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07-08-2011, 01:28 PM   #1
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'A' switch in Manual lenses

I have not used any manual lenses thus far, hence this question. What is the function of the 'A' switch in these lenses as I see in some manual lens reviews people have listed 'absence of 'A' switch' as a minus or con.

So please tell me what i can do with the 'A' switch in manual lens or how it's absence in any particular lens will restrict my usage.

thanks

07-08-2011, 01:32 PM   #2
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If they have the A setting then they fully support auto-exposure (just like a modern lens), but if they don't, then you have to take additional steps to meter, and you won't be able to use P mode. See my article on manual Pentax metering for details.
07-08-2011, 01:39 PM   #3
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thanks Adam!
07-08-2011, 02:16 PM   #4
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QuoteOriginally posted by Adam Quote
If they have the A setting then they fully support auto-exposure (just like a modern lens...
Unless, of course, the reference is to the "A" (a.k.a. "A/M") switch on most M42 screw mount lenses. In that case, the "A" position enables the automatic aperture pin to stop the lens down at exposure time. The "M" position puts the aperture in manual mode where the set aperture is the actual aperture.

Lack of the "A" switch on an M42 lens means that the lens can only be shot wide open on a K-mount camera. The only exceptions are if the lens has been surgically modified to disable the automatic aperture or if a special ($$) adapter is used that depresses the pin when mounted to the lens.


Steve

07-08-2011, 02:51 PM - 2 Likes   #5
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It's like this: M42 screwmount lenses are not like Pentax PK bayonet mount lenses. I haven't seen PK lenses with M/A switches, which select either Manual or Automatic aperture operation. M42 lenses, and some other bayonet mounts like Fujica-X and Ricoh, may or may not have M/A switches.

There are a few types of aperture control for M42 lenses. There are Preset lenses, with no aperture-control pin on the lens base, and Auto lenses, which have such pins. The lens types:

1) One-ring preset. You twist the ring. The aperture closes down to the desired F-stop. You meter and shoot. Nice and simple.

2) Two-ring preset. You set one ring to the desired aperture. You twist the other ring to wide-open, frame your shot with full light, then twist the second ring closed, which is at the desired F-stop, and you meter and shoot. Composing shots is a bit easier in dimmer light.

3) M/A switch. Set the desired aperture. With the switch at M(anual), it acts just like a 1-ring preset. With the switch a A(uto), you can frame a shot wide-open. Then when you press the shutter, the camera stops-down the aperture, meters, and shoots. The stopping-down happens when the aperture pin on the camera base is pressed. Very convenient. But see the WARNING below.

4) Auto only. The lens acts as if it had an M/A switch set to A(uto). But see the WARNING.

WARNING: An M42 Auto lens will ONLY work with stop-down automation on an M42 camera that supports it. (I have heard of, but not seen, an expensive M42-PK adapter with a rim to hold the pin in.) If you mount on your dSLR an Auto-only lens, or a lens with an M/A switch set to A, the lens will ONLY shoot wide-open, no matter what mode the camera is set to.

Some Fujica and Meyer-Pentacon and other M42 lenses are Auto-only, with neither presets nor an M/A switch. The only way to get these lenses to stop down is to alter the pin. Some people remove the pin, but this lowers the lens value and may not be easy nor even possible. Some people glue-down the pin, but this also reduces value if the glue can't be removed later. A bit of metal tape may be enough to hold the pin down, but some lenses have springs that are too strong for this.

So, Auto-only M42 lenses can be trouble. If you have M42 lenses with an M/A switch, or with no pin, then there is no problem. Good luck!
07-08-2011, 06:09 PM   #6
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QuoteOriginally posted by RioRico Quote
It's like this: M42 screwmount lenses are not like Pentax PK bayonet mount lenses. I haven't seen PK lenses with M/A switches, which select either Manual or Automatic aperture operation. M42 lenses, and some other bayonet mounts like Fujica-X and Ricoh, may or may not have M/A switches.

There are a few types of aperture control for M42 lenses. There are Preset lenses, with no aperture-control pin on the lens base, and Auto lenses, which have such pins. The lens types:

1) One-ring preset. You twist the ring. The aperture closes down to the desired F-stop. You meter and shoot. Nice and simple.

2) Two-ring preset. You set one ring to the desired aperture. You twist the other ring to wide-open, frame your shot with full light, then twist the second ring closed, which is at the desired F-stop, and you meter and shoot. Composing shots is a bit easier in dimmer light.

3) M/A switch. Set the desired aperture. With the switch at M(anual), it acts just like a 1-ring preset. With the switch a A(uto), you can frame a shot wide-open. Then when you press the shutter, the camera stops-down the aperture, meters, and shoots. The stopping-down happens when the aperture pin on the camera base is pressed. Very convenient. But see the WARNING below.

4) Auto only. The lens acts as if it had an M/A switch set to A(uto). But see the WARNING.

WARNING: An M42 Auto lens will ONLY work with stop-down automation on an M42 camera that supports it. (I have heard of, but not seen, an expensive M42-PK adapter with a rim to hold the pin in.) If you mount on your dSLR an Auto-only lens, or a lens with an M/A switch set to A, the lens will ONLY shoot wide-open, no matter what mode the camera is set to.

Some Fujica and Meyer-Pentacon and other M42 lenses are Auto-only, with neither presets nor an M/A switch. The only way to get these lenses to stop down is to alter the pin. Some people remove the pin, but this lowers the lens value and may not be easy nor even possible. Some people glue-down the pin, but this also reduces value if the glue can't be removed later. A bit of metal tape may be enough to hold the pin down, but some lenses have springs that are too strong for this.

