Topic: K1000 & stop down metering?

[Jack Johnson]

I picked up a K1000 cheap, and decided it would be fun to put M42 glass on it, so I spent a few more dollars and got a decent quality (but non-Pentax) M42-to-Pentax-K adapter. It fits well, but after taking some meter readings I thought hey, that can't be right, and spent the next hour swapping back and forth between K and M42 lenses. The result: they don't match. Meter is near-perfect with K-mount, several unpredictable stops off with M42. Bummer!

So, out goes the call to the universe. Has anyone else seen where stop down metering on K-mount Pentaxes doesn't behave as expected? Is there a trick? I might just sunny 16 it, but I was hopeful there for a bit.

[Flippy]

Are you putting the M42 lens into manual mode before metering? That is the only obvious problem I can think of.

[SLVR]

x2. I found with my nikon F > M42 adapter that the auto function of the lens didn't work. You have to put the lens into manual then select your aperture then meter. It's kind of a pain in the ass really.

Focus
stop down
meter
shoot

[Francois]

On my Nikon when I use the adapter I have to focus with the lens wide open. Then I turn the exposure ring to close the lens.
On the bodies that use a match need or something similar, I have to make the meter level and I have the proper exposure.

But I know that some cameras need the interlink between the lens and body to meter correctly.

But I have a lens that also doesn't have a manual mode... I had to take it apart and jam the aperture pin down.

Thing is the m42 lenses use a lever that pushes a plunger pin to close down the aperture. None of the adapters out there have one.

[Bryan]

I just ran into this problem when I tried to use a M42 lens on my Zenit-B.  Even though the Zenit-B has an M42 mount it doesn't push the pin in, I need to figure out how to secure the pin to use it.  I don't want to damage the pin, I would like to still be able to use it on other cameras.

[Jack Johnson]

I'm putting the lens in manual mode (I usually keep them that way, I tend to enjoy the DoF preview), and the lens is stopping down, it just doesn't register the same light levels as the K-mount lenses at the same aperture, or my hand-held meter.

At first I thought I might be able to fiddle with the film speed once and get by, but a full stop on the M42 lens is sometimes only about a third of a stop on the match needle. The same lens on an M42 body with a working match needle works fine, so I was chalking it up to an oddity with the K1000, but it would be nice to hear if anyone else has seen the issue.

[Francois]

I think that what you've actually been seeing is the difference between F/stops and T/stops.

The F/stop is a mathematical function based on the lens size and the lens focal length.
The T/stop is a meter verified value that indicates the amount of light that is actually going through a lens.

F Stops vs T Stops - what is the difference?

[Kayos]

Bryan, I have an M42 lens that has an auto/manual switch, seems something like that would be ideal on a Zenit B, as when you switch to manual it closes to wherever the ring is set

[Francois]

For the lens I took apart and modified, I noticed that there was a small lever inside the lens that would move to close the aperture when I pressed the pin. So I took the mount apart and made a small block of wood with a small hole in it that I fit over the pin to keep the lever permanently pressed. It took a bit of adjusting with fine sandpaper to get it to the proper thickness but it worked. I also didn't have to cut my nails for a few weeks 

I still have to put it to the test... it's an old odd-brand 135mm lens.

[johnha]

The metering in the K1000 is quite crude (Cds instead of GPD) even though Pentax had been using GPD cells in earlier, higher speced cameras. As such it reacts slower and, in lower light, is much less sensitive than might be expected. It is also possible to get it to 'wrap-around' where very low light levels cranks the needle to the top temporarily. The metering is also 'averaged' across the whole frame (i.e. not centre-weighted).

The camera meters with the aperture open when using K lenses. I suspect when stopping down, depending on aperture, the light levels seen by the Cds cell may drop too low for accurate (read: 'consistent') metering.

I'd meter at a wide aperture and apply a correction for the shooting aperture - this is what the body usually does with K lenses. I'd try mine with an M42 lens - but it's dark at the moment...

[Jack Johnson]

I think that what you've actually been seeing is the difference between F/stops and T/stops.

That's interesting. Based on the needle movement, I'm not sure that's the whole picture, but it certainly could be part of it.

I had no idea about T/stops!

I'd meter at a wide aperture and apply a correction for the shooting aperture - this is what the body usually does with K lenses. I'd try mine with an M42 lens - but it's dark at the moment...

I'm going to try that.

I had intended on leaving the adapter on, and taking it off over and over again got old really quick, so I'll probably wait until the weekend.

In the interim, I picked up a SMC Pentax-M 50/1.7 for $12, so I'll be happy for a day or two. I originally went hunting for the SMC Pentax-M 40/2.8 (even though it's reported as lackluster, I'd like to try 40mm on an SLR), but at that price I couldn't pass it up.

There should be a 12-step program for GAS.

[Pete_R]

It is also possible to get it to 'wrap-around' where very low light levels cranks the needle to the top temporarily.

That's a feature of the camera and indicates the light level is outside the metering range.

The issue of using manual aperture lenses on cameras intended for open aperture metering is that, without the coupling between lens and camera, the camera will set a couple of default values. One is the set aperture of the lens and the other is the maximum aperture of the lens. If both these values are defaulted to the same value, then you should get an accurate meter reading as the camera will assume you are using the lens at maximum aperture and no offset will be applied. But if they are different, then the camera will always believe you are using the lens at something less than maximum aperture and adjust the reading accordingly and will, therefore, be wrong.

I just tried this by putting an m42 lens on my Contax RXII and the camera consistently underexposes by one stop.

Having said all that, I would expect a K mount camera to expose correctly because the lens only tells the camera how much it is stopped down, not the actual f stop being used. So with no coupling I would expect the camera to assume the lens is wide open. It would be worth checking the adapter you are using to see if it moves the aperture lever on the camera when it's installed. This would give you an error. It's the lever labelled 'stop down coupler' in the pic below.

[johnha]

In the interim, I picked up a SMC Pentax-M 50/1.7 for $12, so I'll be happy for a day or two. I originally went hunting for the SMC Pentax-M 40/2.8 (even though it's reported as lackluster, I'd like to try 40mm on an SLR), but at that price I couldn't pass it up.

There should be a 12-step program for GAS.

The M50/1.7 is a very good lens (especially for $12), in some ways it's better than the f/1.4. There's nothing wrong with the M40/2.8, it's a bit slow as a standard lens in dimmer conditions and being a pancake, the aperture and focus rings are quite shallow. On an MX it makes the camera almost pocketable (it only protrudes 18mm from the lens flange).

As far as GAS goes, the M40/2.8 was my preferred standard lens - until I bought an FA 43mm f/1.9 - expensive but for me, the ideal standard lens.