How to repair your MC Zenitar F2.8 / 16mm lens due to "focus error"

I bought this lens back in 2000 or so.  Used it with my film camera a number of times.  Then I tossed it into my camera bag and forgot about it.  About two years ago I tried it with my newer digital camera and found out it would not focus.  I even tried it by mounting it on my film camera, nope.  It would focus about 10 feet, that's it.  I figured I dropped it somehow and even showed it to my "optical engineer" son.  Nothing bent, loose.  It just won't focus right.  So a few days ago I found the lens case and tried it again because vacation time was coming (Aug. '10).  Nope.

Checking the Internet I find many, many pages on the "Zenitar focusing problem" and they all conclude... Russian quality, not so good.  But if you just remove the rubber focusing ring.  Loosen three screws (3/4 turn is all I needed) hidden there, you can "adjust" the focus.  After 30 min. of trying I got it to focus, clearly, to infinity.  Nice, I can use it again.  In packing it up in the supplied lens case I find the 3 filters it came with.  I found the UV filter that screws in back and cleaned it.   The rear filter still has some tiny haze spot I can't get off.  I guess this is why I took it off.

I screw in the supplied UV filter and voila !  It focused PAST infinity.....

Now I check with the "optical engineer" son and NOW he tells me a rear glass filter will change focus (big time) if made that way.  If you check the "optics" page of this lens, a flat piece of glass is shown in the rear.  That's the filter.

Sooooo... to fix your Zenitar F2.8 $200+ fisheye lens (with digital, it's less of a fisheye) place the "required" supplied UV filter as stated in the instruction page I found at the bottom of the supplied lens bag.

<< from a Nikon 300mm F4 lens with 39mm rear filter>>
The internal filter should remain in place because the presence or absence of the filter changes the effective (optical) distance to the film plane by a tiny amount. Don't worry about this, but please leave the filter holder in place.

So know you know how to fix your lens... read the manual and use the rear mounted filter.
Now I check the NET and see that these are still being sold, remember this are for FILM cameras and will not give you the extreme wide angle due to the smaller sensor in digital cameras.  BUT I only see 3 filters being provided and nothing about the UV filter. So if the company  readjusted something in a newer model.  That I don't know about.

Note the missing rubber focusing ring.

 

See the "unscrewed" supplied UV filter

I knew the lens worked.  Some test photos years back with FILM !