sony/minolta 135/2.8 STF on pentax?

hi all,

has anyone ever tried to get a sony lens on pentax? i understand that it would involve removing the lens mount, so you'd lose af etc.

i'm looking specifically at the 135/2.8 stf, which is a manual focus lens anyway, so i think the functionality loss would be minimal. i'd love to get one of these onto a pentax mount.

even if no one else has done it, does anyone know of any engineer/shop that i might be able to contact within europe that might be able to help out?

i know that others have done similar things with a minolta mc/md mount, but the alpha mount.

thanks for the help!

I don't know about Sony, but the Minolta MC and MD mount does present a challenge because the original lens to film plane distance is shorter than on Pentax by 4.5mm. This means that the new mount needs to be 4.5 mm shorter than the old one, on some lenses that is possible but on others the aperture ring is already on the end of the lens. Without adapting that distance correctly the lens will never be able to focus on infinity, unless there is plenty of spare travel in the focus mechanism that can be used by moving the infinity stop.

Each lens tends to be different. The Sony 135/2.8 may well be physically adaptable, but whether it is actually worth it is another matter!

Inevitably there is a forum for this sort of thing - http://forum.mflenses.com/

the sony alpha mount is about 1mm less than pentax, if i recall correctly, which seems like much less of a daunting task. it's worth it to me, since i've craved an stf for a while!

Needless to say it would be easier to put the lens on a Sony body, but if you are determined to modify a Pentax one I guess you'll do it.

i'd love to see it in a pentax mount. it's a lens that i'd like to use as a general purpose telephoto which i'd keep in my bag, so i'd like to spend some time getting everything *perfect*.

that's 1mm extra space should be salvageable.

if i get it done, i'll be sure to share the results here

Just buy a Jupiter 37A 135mm f/3.5 It's almost as good and only about £25

while i hold the jupiter 135/3.5, and the jupiter 135/4 in high esteem, they are not a patch on the stf...

They work on a Pentax though wheras an STF won't

I also hunger for the soft bokeh of the STF, i wonder if you can not mimic the effect of the extra aperture with the use of filters on the front of lens?
The actual aperture of the lens need to be in the rear of the lens then of course.

What about the Voigtlander Lanthar 125mm f/2.5, seeing that one has stilled my hunger a bit.


http://www.dyxum.com/dforum/diy-version-of-stf_topic55326_page1.html

The 2nd aperture is not the principal thing that affects bokeh. It is the paired lens elements just after the aperture that has the main effect.

http://www.the135stf.net/apodisation.html

Thanks Tyr but still worth a try to see if you can mimic it with a gradient filter.

It has a centre graduated filter in the middle of the lens next to the iris. Clear in the middle, darker to the edges. It is not a coated or dyed filter, but is actually shaped (negative) lens element. By making the lens element out of grey tinted glass, the thicker edges of the minus lens are denser than the thin middle bit. Simple idea, quite brilliant. It means that wide apertures have a graded edge to the iris shape, instead of a clean circle, but small apertures are not affected and there is less speed loss. It looks a bit like a 'centre filter' as sold for Super Angulon wide angle lenses.

This looks promising.
http://forum.xitek.com/showthread.php?threadid=635957&pagenumber=1

if you look at that post, you'll see that it was started by me.

i've messed around with the idea, and putting an it anywhere but where the aperture of the lens is results in vignetting that's way too severe.

i think it's possible to do a mount swap on the stf. i know that it's been moved to canon, which is an easier task, given canon's longer registration distance. but i reckon 1mm is fine for a mount swap...

i'm having difficulty in finding a technician to do it for me, so i might have to have a go myself. i've done some conversions before, but i just get butterflies when it comes to doing it with an expensive lens...

Isn't that lens generally used for portraits? Do you really need infinity focus? Although I'm not sure where the new 'infinity' point will be if the registration is 1mm too long, perhaps it will be much too close for portraits.

Have you made it gradient?
You will think it should give vignetting but the aperture also makes the beam of light smaller so you would think it would vignette as well... still don't understand the workings behind that...
But enough talked about that...

Do you know if this lens focus past infinite if so that 1mm might not be so problematic to swap mounts. Since it's an manual focus lens i doubt it will though but you never know.

You might even be able to shave some of the back of the k-mount you want to use maybe?

Hello Stefan