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12-19-2010, 01:20 AM   #1
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M - A switch on the M42 lenses

What is the function of this switch. I see that it opens the aperature blades wide open but why?

12-19-2010, 02:24 AM   #2
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Originally the A/M switch was to support both older m42 cameras with no aperture control and newer m42 cameras with automatic aperture control.

M (manual) mode for older cameras that had no control of aperture on the lens. The lens is stopped down when aperture is set.
A (auto) mode for newer cameras with automatic stop down of lens. Then the lens would be kept wide open until shutter is released. (just like K-mount lenses on K-mount cameras)

No DSLR fully support A-mode on m42 lenses so it will only be a way of switching the lens between stopped down and wide open aperture.
12-19-2010, 03:23 AM   #3
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There are 4 basic types of M42 lenses:

1) The lens has no M/A switch and no auto-aperture pin, and a single aperture ring.
2) The lens has no M/A switch and no auto-aperture pin, and two aperture rings.
3) The lens has no M/A switch and does have an auto-aperture pin.
4) The lens has an M/A switch -- that works!

Type (1), a totally manual lens, is simple to use on a dSLR: set the aperture and shoot.

Type (2), known as a PRESET lens, lets you set one ring to a desired aperture, and the other to wide-open. You can focus on a subject wide-open, then spin the ring to stop-down for shooting.

Type (3), an auto-only lens, ONLY stops down on an M42-auto body. On your dSLR it is always wide open unless you glue or jam or remove the auto-pin, which reduces the lens' usability and value. Or buy an expensive adapter.

Type (4) lets the lens stop down automatically on an M42-auto body, or be used manually on any film or digital SLR. Very simple: set the switch to M, set the aperture, and shoot.

None of these will automatically stop down on your dSLR. I have heard of (but not tried) an expensive (over US$100) M42-PK adapter with an internal flange that holds down the auto-pin; I do not know if this allows auto-stop-down on a dSLR. On the type (3) lenses I bought a couple years ago, I superglued the auto-pins, but I find I can't un-glue them. I now avoid buying such lenses, just as I avoid lenses with apertures stuck wide open. Plenty of good manual, preset, and M/A lenses exist. The M/A switch is your friend. Cherish it.
12-19-2010, 11:23 AM   #4
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Thanks for the info! RioRico you've been a wealth of knowledge the past couple days its very helpful!

12-19-2010, 12:20 PM   #5
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The M-A switch is also used on bellows and non-auto extension tubes. An M42 lens is wide open by default, unlike a k-mount lens, in which the lens is stopped down by default. On an M42 lens, the camera must push the pin to stop the lens down. On a k-mount lens, the camera holds the aperture open. If you remove both from their respective cameras, the M42 will be at its widest aperture (if the M-A switch is set to A) regardless of the aperture ring, while the k-mount will stop down to the selected aperture.

Most bellows, and some M42 extension tubes lack the linkage to push the aperture pin. This is where the M-A switch comes in handy.

The M-A switch is also mandatory (without modifying the lens) if you want to use an M42 lens on a dslr, which lacks the mechanism to stop the lens down to the selected aperture.

I use the 50mm, f/1.4 Super Takumar from my Spotmatic on my K10D. I set the M-A switch on the lens to M, put the camera in Av mode and shoot using the stop-down metering method. In Av mode, I just focus, select the aperture I want, and the camera chooses a shutter speed. In M mode (on the camera), I select the aperture and then use the e-dial to select a shutter speed, watching the led's in the viewfinder for the "correct" exposure. Ev compensation is especially easy. If I want to add a stop, its easy to simply open up a stop from the camera's choice of "correct" exposure. Vice versa for subtracting a stop.
12-19-2010, 01:17 PM - 1 Like   #6
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QuoteOriginally posted by littledrawe Quote
Thanks for the info! RioRico you've been a wealth of knowledge the past couple days its very helpful!
Hit my rep, eh?

Ah, noblepa is quite right. Oops, I forgot about the M/A switch as a preset-enabler. [/me slaps head.] Stop-down to a desired aperture; flick the M/A to A to compose the shot wide-open; flick the M/A to M to stop-down for metering and shooting. Like I said, the M/A switch is your friend, like a loyal dog or a bribe-able tax assessor.
12-22-2010, 10:29 AM   #7
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QuoteOriginally posted by noblepa Quote
I use the 50mm, f/1.4 Super Takumar from my Spotmatic on my K10D. I set the M-A switch on the lens to M, put the camera in Av mode and shoot using the stop-down metering method.
Might be a dumb question, but does this mean you slide the switch to cover the man, leaving the auto visible, or is it the other way?
12-22-2010, 10:34 AM   #8
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QuoteOriginally posted by cardinal43 Quote
Might be a dumb question, but does this mean you slide the switch to cover the man, leaving the auto visible, or is it the other way?
It's easy to test. Set the switch and turn the aperture ring. Does the aperture close? If it does it's set to man. If you have to push the pin it's set to auto.

12-22-2010, 10:36 AM   #9
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QuoteOriginally posted by cardinal43 Quote
Might be a dumb question, but does this mean you slide the switch to cover the man, leaving the auto visible, or is it the other way?
No dumb questions, just dumb answers. Duh.

Anyway, the setting you want should be VISIBLE. So to switch to M(an), slide the switch so A(uto) is covered and M(an) is displayed. You can verify that you've done this right by setting the aperture to f/8 or thereabouts. With A(uto), the iris will stay wide open. With M(an), the iris shuts to f/8. Also, in M(an), the diaphragm linkage pin won't slide in. Cheers!
12-22-2010, 12:58 PM   #10
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QuoteOriginally posted by cardinal43 Quote
Might be a dumb question, but does this mean you slide the switch to cover the man, leaving the auto visible, or is it the other way?
This is a VERY common question, because it's so weird.

Not a stupid question at all.
12-23-2010, 02:05 PM   #11
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Thank you gentlemen for your replies. Now, let's see if I can take some pics.
11-13-2014, 09:19 AM   #12
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QuoteOriginally posted by RioRico Quote
1) The lens has no M/A switch and no auto-aperture pin, and a single aperture ring. 2) The lens has no M/A switch and no auto-aperture pin, and two aperture rings. 3) The lens has no M/A switch and does have an auto-aperture pin. 4) The lens has an M/A switch -- that works!
thanks a lot, I just got a Helios 44 and also get confused with the A/M switch, it's so useful now understanding my lens inside-out
11-15-2014, 07:41 AM   #13
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just make sure the aperture pin is inside the lens when you mount it. It can bump into the af screw otherwise
11-15-2014, 09:30 AM   #14
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QuoteOriginally posted by Na Horuk Quote
just make sure the aperture pin is inside the lens when you mount it. It can bump into the af screw otherwise
Ummmmm...The aperture actuator pin on all M42 lenses is inward of the adapter and is no where near the AF drive on any Pentax camera.


Steve
11-15-2014, 10:15 AM   #15
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QuoteOriginally posted by stevebrot Quote
Ummmmm...The aperture actuator pin on all M42 lenses is inward of the adapter and is no where near the AF drive on any Pentax camera.


Steve
I believe he is referring to a small pin on later SMC tak's which was part of the modification to provide open aperture metering on the ES II and spotmatic F bodies
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