manual lens (M 120mm f2.8) infinity focus problem

I have a pentax M 120mm F2.8

I've just noticed it doesn't focus to infinity on my ist DS

I'm actually not sure if its always been like that or if it has changed.

I'm looking to sell it since i just bought the DA* 50-135 but I need to find out about the infinity focus first.

any clues?

First check it's not focussing *past* infinity. Many older lenses will go slightly beyond infinity if you turn them all the way.

If that's not the case then does the focus feel rough? It's possible that grit can get inside the helicoid and stop it from moving along its entire length.

Also check that the lens base is fully seated against the camera body (i.e. the adaptor is fully flush with the body.

thanks, but nope its none of those things. its very smooth, sits flush and fits on the body easily, and doesn't go past infinity.

the dial reads infinity, but the actual focus is about 10m

Sounds like a chat with Harrow Technical might be next stage. Robin'll give you a steer as to what might be the problem and a guide as to the cost of getting it sorted.

M 120mm F2.8 isn't a v common lens - so if is in good condition, worth getting fixed IMHO


Steve

I have a dim memory of opening up a pentax M 50mm to clean it and finding a complicated thread system that had to be reassembled on exactly the right thread to get the focus right, maybe this has been disassembled and reassembled slightly wrong at some point.

On some lenses it is just a matter of undoing one or more screws around the barrel. These are unfortunately often located beneath the rubber grip material which will need to be peeled back first.

hmm, took it apart and put it back together and its fixed. dunno quite what I did. something must have been not quite seated right. all fine now, focuses just a hair past infinity.