    
John_shriver
Tinkerer
Username: John_shriver
Post Number: 58
Registered: 12-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, June 12, 2010 - 01:00 pm: | 
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At least in very expensive lenses like Leica's, the set of elements in one lens is selected to go together, so that the production tolerances balance each other out.
Did Pentax do the same sort of things? I have two Pentax-M 85/2.0 lenses, both bought rather cheaply.
One was dropped on it's nose, and has a quite dinged-up filter ring. The focusing helical is not particularly smooth either, and the distance scale is off. But I can't quite get the filter ring smooth enough to get the dress ring off, and that's the "front door" to a Pentax lens. But all the glass looks nice, but probably the aluminum of the front group is slightly bent.
The other has not been dropped. But prior service has taken a chip out of the inside surface of the rear group, about 1mm diameter and 3mm from the edge. It's not "out of the way" until f/4. I can paint it block, but it's a fairly substantial defect.
So the obvious temptation is to swap the rear group from the dropped lens into the back of the presently chipped lens.
I expect that I'd have to reset the infinity stop, since the actual focal length of the lens might change.
But I don't want to do this if I'm going to make the lens less sharp.
Does anyone have any Pentax lens service manuals? Are the front and rear groups called out as separate parts? Any notes about replacing them as a matched pair?
Of course, I could just shoot test charts before and after and see if there's a difference. |
