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08-19-2011, 07:37 AM   #1
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Will Pentax M50/2 rear element group fit M50/1.7?

I've got a Pentax M50/2 in good condition, but as we al know, it is not the sharpest lens around. I've also got a M50/1.7, with haze on the inner element of the back group. It seems like lens seperation in progress. I would like to know if anyone can tell me if it would be possible to replace the back group of the 1.7 with the one from the f2, and if possible, what would the effect be on the "new' lens; softness of f2, coupled with more light from f1.7?

08-19-2011, 12:06 PM   #2
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Hi

I have a 2.0 and 1.7, the elements are not the same size.
08-19-2011, 09:31 PM   #3
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Thanks bobpur
08-19-2011, 10:19 PM   #4
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This aroused my curiosity, so I pulled out my f/1.7 and one of my f/2's and a caliper. No, the rear elements are different diameters: about 23.5mm on the f/1.7, about 20.5mm on the f/2. Spacing between front and rear elements are also different, with the f/1.7 being thicker. But the lens throats in back are the same diameter, about 43mm, to fit within the PK bayonet mount.

What are the implications? I dunno for sure, I haven't tried disassembling either of these -- although since I have surplus f/2's, I've been thinking of trying some experimental surgery, to readjust the field curvature. But it *might* be possible to swap assemblies. What would result? Again, I dunno. But in your position, I'd try for a Frankenlens. But I'm nutz.

08-19-2011, 10:27 PM   #5
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Thanks for taking the time Rico, I haven't got much to do this weekend, so I might give it a try. Will let you know what happened
08-20-2011, 01:31 AM   #6
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Ok, I couldn't wait, and took out the rear elements of the 1.7 and the 2, and unfortunately they are not interchangeable. Although the group fitting size, as seen at the back of the lens, is the same on the 1.4, 1.7 and 2, the thread on the inside, once removed, differs a lot. I haven't stripped the 1.4 as it is in pristine condition, but the thread diameter on the inside of the 1.7 is about 30.43mm and on the 2, about 26.92mm. This conical shape seems to be directly related to the diameter of the actual lens element itself, which in the 1.7 is about 24.80mm, and the 2, about 20.70mm.
This all means I am still looking for a good 1.7!
08-20-2011, 06:13 AM   #7
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Ah, another learning experience...
08-20-2011, 08:22 AM   #8
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I have not yet seen the reason why?

08-20-2011, 09:22 AM   #9
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Lowell, the thread is at the bottom end of the group, where the difference in diameter is, as you can see from the measurements, which is related to the lens element diameter. Even if you can get an adapter between the group thread and the body, they are also not the same length, the 1.7 is 14.9mm and the 2 is 11.9mm, so without some serious engineering, I can't see any way it can be done

Last edited by altopiet; 08-20-2011 at 09:28 AM.
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