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02-11-2012, 09:01 PM   #1
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Understanding M42 lenses

I've been trying to understand old lenses and how they can be used with cameras like the K-5. To that extent I've read Adam's excellent article on the topic, which can be found in a few places on the forums, such as https://www.pentaxforums.com/forums/pentax-beginners-corner-q/110658-using-ma...x-dslrs-f.html.

The guide mentions:
[Screwmount lenses only] Switch the diaphragm clutch on your lens to "Manual" (you can leave it on Auto when composing and focusing).

On the one hand this makes it seem like M42 lenses offer more flexibility than K lenses which can only operate in manual. But I'm left with more basic questions since I don't come from a Pentax background. What is the diaphragm clutch? What is the difference between having the clutch in Auto and Manual, and do all M42 lenses have a clutch?

Marc

02-11-2012, 09:09 PM   #2
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From the M42 Wikipedia article.

QuoteQuote:
The first lenses were plain stop-down design but many manufacturers extended the M42 lens mount to provide extra features. The first innovation was the pre-select type, which allowed an aperture value to be pre-selected without actually closing the aperture, with a separate ring to close down the aperture quickly to the chosen value. This gave the user the benefit of comfortable framing and focusing with a bright viewfinder and clear focus separation, and then closing the aperture without the need to remove the eye from the eyepiece. A further development followed with "auto" lenses, which have a pin in the mount which closes the aperture against a spring to the chosen setting when it is pushed. This was adopted as a common standard by virtually all lens manufacturers. Cameras designed for these lenses have a bar in the bottom of the mount which depresses the pin when the shutter is released. The first cameras, such as the Praktica Nova range, used physical finger pressure on the shutter button to operate the bar and close the aperture, allowing a stopped-down preview of the depth of field before the shutter fired. However, this function was removed in later Praktica models because some users found it was possible, with longer exposure times, to release the shutter button and open the aperture before the shutter had closed. The bar on Pentax Spotmatic cameras is operated by spring pressure with timing linked to the shutter, but these cameras also had a separate switch for the light meter circuit which closed the aperture and gave the depth of field preview in this way. To allow auto lenses to be used on earlier cameras without the bar, many lenses were provided with a switch or button to put the lens into stop-down mode, commonly referred to as the Auto-Manual or A/M switch.
M42 lens mount - Wikipedia, the free encyclopedia

The diaphragm clutch is also known as the A/M or Auto-Manual switch. Some M42 lenses are auto only and do not have the switch. These lenses have to be modified or used wide open only. Early M42 lenses are manual only (Pre-set). There are also a few semi-auto lenses out there like the early Auto-Takumars.
02-11-2012, 09:22 PM   #3
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The history isn't relevant for use on modern DSLRs
(except to know that the "auto-only" lenses are not convenient for DSLR use).
M42 lenses that are not "auto-only" all have an equivalent
of the so-called "diaphragm clutch" in one form or another,
although there are lots of variants of the actual mechanism
-- RioRico attempted a classification of them in a recent thread.

On modern DSLRs, you set the lens' aperture with its aperture ring.
With the "clutch in Auto," or whatever, the lens is wide open - good for focussing.
Then if you've chosen an aperture narrower than fully open,
you need to "switch to Manual" to stop the lens down to that aperture
for metering (maybe Av) and taking the photograph.
02-11-2012, 09:29 PM   #4
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Can you typically switch the aperture in manual? If you only wanted to shift by one stop, it would seem tedious if you had to go back to auto first...?

02-11-2012, 09:32 PM   #5
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You can switch whenever you like.
02-11-2012, 09:36 PM   #6
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There is no need to switch back and forth at all. It's really best to put the A/M switch on manual and ignore it. The downside to leaving it in manual is that the viewfinder dims when you stop down. This is why some people use the A/M switch to stop down. You can set the aperture and focus with the lens wide open and then switch to m and the lens stops down.
02-11-2012, 09:37 PM   #7
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In auto mode, the lens doesn't actually stop down when you move the aperture ring as the diaphragm is controlled by a pin which only M42 cameras can trigger. Thus, you basically need to stay in manual mode when shooting with an M42 camera, or you can keep it on auto until it's time to meter if you want to keep your VF bright.

