I own a pentax smc 80-200 f4.7-5.6 lens. I have a problem in infinity focus when the focal length goes above 100mm. The images appear out of focus after 100 mm. I dont know whether its the problem with the lens or I am doing something wrong. If anyone have any idea about this your help is much appreciated. Thank you.

[MarkJerling]

Hi Chris
I'd recommend that you post some photos as that may help members to help diagnose the likely causes of the problem you're experiencing.
Kind regards,
Mark

[Suzhouren]

Hi Chris,

It may be a focus calibration issue with the camera (can be checked with a suitable target), or it could be that the lens itself isn't quite reaching infinity. Does the lens hit the hard stop at the end of its travel? I had to adjust my copy of this lens after taking out the front element to clean off some fungus. From what I remember, it's very simple to adjust: there's a plastic trim ring surrounding the front element that simply pops off, there's then a retaining ring which must be loosened. The actual front element is mounted in a plastic surround which is just threaded into the barrel -- the threads are reverse here as far as I recall. There are ridges on the surround which help you to turn it -- you want to make it move slightly nearer to the camera if the lens won't hit infinity. A few tests with a suitably distant target should get it dialed in nicely.

Another issue which causes focus problems on this lens is if the little metal comb which shorts the distance coding tracks isn't making clean contact. My copy had this problem too (hence why I got it for next to nothing!), though the symptoms were that it simply wouldn't lock focus on the long end and just kept hunting back and forth. This too can be fixed, but is harder as the metal fingers are very delicate and easily damaged.

Once set up properly, this lens is great. My copy, while poor close up, is great at longer distances -- sharper at infinity than my F70-210.