[RioRico]

@xjjohnno - That's interesting. So the DX 50mm f1.4 can damage the mirror. But the Super Tak 50mm f1.4 doesn't seem to as far as I can tell. This must mean that the f1.4 Yashica extends further than the equivalent 1.4 Takumar.

I've the DX50/1.7 and the ML50/1.4 but not the DX50/1.4. The ML50/1.4 (in C/Y mount, chopped slightly to mod for PK) has much larger elements than the DX50/1.7 but the rear element only extends slightly less than 8mm. No fouling there!

Consider that M42 and PK mirrorboxes should have the same depth, since PK is derived from M42. But no; I just checked my M42's, two Prakticas and a Spotty (I was too lazy to break down my Chinon rig). All have mount-to-mirror depths right around 10mm. I think ANY M42 lens with extension greater than about 9.5mm is going to foul the mirror on ANY M42 SLR body. So if a lens fits an M42 mount without fouling, there's no chance it could foul a PK mirror, given the PK's 13mm depth.

[ducdao]

I've the DX50/1.7 and the ML50/1.4 but not the DX50/1.4. The ML50/1.4 (in C/Y mount, chopped slightly to mod for PK) has much larger elements than the DX50/1.7 but the rear element only extends slightly less than 8mm. No fouling there!

Consider that M42 and PK mirrorboxes should have the same depth, since PK is derived from M42. But no; I just checked my M42's, two Prakticas and a Spotty (I was too lazy to break down my Chinon rig). All have mount-to-mirror depths right around 10mm. I think ANY M42 lens with extension greater than about 9.5mm is going to foul the mirror on ANY M42 SLR body. So if a lens fits an M42 mount without fouling, there's no chance it could foul a PK mirror, given the PK's 13mm depth.

I have both DX and DS 50/1.4 but never tried them on my Pentax bodies. Will check them out tonight, if I can find them...

[stevebrot]

I have yet to find an M42 lens that extends more than about 9mm from the base.

For the most part, this is true and for APS-C it would be highly unlikely to have an issue with any M42 lens. That being said, with film bodies and FF digital, mirror interference is a historic concern with some lenses. There is a recent thread regarding Zeiss Jena Biotar 58/2 being incompatible with certain Pentax M42 film SLRs.


Steve

[RioRico]

... with film bodies and FF digital, mirror interference is a historic concern with some lenses. There is a recent thread regarding Zeiss Jena Biotar 58/2 being incompatible with certain Pentax M42 film SLRs.

I noticed that thread but didn't really follow it. My Biotar 58/2 was Exakta mount and was no problem on my Spotty except for no infinity focus. My Biotar-clone Helios-44 is M39 and doesn't extend into any danger zone. And of course FF Canons have a problem because their register is so short. I suppose the safest course for Pentaxians is to just measure the rear-element extension vs the mount-to-mirror depth of whatever camera they'll want to mount it on.

[altopiet]

Here is another thread of a bloke having a problem with the same lens and a K20D, https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/71753-m42-lens-caution.html

[RioRico]

Here is another thread of a bloke having a problem with the same lens and a K20D, https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/71753-m42-lens-caution.html

Excellent discussion. But I am perplexed. One post there has "a quote from some old Yashica body manual:

"Because of the special construction of the mirror, the following Yashinon interchangeable lenses cannot be mounted on the Yashica Electro AX:

Yashinon-DX 21mm f/3.3
Auto Yashinon-DX 28mm f/2.8
Auto Yashinon-DX 50mm f/2, f/1.7, f/1.4"

So it would seem that those are possibly troublesome lenses. But my DX50/1.7 works fine on my K20D and my Spotty, no fouling at infinity focus. Is a puzzlement.

[xjjohnno]

Rico, perhaps your DX 50 f1.7 had some adjustments made to it by a previous owner?

[Spodeworld]

Wow...now I am getting a little concerned again. The lens just showed up today...I did notice that the barrel extends beyond the auto pin on the back (I'll have to check some other lenses to see if they do too). hmmmm, what to do....

