Any Pentax MG experts? Help me understand how exactly the autoexposure works please .

[dmr]

Yes, I know the Pentax MG is a strange bird.

I recently ordered one of those K-Mount to M42 adapters, mainly intending to use it with my MX and a Vivitar 28mm lens which I really like. (It does not barrel-distort as one zoom which I have and otherwise like does when zoomed to wide.) On the MX everything behaves as expected. Yes, metering only works properly when stopped down and I expected that.

Anyway, just for grins and giggles I put the adapter and the Vivitar on the MG to see if and how it would work. I was not expecting the auto-exposure to work at all, but I first aimed at a bright scene, clicked the shutter, and it fired with what sounded like a sane speed.

Then I moved it to a darker scene, and to my surprise, the shutter was obviously slower. So, it looks like something is working. Encouraging!

However, I found that, unlike with the usual K-Mount lenses, the F stop has no effect at all on the shutter speed. The shutter is obviously the same speed whether I set it to f/2.5 or f/22, which obviously fails the sanity check.

I realize there is probably no work-around, other than using the MG in the 1/100 "dumb as a rock" mode, but I would like to know more about the "why" of the situation.

I'm wondering if anyone here knows enough about the MG to explain exactly how the aperture-priority metering works. Is there maybe a "default" f stop for lenses that are not fully coupled?

Thanks gang!

 

[johnf04]

If the lens is not coupled to the aperture simulator in the camera, you have to manually stop down the lens to get an exposure reading.

The camera manual is here: https://www.butkus.org/chinon/pentax/pentax_mg/pentax_mg.htm

 

[dmr]

If the lens is not coupled to the aperture simulator in the camera, you have to manually stop down the lens to get an exposure reading.

The camera manual is here: https://www.butkus.org/chinon/pentax/pentax_mg/pentax_mg.htm

Thanks for the link, which does have a copy of TFM.

My point is, stopping down the lens on the MG does not change the shutter speed at all. It works as expected on the MX, which is the one I intend to use with it. I'm curious on the "why" of the thing. Yes, I do understand the difference between the coupled open-wide metering (like the K1000) and the stopped-down metering (like the Spotmatic).

 

[Swift1]

My point is, stopping down the lens on the MG does not change the shutter speed at all.

Did you manually stop the lens down with the A/M switch on the lens?

 

[Dwig]

Did you manually stop the lens down with the A/M switch on the lens?

... which is not present on most Vivitar M42 lenses. In fact, most non-Takumar lenses made after the early to mid 1960s lack the function as they expected the body to have a stop-down metering function.

Some adapters will "push the pin" to force the lens to always stop down, but very few M42>K-mount adapters do.

 

[dmr]

Did you manually stop the lens down with the A/M switch on the lens?

Yes, and from reading TFM it implies that it should work. I'll give it another try when I'm back home tomorrow.

Thanks.

 

[Swift1]

Yes, and from reading TFM it implies that it should work. I'll give it another try when I'm back home tomorrow.

Thanks.

The Pentax MG should meter the same as the Pentax ME. When using lenses that aren't K mount, the MG should accurately meter whatever light is coming through the lens. As you manually stop the lens down, the meter should adjust the exposure as the light coming through the lens changes.
The meter should actually work with no lens mounted, I believe .

 

[Crazy Fedya]

You will need to jam the stop down pin somehow, causing the lens to stop down to whatever aperture value is selected.

 

[dmr]

You will need to jam the stop down pin somehow, causing the lens to stop down to whatever aperture value is selected.

This lens has a M-A switch.

Someone on "another network" noted that if it changes shutter speed by pointing to a dimmer scene, it should as well when stopped down. When I'm home tomorrow I'll test again and pay more attention to detail.

 

[dmr]

Ok, gang, a follow-up here, now that I've had time to relax, think about things, and pay more attention to detail.

Thanks again to all who responded!

Under carefully-controlled laboratory conditions, meaning my test set-up of the blank bathroom wall with the overhead and vanity lights on dimmers, the MG **DOES** indeed behave as it should.

Dimming the lights does, as expected, result in a slower shutter speed. Keeping the lights on full and stopping down does, again as TFM and everybody implied, result in a slower shutter speed.

I don't know what happened! (Going prematurely blonde?) I was kind of sitting on the floor playing with the cameras and shooting random things and listening to the shutter speed and I would SWEAR that on the MG, I stopped it down and the shutter sounded exactly the same as it did wide open. ?????? One of those great mysteries of life, I guess?

Now that I'm more confident that it's working, I'll now plan to shoot at least one roll of the new Ektachrome in the MG using auto-exposure and using the Vivitar wide for at least some of the shots.

As an aside, the K to M42 adapter is a wee bit tricky to get on and off of the cameras. It comes with a tool, kind of like a huge wind-up key with two tabs which go into two slots in the adapter.

To put the adapter on the camera, you need to get it EXACTLY in place and push against a spring thing on the left (as looking toward the lens mount) side of the mount and get the opposite slot carefully lined up with the f-stop actuator tab.

That's the easy part!

You would think it's just the reverse to get it off, huh? Wrong!

You first use the tool with the one extended tab to carefully compress that spring thingy and partially turn counter-clockwise. Then you have to pull the tool out somewhat and continue turning to get it fully out.

Oh well, I think things are looking good for that little project.

Thanks again!

 

[Swift1]

Good to hear the camera is working properly.
Do you have the genuine Pentax m42-K mount adapter?
I used to use one quite often on my Pentax K10D . I never had the tool, so I just unlatched the spring bit using a pen, and the then twisted out the adapter with my finger tips. IIRC, it was not too difficult, but it's been years since I did it so I may be remembering wrong.

 

[dmr]

Good to hear the camera is working properly.

Thanks.

Do you have the genuine Pentax m42-K mount adapter?
I used to use one quite often on my Pentax K10D . I never had the tool, so I just unlatched the spring bit using a pen, and the then twisted out the adapter with my finger tips.

No, this is a white-box no-brand generic one. However it looks exactly like the one shown in TFM except it says "M42 Pentax" instead of "Asahi Pentax" on it. It fits securely and the lens screws on smoothly.

Below is the page that discusses the adaptor in TFM. In the middle is the one I got, mounted on the MG. The bottom is a close-up of the tool that was included.

The manual says to put it on the lens before mounting to the camera. I did not try that and it may be easier that way, so I may try it.

Because of the way that spring thing works, I don't see any way to reverse the process. TFM says to unscrew the lens and use "a pointed object" to release the spring.

The tool has the extended tab on the top in the view below. It's fairly easy to push all the way in, turn once, back out, and turn the rest of the way.

 

[Oren Grad]

Happy stopped-down snapping!

Yeah, getting the adapter out of the body is a pain, sometimes literally. I have the original Pentax-branded adapter, but no tool. Usually I can get it out by poking a fingernail in by the spring and squeezing it just so, but my fingertip does take some abuse in the process.