Using a M42 lens on a Pentax K camera...

[ChrisR]

OK I'm confused. I bought a M42 zoom lens for my two Pentaxes (ME and MX), and splashed out on the proper Pentax M42 adapter, allowing infinity focus. Actually it seems to focus a bit past infinity which is kind of odd. But that's not my question...

Today as it was a nice(ish) day I took ALL my Pentax lenses and a tripod out near the Castle and tried them all (on HP5 film). It'll be interesting to see the results. The last was this M42 zoom, which I've never used before.

I know that the "wide-open meter, automatically stop down on shoot" thingy wouldn't work with the new lens. I was expecting to have to compose and focus with the lens wide open, then stop it down to meter and shoot. However, the lens appeared to stay wide open throughout, and moving the aperture ring had no effect on the (over) exposure. I set the aperture from the previous shots and took my shots anyway (perhaps I should have adjusted the speed from 1/500 to 1/1000, but I guess it wouldn't have made that much difference).

When I got back I looked more closely at the lens, thinking there might be some sort of "stop down" button, but nothing. I took the lens off the camera, and I could see that the aperture stays open unless the pin in the lens base is pressed. But there's nothing in the lens mount or the MX that will press on that pin. So I'm kind of stumped as to how the aperture is ever going to close on the K mount adapter.

Now either this lens is of an unusual design or I'm doing something stupid. I'm always inclined to expect the latter. Any ideas what I've done?

 

[freecom2]

Just had a quick look - the order is:

Focus and frame at the largest aperture setting
Select aperture (which darkens the screen, depending on how small your f/stop is)
Adjust the shutter speed accordingly
Take shot

This relies on something that was patchy on M42 lenses - M/A switches. You should be able to make it out in this picture:

With 3rd party M42 lenses it was a bit hit and miss whether it was included.

 

[s162216]

As above the lens needs to have an Auto and Manual switch to stop down the aperture, later ones tended to not include it and only had the auto mode because the function was included on nearly every M42 camera and most provided the ability to stop down using the depth of field preview.

You can just use Aperture Priority (theres no manual mode on the ME anyway) with the lens stopped down although obviously the viewfinder will be dark, so focus wide open and then change the aperture to take the shot.

 

[ChrisR]

As above the lens needs to have an Auto and Manual switch to stop down the aperture, later ones tended to not include it and only had the auto mode because the function was included on nearly every M42 camera and most provided the ability to stop down using the depth of field preview.

No sign of any Auto/Manual switch on the lens.

You can just use Aperture Priority (theres no manual mode on the ME anyway) with the lens stopped down although obviously the viewfinder will be dark, so focus wide open and then change the aperture to take the shot.

That's what I expected to happen, but when I adjusted the aperture (to f/11), the viewfinder did NOT get dark; nothing changed at all. As above, from later inspection this is because the aperture pin on the lens mount was not pressed in. Can't work out why, or what should have pressed it.:bang:

(I was actually using the MX in this case.)

 

[dizidav]

None of the 'K' mount cameras have the ability to press the pin, you will have to modify the lens, to get the aperture ring to work.

 

[s162216]

None of the 'K' mount cameras have the ability to press the pin, you will have to modify the lens, to get the aperture ring to work.

Or preferentially get an adapter that presses it in for you, but unfortunately you can't use them with SMC Takumar lenses as the aperture lug gets caught on the adapter.

Just get an M42 lens that has an A/M switch so that the lens is stopped down when switched to 'M', theres plenty of them around quite cheaply, unless you use an adapter that presses the pin in or you keep the pin stuck in (which will likely make the auto mechanism not work ever again) you can't use an auto only lens on a K mount camera.

 

[ChrisR]

Yes, I'm gradually coming round to the realisation that the problem is with the lens, which will have to go. I think I'll hang on to the adapter, on the possibility of getting another lens, but next time I'll be more careful!

 

[excalibur2]

With no M or A switch, you either have to glue the pin in or use a M42 camera.....but there still could be a problem as for example, the very good M42 Meyer 50mm f1.8 lens that used to go for peanuts (maybe it still does) is a damn nuisance as it has the M or A switch but it's spring loaded so if I want to use it on say my Canon with adapter for taking a shot or taking an exposure reading, I have to either hold it down with a third hand or stick it down with some tape.