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Last edited by Spodeworld; 09-28-2011 at 10:07 AM.

[RioRico]

Sorry for the delay, I just woke up.

Rico, perhaps your DX 50 f1.7 had some adjustments made to it by a previous owner?

Mine shows no sign of any modification or tampering.

Wow...now I am getting a little concerned again. The lens just showed up today...I did notice that the barrel extends beyond the auto pin on the back (I'll have to check some other lenses to see if they do too). hmmmm, what to do....

I look at mine and I see the barrel extends about slight over 1mm beyond the pin. I stick a metric ruler against the base and see that the extension is about 9.5mm beyond the base. I measure the mount-to-mirror distance on my Spotmatic as being just about 10mm. I thread the DX50/1.7 tightly into my Spotmatic, set it to Bulb, open the back, press the shutter... and the mirror DOES seem to foul on the rear element. Un-threading the lens about 1/2 turn frees the mirror.

I also used the DX50/1.7 as my lens-of-the-day on my K20D yesterday (with a cheap clone M42-PK adapter) with no problems. I mounted it on my K1000 also, with no problems. I had measured mount-to-mirror on my various K-mount bodies as being 13mm, so their mirrors should clear the lens barrel by 3mm. I'll tag the lens as for K-bodies only, not for M42.

[Spodeworld]

When you say foul the mirror...do you mean doing permanent damage to it? Or just not allowing it to function but not doing damage to it?

Also, I'm looking at the barrel too, and it seems it is a bit over 1mm past the pin.

[RioRico]

When you say foul the mirror...do you mean doing permanent damage to it? Or just not allowing it to function but not doing damage to it?

I don't see any damage to the Spotty's mirror. It looks like the edge of the mirror just barely doesn't clear the edge of the lens barrel, and so doesn't sit flat against the mirrporbox roof (groundglass). (It sometimes doesn't allow the shutter curtain to move either.) But it's not like the body of the mirror is whacking the barrel and smashing the mirror. As I said, loosening the lens slightly frees the mirror.

I try this test on an M42 Argus (Chinon) CR-3E body, and it fouls, just like on the Spotty. I try this test on a couple old M42 Prakticas (L2 and FX3) and there's no fouling. So I guess M42 mirrorbox specs weren't quite the same with various camera makers. The Chinon and Spotty are Japanese, the Prakticas are E.German -- is that significant? I have no other M42 bodies to play with so I'll have to say, Insufficient Data.

[Spodeworld]

OK...I held my breath...mounted the lens on my K20D....set the focus to infinity...and.......fired off a bunch of shots without any problems!

Maybe I got lucky.

[RioRico]

OK...I held my breath...mounted the lens on my K20D....set the focus to infinity...and.......fired off a bunch of shots without any problems!

Maybe I got lucky.

Not luck. But congratulations anyway! No, not luck -- unless an ML50 has been weirdly modded, there should be at least 3mm clearance in the K20D. Have fun!

[Bismuth]

I have also a K20D and the Yashinon DX F/1.7. At infinity I can take a photo, so the mirrors goes up, but its not going down afterwards and the viewfinder remains black, so the mirror is apparently stuck. If I Turn the focus ring away from infinity then the mirror goes down and everything is alright. My camera is still working so if the mirror is stuck only once, it will unlikly cause any damage.
I have read that you can avoid this problem with some "adjustments" to the fringe of the rear lens. You only have to file off 1 or 2 mm of the fringe.
(I haven´t tried it yet, because you need some skills with a file if you don't want to damage the lens.)

[altopiet]

If I Turn the focus ring away from infinity then the mirror goes down and everything is alright

Bismuth, how far do you have to turn the ring away from infinity, because if it goes up without a problem, then you might be talking about a fraction of a mm and not 1 or 2?
I made myself some shims from soft drink cans that I use on my M42 lenses, to enable me to use CIF. The shims do not have any influence on my infinity focus. I would suggest trying something like that first, before starting to file?