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01-06-2011, 11:31 PM   #1
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lens mount trading

I recently rescued a Nikon 180mm f:2.8 ED AF IF lens from a junk pile(a freebie). All of the elements are still there and the lens is in fantastic shape. not even a scuff... but... it was used in some sort of an instrument at some point and for some reason all of the electronics were gutted. mostly what I am into is astrophotography so this would make a perfect lens... if I can get it to fit

I have heard rumors that you can mount this directly on the pentax mount and it kinda fits... but... I don't trust that this would be sturdy enough. i looked at the lens mount adapters but they all use an ~1.5x corrector and I really want to stay at f:2.8. Is there a way to just replace the nikon mount with a pentax mount? has anyone had any luck with this?

01-07-2011, 02:08 AM   #2
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Yes, the conversion can be done. It should be fairly straight forward if the lens is complete mechanically and has an aperture ring. There are several threads in the lens forum on converting Nikon lenses. Essentially you grind off the AI metering lug on the aperture ring, unscrew the Nikon mount and attach a Pentax mount that has the correct screw spacing. Depending on the thickness of the Pentax mount used you may need a shim to adjust infinity focus.

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/60225-nikkor-lens-club.html
01-07-2011, 10:53 AM   #3
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I have used several converted and virgin Nikon lenses on PK-mount. I have not replaced the mount with a PK, just modded it (or not). Some NI's have a full-circumference lip behind the aperture ring. Such is not amenable to dremeling, and I've just force-fit the lens onto a PK mount. I have a 200/3.5 which I think is about the limit weight-wise for the amount of strain on the mount -- I handle that one very carefully.

Other NI's have an metering lug and a partial-circumference lip behind the aperture ring. I call this the nub-and-arc architecture. I have modded 4 lenses of this type. My first and most-used is a Nikkor 85/2 I'd bought for US$9. The procedure: use a Dremel metal-cutting blade to cut off both nub and arc, slightly sanding the aperture ring base to leave it smooth. Blow away the metal dust and mount the lens -- it's that simple! I fretted over the first lens for a couple days, working slowly, carefully... Later mods took about 10 minutes each, working on a back porch railing (my hi-tech workshop, heh). No big thang...

This mod is pretty secure, but not totally. My next step will be to cut a slot 1mm wide and deep and 5mm long in a position on the NI's base for the PK camera's locking pin to engage. That should, with minimal effort and mechanical expertise, make a rock-solid mount, even for a lens as heavy as the 180/2.8. I have had to make no adjustments for infinity focus. The 1mm difference in lens registration might push near-focus back slightly but should have no impact on astro work.

Yes, other mods require replacing the lens mount. I hesitate to delve that far into a lens' innards. I've already accidentally disabled (wrecked) the aperture linkages in a couple non-Nikon lenses and don't wish to junk any more of them. I prefer my modest butchery.
01-07-2011, 02:20 PM   #4
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QuoteOriginally posted by flyznest Quote
I recently rescued a Nikon 180mm f:2.8 ED AF IF lens from a junk pile(a freebie). All of the elements are still there and the lens is in fantastic shape. not even a scuff... but... it was used in some sort of an instrument at some point and for some reason all of the electronics were gutted. mostly what I am into is astrophotography so this would make a perfect lens... if I can get it to fit

I have heard rumors that you can mount this directly on the pentax mount and it kinda fits... but... I don't trust that this would be sturdy enough. i looked at the lens mount adapters but they all use an ~1.5x corrector and I really want to stay at f:2.8. Is there a way to just replace the nikon mount with a pentax mount? has anyone had any luck with this?
I have converted lenses for both Nikon AI and Minolta MD mount to K mount with stop down aperture.

Basically what you need is a K mount flange, either from a doner lens or by using a flanged M42 to K mount adaptor.

Doner lens mounts simetimes work better because they are thicker, and it is easier to countersink the screw heads, M42 - K mount flanged adaptors are only 0.8mm thick. I usually need to file th escrew heads a little so they don't scrape my body side mount.

the Nikon lenses have longer regestration distance also, so the thicker the doner mount the better. Never tried making the aperture coupling work

01-08-2011, 01:04 PM   #5
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I suppose what i will do is go to the used camera shop here in town and see what he has for a donor. I have a 28mm Albinar that has all sorts of issues that I wouldn't mind scrapping... As far as using it unmodified... I've tried twisting it on there and it sorta fits but i wouldn't trust it.

Last edited by flyznest; 01-08-2011 at 02:23 PM.
01-08-2011, 08:17 PM   #6
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I didn't get to the camera shop today but I think I found a fairly simple solution just looking around at pics on the web. The flange on my Nikon lens uses 60 degree screw spacing. While most of the pentax lenses use 4 and 5 hole patterns(90 degree or 72 degree spacing), I found some teleconverters with 3 screws... thats 120 degrees.... There will likely be a radius difference but this should put me close... and I can have the last little bit machined.
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