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01-27-2012, 05:08 PM   #1
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stop-down metering with MX

I understand completely the stop-down metering principle and means with Nikon cameras, which if not getting any aperture information from the lens, the cameras measure the actual light without diaphram compensation. I now have an MX and after reading online and in the instruction book, all it explains is that stop-down metering is needed with Takumar lenses on an adapter M42 to Pentax Bayonet. OK, am planning on getting the adapter and trying it, but one question. If the body gets no diaphram position information from the lens, other than the actual light coming in from the scene thru the stopped-down iris, will its lights read accurately ? or is there some additional setting or control. My guess is not, but I'd just like some confirmation.

01-27-2012, 05:17 PM   #2
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The key is the lens: there is an auto/manual switch. On Auto, the shooting aperture is achieved by pushing in the pin on the mount- by a M42 camera. On Manual, turning the aperture ring directly affects the aperture blades, i.e. you dial in f/8 and the lens is at f/8 as you look through it. So in effect you either compose or at least focus wide open, then turn the aperture to your shooting value by hand. Not as convenient as the nikon stop down method.
01-27-2012, 06:05 PM   #3
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ok ..... the question was about the MX body, would it know there was no aperture value to compute, and just measure the light values ? Is there something in the system to tell it there is no aperture info to calculate. ?
01-27-2012, 06:28 PM   #4
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Because the M42 lens leaves the aperture control untouched, the body assumes it is at full aperture. (These bodies never know the actual aperture, just how many stops closed the K series lens would go.) So it works fine on the MX.

01-27-2012, 06:35 PM   #5
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For m42 lenses, you have to put them in manual mode. You'll then be able to use them as you would a K or M lens, except the VF will get darker as you stop down.

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01-27-2012, 06:42 PM   #6
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Actually, the M42 adapter is responsable for pushing the camera's lever for aperture detecting to the max, making it think that it has an (f/1.2, I suppose) lens attached. Or maybe it goes even further than that, going to a position reserved only for M42. I am talking about the lever that got crippled away from the later K mount. Anyway, the camera still reads and meters very accurately, and having LEDs in the viewfinder is such a nice thing compared to the plain Spotmatic. I wish I had LEDs myself.
01-27-2012, 07:39 PM   #7

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Pentax M42 lens + adapter to kmount + MX. The M42 lens must have the auto/manual switch or I understand there are M42 to Kmount adapters that push the pin in. Focus and compose the lens then turn the aperture and adjust the shutter speed to match desired exposure.

01-28-2012, 12:23 AM   #8
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Thank you for the confirming info. That's what I was hoping, that the lights in the MX would read correctly at whatever aperture was actually set on the Takumar, if the lens' switch was set to M.
01-28-2012, 06:11 AM   #9
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Exposure would be correct even if the lens were set to "A" - but the lens would stay wide open, since the K mount won't push the pin on the lens to stop it down.
05-20-2014, 02:48 PM   #10
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OK maybe the M42 adapter does something but what about other K mount lenses without aperture control? I have an old T2 mount lens with a K adapter that mounts directly on the MX but it's K bayonet does not have any lever or pin to 'talk' to the body. The iris is controlled on the lens itself (it's a preset diaphragm but that is not relevant I think in this discussion). What I see is that the MX LEDs show a "green" light corresponding to about +2 stops (overexposure). For example the "right" F stop might be around f/8 as metered with a normal Pentax-M lens, but with this T2/K lens I need to open up the iris to about f/4 to get the same green indication. Any idea what's happening here?
05-20-2014, 05:30 PM   #11
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QuoteOriginally posted by Luc More Quote
does not have any lever or pin to 'talk' to the body
But they should have a cut-out or slot with inner pin to engage the aperture follower on a non-crippled K-mount body (e.g. your MX). The follower is visible at about 1:30 on the body's mount face. In its baseline position with non-coupled lens or adapter mounted, it is shifted about 30 degrees clockwise to indicate that the lens is full open. (With stop-down metering that is where you want bias on the meter circuit.) Automatic aperture* K-mount lenses with aperture ring have a slot with a pin attached to the aperture ring such that the follower's position reflects x-number-of-stops from full open. (The follower drives a ring in the body with a strip-type variable resistor.) Without engaging that inner coupler, the meter will default to a bias equivalent to (IIRC) f/32 and metering will be essentially non-functional.

Might it be that your T-2 adapter is damaged or non-compliant?


* Most K-mount lenses having aperture rings...Different from automated aperture control where the body controls the aperture width.

BTW...this is an ancient ZOMBIE thread. In the future you might want to start a new thread and link back to older thread.

Last edited by stevebrot; 05-20-2014 at 05:58 PM.

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