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02-11-2012, 10:43 PM   #8
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The K-5 can't control the aperture on an M42 lens. You can use the M mode or Av mode where metering is done in real time. You control the aperture with the ring, and as mentioned, if you choose a smaller aperture the viewfinder will be dark making it difficult to compose and focus. So your shooting would go like this in Av mode:

Set aperture to maximum. Compose and focus. Set your aperture as desired. The shutter speed will adjust based on the metering. You can adjust the iso with one of the wheels to change the shutter speed.

Presumably some lenses can be set to wide open by the auto-manual switch. The one I have can't, it requires depressing a pin.

It works pretty well.

02-11-2012, 11:35 PM - 3 Likes   #9
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I don't know if it is clear from the above comments, but the word Auto means automatic aperture stop-down at exposure time. It does not mean support for exposure automation or programmed exposure automation (the "A" position on some K-mount lenses). M42 lenses come in four flavors:
  • Manual aperture -- the iris opening is always coupled with the aperture ring
  • Preset aperture -- the iris opening is full open unless the preset ring or switch is moved to the stop-down position
  • Semi-auto aperture -- The iris is open until a pin at the rear of the lens is depressed, at which point it stops down to the position set by the aperture ring. The iris is reopened with a lever to be ready for the next exposure.
  • Auto aperture -- Like semi-auto except that the iris reopens by itself after the exposure
Auto and semi-auto aperture lenses often featured prominent labeling (e.g. the Auto-Rikenon 50/1.7 that came with my Singlex TLS). It was a huge convenience factor and a selling point back-in-the-day and by 1970 most lenses on the market featured auto-aperture. Almost all K-mount lenses have automatic apertures as do most M42 Asahi Takumars. A feature on most "Auto" M42 lenses is a switch or slider near the mount on the side of the lens that disengages the auto-aperture mechanism and puts the lens iris in fully manual mode. It is that switch that allows forward compatibility for adaptation to K-mount bodies.

The A/M switch must be in the "M" (manual) position at the time of exposure in order to use the lens other than wide open on a K-mount body. Some M42 lenses lack the A/M switch (e.g. Helios-44M-4) and require a little surgical modification to make them adaptable to K-mount. That is, unless you have one a very few very expensive adapters that have a flange to depress the aperture actuation pin. Metering and exposure mode support varies by camera body with a general rule that shutter-priority and program exposure automation are never supported.

Are we confused yet?

Here is a quick cheat sheet to determine if a lens will work on your K-mount body:
  • No silver pin on the base? The lens will work.
  • Silver pin and A/M switch? The lens will work.
  • Silver pin and no A/M switch? The lens will not work without modification or special adapter.
Note that there are still a few incompatibilities based on physical projections (some later Mamiya/Sekor) or non-standard dimensions (some Helios-44-3), but for the most part the three bullet points are pretty dependable.


Steve

BTW...countrary to the advice in a post above, Av mode is not recommended for stop-down metering on the K-5. M-mode with green button will provide consistent stop-down meter accuracy with M42, Pentax-K, and Pentax-M series lenses. Av mode generally will not. Why this is so is a long story and is discussed elsewhere on this site.

Last edited by stevebrot; 02-11-2012 at 11:52 PM.
02-12-2012, 09:21 AM   #10
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QuoteOriginally posted by stevebrot Quote
........

Here is a quick cheat sheet to determine if a lens will work on your K-mount body:
  • No silver pin on the base? The lens will work.
  • Silver pin and A/M switch? The lens will work.
  • Silver pin and no A/M switch? The lens will not work without modification or special adapter.
...

Steve
Great summary - which can be summarized further:

The lens will work as-is unless it has a silver pin and no A/M switch.

Old Dave
02-12-2012, 10:08 AM   #11
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QuoteOriginally posted by stevebrot Quote
M42 lenses come in four flavors:
  • Manual aperture -- the iris opening is always coupled with the aperture ring
  • Preset aperture -- the iris opening is full open unless the preset ring or switch is moved to the stop-down position
  • Semi-auto aperture -- The iris is open until a pin at the rear of the lens is depressed, at which point it stops down to the position set by the aperture ring. The iris is reopened with a lever to be ready for the next exposure.
  • Auto aperture -- Like semi-auto except that the iris reopens by itself after the exposure
There are more "flavors" than this, like the Ludwig Meritar, a sort of Manual/Preset hybrid.
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