So, Auto-only M42 lenses can be trouble. If you have M42 lenses with an M/A switch, or with no pin, then there is no problem. Good luck!
+1 to you for that detailed reply - that answered the question in my mind clearly!
07-09-2011, 12:08 AM   #7
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Except there are exceptions to the rule apparently. I have a Vivitar 75-300MM K mount that has the "A" but that doesn't work just like an auto lens. Curious to see if it was just my copy I looked it up and apparently it just doesn't, period. It in no way acts like a true modern auto lens. It's a great lens, optically speaking I like it, but I really wish it had worked just like an auto lens. I do a lot of birding with that one and it would have come in handy.
07-09-2011, 08:50 AM   #8
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QuoteOriginally posted by magkelly Quote
Except there are exceptions to the rule apparently. I have a Vivitar 75-300MM K mount that has the "A" but that doesn't work just like an auto lens.
Like I said, I haven't seen PK-mount glass with M/A switches, but Vivitar may have sold lenses for multiple mounts that include switches with varying degrees of functionality. (How is THAT for blather, eh?) And of course an M/A lens isn't the same as PK-A with the mount contacts. Is your Viv a native PK-mount, or with a TX/T4 or other adapter? What happens when you flick the switch?

07-09-2011, 09:42 AM   #9
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QuoteOriginally posted by RioRico Quote
[...]when you press the shutter, the camera stops-down the aperture, meters, and shoots.[...]
RioRico, I know of no camera that meters after shutter is pressed. Spot F & ES bodies meter with lens wide open, adjusting according to aperture set on lens, communicated to the body by lever.
07-09-2011, 10:11 AM   #10
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QuoteOriginally posted by rhodopsin Quote
RioRico, I know of no camera that meters after shutter is pressed.
My M42 Argus-Chinon (also branded an Agfa and others) CR-3E does exactly that. I believe the Spotmatic and all other TTL-metering SLRs in the days before onboard computers took that approach. A calculation can be done only if the camera (thinks it) knows the aperture, ie if the aperture information is transmitted as with PK-A contacts. Such just doesn't exist with standard M42 lenses. TTL metering can only happen when the shutter button is pressed and the iris stops down.

On another thread is discussion of varied 'efficiency' of Pentax vs Zeiss lenses. We know that T-stops (light actually transmitted) and f-stops (ration of aperture to focal length) are not the same due to 'efficiency': coatings, design, etc. So a metering system that reads wide-open, then computes for stopped-down setting, will give different results with lenses of different transmission rates. For metering to be accurate, it MUST read the light that is actually transmitted.
07-09-2011, 10:23 PM   #11
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QuoteOriginally posted by magkelly Quote
Except there are exceptions to the rule apparently. I have a Vivitar 75-300MM K mount that has the "A" but that doesn't work just like an auto lens. Curious to see if it was just my copy I looked it up and apparently it just doesn't, period. It in no way acts like a true modern auto lens. It's a great lens, optically speaking I like it, but I really wish it had worked just like an auto lens. I do a lot of birding with that one and it would have come in handy.
I'm wondering if you're referring to "A" on the aperture ring, in which case the lens ought to have been able to operate just like a modern lens (but without the AF). Interesting that it doesn't - maybe someone has been "doctoring" it...
07-10-2011, 03:17 AM   #12
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QuoteOriginally posted by Adam Quote
If they have the A setting then they fully support auto-exposure (just like a modern lens), but if they don't, then you have to take additional steps to meter, and you won't be able to use P mode. See my article on manual Pentax metering for details.
Also worth mentioning is that aperture data is saved in the EXIF for future reference.

edit - typically for Pentax lenses the A setting doubles the price. I'm undecided if it is actually worth it. It's nice to have but think of how many more lenses you can buy. The slippery slope...

Last edited by abacus07; 07-10-2011 at 03:24 AM.
07-10-2011, 08:12 AM   #13
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QuoteOriginally posted by abacus07 Quote
Also worth mentioning is that aperture data is saved in the EXIF for future reference.

edit - typically for Pentax lenses the A setting doubles the price.
I think you may be talking about A-type lenses, not lenses with A/M switches. The A setting on an aperture ring is quite distinct from an A switch. Yes, A-type Pentax lenses (PKA) are mostly valued more than M-type (PKM) or screwmount (M42) -- and it's the M42's that mostly have the M/A switches. But some PKA's are thought LESS of than their corresponding M and K kin, as many A's are more plastic. And decent A-type lenses from other makers (like Ricoh) may still be inexpensive.

So I need to ask Sany the OP: When you asked about the "A-switch", did you mean the A/M switch on older lenses, or the aperture ring A-setting on newer lenses?
07-10-2011, 09:07 AM   #14
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QuoteOriginally posted by RioRico Quote
So I need to ask Sany the OP: When you asked about the "A-switch", did you mean the A/M switch on older lenses, or the aperture ring A-setting on newer lenses?
I was guessing the OP meant A setting (referencing aperture) but I actually misread Adam's post. And I know just how confusing all of the different levels of the lenses are since I'm a relative newb to this world.
07-12-2011, 02:31 AM   #15
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That's right it must be the aperture - because I saw this in one of the 'M' lens reviews when one of it goes like<i><font color="red"> '.. this lens has neither <b>A-functionality</b>, nor autofocus - but when you buy a purely manual lens,...' also one more review in pentax M100 f4 where under cons somebody has mentioned 'no A setting'</font></i><br />
<br />
<font color="black">I was browsing the review for buying the manual lenses and was concerned what it was.</font><br />
<br />
<font color="#000000">thanks all for taking time to help!</font>

Last edited by sany; 07-12-2011 at 03:15 AM.
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