Focus off on Serenar 1.8/50 lens.

[tunalegs]

I bought a serenar 1.8/50 on ebay, and found once I had it that it does not focus properly. I tried it on a couple of LTM cameras and on a Zorki 4k, and in each instance the focus was off. At close focus it is relatively close, maybe off by an inch or two - but by the time you get to an indicated 6 or so feet the focus is off by a few feet.

Before I send the lens back I want to know if this is an easily solved problem. It looks to me like somebody opened the lens to clean it and maybe something just wasn't put back to spec perfectly. But I don't want to fiddle with it if I'm going to send it back.

So is this a common problem? Is it fixable? Or am I best just sending it back and getting my money back to me?

 

[02Pilot]

If someone opened it and forgot to replace a shim, you could certainly see the sort of problem you're describing. It's not hard to fix (replace shim), but it would require some fiddling to get the thickness right. How much is your time and effort worth in relation to what you paid for the lens?

 

[peterm1]

You need to ascertain if it is focusing to the back (back focus) or to the front (front focus) of the point of focus. This will help ascertain if its a shim or something else (a missing shim will cause back focus). It is unlikely someone put in an extra shim so if it is front focusing that is probably off the table as an issue.

A more likely problem may well be that when lenses are fully demounted and the helical coils are unscrewed from each other if they do not get reassembled in the correct way the focus will be off in one direction or the other. An experienced tech knows that when unscrewing the helicals they need to carefully note and mark the position at which the components come apart. A scratch is made on the parts at this point so the starting position of the thread is known and they align correctly. There is said to be only one way to correctly screw the helical components together and many ways to get it wrong. If it is the issue and no marks have been scratched then its a matter of trial and error. It seems to be a common issue when a lens is serviced by a do it your-selfer who is inexperienced. If this is the problem and I had it on my lens I would expect the cost to be anything from $100 upwards to fix depending on how long it takes for the camera guy to find the correct alignment and reassemble the camera. Given its a common Serenar 50 f1.8 I think i would advise sending it back.

 

[tunalegs]

It is indeed back focusing, and I can see from an online article how the lens is shimmed. I am going to send it back anyway (I don't imagine finding suitable shims would be easy) and just look for a better example. Thanks for the insights.