out how to move it + or - on the scale. The 4 way navigation bar moves the menu to #26 or #28. How do you make the lens adjustment?