M - A switch on the M42 lenses

littledrawe · 12-19-2010, 01:20 AM

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

Fogel70 · 12-19-2010, 02:24 AM

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.

RioRico · 12-19-2010, 03:23 AM - **1 Like**

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.

littledrawe · 12-19-2010, 11:23 AM

Thanks for the info! RioRico you've been a wealth of knowledge the past couple days its very helpful!

noblepa · 12-19-2010, 12:20 PM - **1 Like**

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.

RioRico · 12-19-2010, 01:17 PM - **1 Like**

Originally posted by littledrawe

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.

cardinal43 · 12-22-2010, 10:29 AM

Originally posted by noblepa

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?

**boriscleto** · 12-22-2010, 10:34 AM

Originally posted by cardinal43

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.

RioRico · 12-22-2010, 10:36 AM

Originally posted by cardinal43

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!

Ira · 12-22-2010, 12:58 PM

Originally posted by cardinal43

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.

cardinal43 · 12-23-2010, 02:05 PM

Thank you gentlemen for your replies. Now, let's see if I can take some pics.

redcat · 11-13-2014, 09:19 AM

Originally posted by RioRico

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

Na Horuk · 11-15-2014, 07:41 AM

just make sure the aperture pin is inside the lens when you mount it. It can bump into the af screw otherwise

stevebrot · 11-15-2014, 09:30 AM

Originally posted by Na Horuk

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

**Lowell Goudge** · 11-15-2014, 10:15 AM

Originally posted by stevebrot

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

12-19-2010, 01:20 AM	#1
littledrawe Veteran Member Join Date: Jan 2010 Location: Red Rock Photos: Gallery \| Albums Posts: 2,514	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?

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12-19-2010, 02:24 AM	#2
Fogel70 Pentaxian Join Date: Sep 2009 Location: Stockholm, Sweden Photos: Albums Posts: 3,062	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 - 1 Like	#3
RioRico Veteran Member Join Date: Jun 2008 Location: Limbo, California Posts: 11,263	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
littledrawe Veteran Member Join Date: Jan 2010 Location: Red Rock Photos: Gallery \| Albums Posts: 2,514 Original Poster	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 - 1 Like	#5
noblepa Veteran Member Join Date: Sep 2007 Location: Bay Village, Ohio USA Posts: 1,142	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
RioRico Veteran Member Join Date: Jun 2008 Location: Limbo, California Posts: 11,263	Originally posted by littledrawe 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
cardinal43 Veteran Member Join Date: Oct 2007 Location: Virginia Photos: Gallery Posts: 2,412	Originally posted by noblepa 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
boriscleto Loyal Site Supporter Join Date: Dec 2009 Location: North Syracuse, NY Photos: Gallery Posts: 16,475	Originally posted by cardinal43 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
RioRico Veteran Member Join Date: Jun 2008 Location: Limbo, California Posts: 11,263	Originally posted by cardinal43 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
Ira Inactive Account Join Date: Mar 2009 Location: Coral Springs, FL Photos: Gallery \| Albums Posts: 5,216	Originally posted by cardinal43 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
cardinal43 Veteran Member Join Date: Oct 2007 Location: Virginia Photos: Gallery Posts: 2,412	Thank you gentlemen for your replies. Now, let's see if I can take some pics.

11-13-2014, 09:19 AM	#12
redcat Veteran Member Join Date: Jul 2014 Location: Paris Photos: Gallery Posts: 1,939	Originally posted by RioRico 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
Na Horuk Veteran Member Join Date: Feb 2012 Location: Slovenia, probably Photos: Gallery \| Albums Posts: 11,186	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
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by Na Horuk 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
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,888	Originally posted by stevebrot 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