[DrMindbender]

Since I decided to start playing the "as is" game on eBay, and I realized that my Sigma 75-300mm needs work, I began to wonder which lens mount versions would have the same components as the K-mount version with third party lenses? Basically, and it probably is easier said than done, if you were to replace the existing mount on a lens with a K-mount, it would more or less be identical to the k-mount version of that lens. The idea is that if I need parts, I don't have to wait for another k-mount version of that lens and I only have to worry about throwing out the mount components.


I have read that the Nikon AI/AI-S version of third party lenses might achieve what I want, but I want to be sure and to see what other options I have.

[Just1MoreDave]

I don't know for sure about Sigma, but I was nearly always disappointed when I tried this with Vivitar lenses. Vivitar didn't make anything so that was partly the problem. I expected that it would be far easier to switch from another brand to K - like a few screws, a spacer for correct registration, done. It never was that easy. I also don't think rigid part standardization was important to lens makers. Even Takumar lenses that look externally similar may not have interchangeable parts. Pentax lenses are usually OK.

[DrMindbender]

In this case, I mostly need the front focus group (I'm not sure if it's fungus or the coating) and the mount screws. So those are the most needed if I have to junk everything else.

[UncleVanya]

It will be likely that the optics are quite similar. The screws and the mechanical linkages may be less likely to interchange.

[DrMindbender]

It will be likely that the optics are quite similar. The screws and the mechanical linkages may be less likely to interchange.

That's what worries me the most since it's possible that some lenses might have reverse threading on the front group even though they are also Sigma λII's.

[Not a Number]

Screws are probably stock items. Just get a decent set of calipers and measure them and search online for them.

For instance if the diameter is 1.7mm and the screw is 5mm long (common size on Pentax MF lenses) search for "M1.7 x 5 microscrews". There should be a number of vendors online selling them.

[PJ1]

On a manual lens it might work if the flange focal distance (FFD) was established by the mount section. Assuming you have a third party lens body of set length (call it "A") and a mount piece with thickness "N" (say it is for a Nikon DSLR) which gives you the required Nikon (F mount) FFD of 46.5mm. It may be possible to swap out the mount piece for a Pentax "K" mount piece of thickness "K" (a mount piece meant for the same lens body) to give you the required Pentax K mount FFD of 45.46mm. The same could work for any mount that the particular lens was manufactured for - as long as "A" remained the same. Issues such as throat diameter could complicate things a bit, as would aperture levers, etc.

[pentasonic49]

The front end would probably be similar, but aperture mechanism and electronics would not. Also, the Pentax version of some lenses does not always have optical shake reduction as fitted to recent Nikon and Canon fit models.

[DrMindbender]

The front end would probably be similar, but aperture mechanism and electronics would not. Also, the Pentax version of some lenses does not always have optical shake reduction as fitted to recent Nikon and Canon fit models.

I should point out that my interest is nothing more advanced than KA lenses around the early 90's. So, aside from the differences in how each company implements camera aperture control, electronics shouldn't be much of an issue.