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

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!

Basically with rather modern cameras there are two kinds of autoexposure:

-) open aperture AE
For this there must be a two-way coupling: the body must get information on the lens speed, and which aperture has been preset (aperture priority) or the lens must know which aperture to set (shutter speed priority)

-) stopped down AE.
Here no coupling at all is necessary

Seemingly your camera did the latter (I'm ignorant on your model.)

AgX said:

-) stopped down AE.
Here no coupling at all is necessary

Seemingly your camera did the latter (I'm ignorant on your model.)

Click to expand...

Actually, it looks like the MX is doing just this properly, but the MG is not. Scene brightness obviously affects the exposure, but stopped-down f stop does not.

At least per the MG instruction manual, the camera meter should work in stop-down mode with lenses used on an M42 adapter, just as on the MX - there are a couple of pages devoted to using Super Takumars and SMC Takumars that way. Make sure the diaphragm is actually stopping down when you twist the aperture ring - by chance does the lens have an auto/manual switch that inadvertently got flipped when you transferred the lens and adapter to the MG?

Oren Grad said:

At least per the MG instruction manual, the camera meter should work in stop-down mode with lenses used on an M42 adapter, just as on the MX - there are a couple of pages devoted to using Super Takumars and SMC Takumars that way. Make sure the diaphragm is actually stopping down when you twist the aperture ring - by chance does the lens have an auto/manual switch that inadvertently got flipped when you transferred the lens and adapter to the MG?

Click to expand...

Thanks. I did get a copy of TFM and yes, it implies that this should work. I'm very sure the switch was in manual mode, as the viewfinder darkened when I stopped down.

Someone on "another network" remarked that if it changed when moving from a bright to a dimmer scene, it should also change when stopped down, as both will give less light to the sensor.

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!