infinity mark alignment

I have a lens that seems to be struggling to achieve focus on distant objects. Above f4 it is ok but between f4 and f1.8 I can hear the motor trying to focus the lens even after the lens has stopped moving. When I focus the lens manually it achieves focus exactly when the lens hits the stop. WhenI turn the barrel to focus to infinity the infinity symbol aligns exactly with the red line on the barrel but will turn no further. Is this normal?

I am worried that the screw drive or the motor in the camera could get damaged

Any advice would be much appreciated.

Thanks

On a random sample of what's at hand, my old M/F lenses stop at 'infinity' but my A/F have a little slack beyond it, although the scale doesn't move round any more.

May be helpful if you could confirm which lens it is.


Steve

Thanks for the reply. The lens is the 77 Ltd that I can see from your signature you have also. I also checked my da 70 and that has more play around the infinity mark. The reason I notice this is because the images seemed a little soft below f4 but shooting distant objects at f4 or below is not something I would normally do.

Is this lens known for being soft when shooting distant objects? I know some portrait lenses are.

Tried my 77 Ltd on the K7 & K20 - pointed it at hills in the distance and it locked on immediately - no sign of any hunting or similar focus related issues. Tried it at f11 and f4 with AF on SEL - no difference. No play beyond infinity either.

Never heard anything bad said about this lens's IQ.

Has it had a knock do you think? That might account for the AF motor still trying to move the lens's focusing elements - doesn't think it's at infinity yet. Looks like a trip to the service centre is needed.

If it's not sharp then something is clearly wrong. However, bear in mind the subject matter and make sure that the infinity subject isn't shrouded in haze!

Buildings are a good subject for distant shooting tests as they have very clear lines and seeing whether or not they are sharp is easier.

Also use a tripod for sharpness tests, aling with the 2 second self timer so the mirror is raised before the exposure. This ensures there is no vibration from the mirror to take the edge off the critical sharpness.

In the past 2 months it has been to pentax once because it was soft at all apertures which they fixed under gaurantee and then 2 weeks after it’s return I noticed a scratch on an aperture blade so they replaced that blade and now this.

Are you suggesting that the elements could be misaligned?

Thanks again

Do you get any lateral image shifting at certain focussed distances. I once saw a lens that shifted the image slightly to the left when focussed near infinity. It was hard to detect but easily seen in Live View magnification. that could confuse the AF system.

I had this sort of problem with my DA50-200 which went really badly out of alignment so today I decided to rebuild it and see if I could improve it.

By the way, not a task I'd recommend as it took about two hours to put it back together.

It had 20px CA and could not focus to infinity so I couldn't sell it and it would cost more than the value of the lens to repair. Decided to take it apart and try and fix it myself.

Infinity focus now works fine but it is no longer para-focal. Infinity is just before the infinity mark at 200mm but at 50mm it is at 3m. It also focuses closer at 200mm than it does at 50mm. I have to step back if I want to use the 50mm end. I just fixed it up as a backup, probably won't get used for years.

I find the fact that it behaves differently according to the aperture a bit odd, leading me to suspect that there's something funny happening there (rather than just a misaligned infinity setting). The lens is held at f1.8 for focusing so the aperture you set shouldn't make any difference, as this is only stopped down to when you actually press the shutter.

Yes you are correct. I’d forgotten about that. I can’t remember if it does behave differently. It is just that the images are soft below f4.

Can I attach images on this forum or do I have to provide a link?

Photobucket is a good place to host images and then they can be linked to. No file attachments though.

link

The top 4 images at the above link my be of interest.

The first 3 are 100 percent centre crops of well I was pretty much photographing anything in the distance to check the lens. I think they show some softness.

The 4th image taken at f4.5 should show that the lens is softer on the right hand side but for some reason it is not that obvious in this jpeg. I can assure in lightroom at 100 percent it is obvious.