A little help understanding k mount lenses.

Hi,

Great site.

I currently use all M42 lenses and am thinking about getting a few k-mounts (by the year 2075 I might have something in AF!) All my M42's have an A/M switch. Is it correct to say that K lenses are normally stopped down, basically the opposite of the M42s? Any interesting tricks to have them stopped down continuously without damaging the lens or camera?

Thanks
Lars

Hi, welcome to the forum, for all thinks K mount related check out this site.

http://www.bdimitrov.de/kmp/lenses/index.html

Steve

I presume you are using a camera with a K mount however you say, "Is it correct to say that K lenses are normally stopped down, basically the opposite of the M42s?" which makes me wonder?

Basically, here is how it works.

You cannot use K mount lenses on a screw mount body.

You can use M42 mount lenses on a K mount body via an appropriate adapter. Focus with the A/M switch in the A position but meter and take pictures with it in the M position - stopped down in other words.

You can use M and K series lenses (these are the ones without the 'A' setting on the aperture) in manual stopped down mode all the time on a Pentax DSLR - not very useful IMO.

You can use the later 'A' series lenses (the ones with the 'A' setting on the aperture ring) perfectly well, these are the ones to go for.

Ken

A bit off topic (sorry ) but doesn't that depend on the lens? What I mean is the A series 135mm got very bad reviews whereas the K series f/2.5 got very nice reviews which makes it worth having to push the little green button. Obviously, the way to go is to buy a DA* 50-135mm but that is the difference between 80 quid or so and 500 I just undermined my argument by having both

Cheers, Kris.

When I fit a K mount lens to my K mount camera, it doesn't stay stopped down, it opens wide until the shutter is released (or DOF Preview or stop-down meter operated).

I don't know if that's what you were asking about, Lars, or something else.

Hi guys,

Thanks for the replies.

I should have been more clear. I meant when using a K mount camera. I just purchased a K200 and plan on picking up 1 or 2 others like a Spotmatic and a K1000. All my stuff is M42 now and the reason I bought the K200 was everyone said how well it handled the old lenses. I'm very impressed with it so far. Every piece of equipment I have has fit and worked perfectly. I bought the kit lens(18-55II), it's ok, but not what I'm looking for. The camera is always in manual mode and I don't mind pushing buttons. I don't like buying body specific lenses, they need to fit a range of cameras that's why I was asking about how they work. I think I get it now. You also answered my next question about the DSLR's.

Thanks guys!

On an M42 lens the internal spring mechanism holds the iris open, to close it down you must pull against the spring in some way.

On a K-mount lens the internal spring mechanism holds the iris shut, to open it up you must pull against the spring in some way.

An M42 lens (in auto mode) remains open until the camera places pressure on the internal spring to close it. Modern cameras do not have the flip-up plate to operate the auto "pin" and hence they must be used in manual mode on a DSLR (where the Auto/Manual switch places the pressure instead of the camera). A cam system linked to the aperture ring determines how far the lens will close. After capture the lens will again tension the spring to keep the iris open.

A K-mount lens is opened up by the act of putting it on a camera; a lever is engaged by the camera mount which acts *against* the spring. In order to close the lens down (for metering and shooting) the pressure on the spring is relaxed. A cam system linked to the aperture ring determines how far the lens will close. After capture the camera will again tension the spring to keep the iris open.

So in answer to your original question, yes they are almost complete opposites. Conversion between one method of working and the other would be possible, but neither easy nor economical as it would require bespoke fabrication of almost the entire inner workings of the lens (minus the glass). If you possess a watchmaker's lathe, the patience of a saint and a hundred or so spare hours you could give it a go - but I don't think the results would justify the trouble.

M42 lenses are easy to use on a K-mount camera so I'd advise leaving them as they are and enjoying them the way they were intended.

Hey,

. No, I don't have the patience of a saint, nor do I have the eyesight required. I realized the eyesight part when all the damn cameras and lenses wouldn't focus like they used to! Well, the other eye is still good for now.

Yeah, it was the lever that I needed to understand. Thanks for the detail.

Don't worry, I would never wreck anything old unless it was broken already.

Thanks again.

Thankfully there's no such thing as a body specific lens in the Pentax system, unless you count the System 10 lenses for the Auto 110. The SMC-AF 35-70 F2.8 could be called body specific but even that will work on cameras other than the ME-F it was intended for, just not auto focus.

Even the DA's will mount and fire quite happily on anything with P, Tv or body centric aperture control (so almost anything after the A series bodies to some extent or another), although there will be a certain amount of vignetting with most of them in at least part of the range.

I have used an FA-80-320 (circa 1999) on a K1000 (circa 1975) on a number of occasions with very pleasing results.