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08-11-2013, 01:07 PM   #1
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How do old A lenses work mechanically?

I'm curious about this. I just acquired a Pentax A lens and have been playing around with it on a K200d. It seems to work perfectly when setting the aperture ring to "A" and controlling aperture using the camera's scroll wheel.

So a couple things I'm wondering:

- How does the camera know which aperture is being used? There do not seem to be any electrical contacts on the lens, but there is a small lever as well as a tiny push button.

- I've tried to physically see how the aperture is being changed by the camera, but I can't tell. For example: I take the lens off the camera and manually set the aperture to 22. Then I mount it, change it to 2.8, and take a picture. Then I remove the lens, expecting the aperture to be at 2.8, but it's back at 22. I can see in the photos that the DOF is changing, and the camera is changing shutter speed. Does the camera just temporarily change the aperture during the photo, and then it reverts back to the physical setting?

Again, seems to be working great. But I'm wondering how it works.

08-11-2013, 01:14 PM - 2 Likes   #2
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The way A lenses work is quite low-tech, but consistently gets the job done as you've already observed. The contacts on the lens mount only tell the camera what the maximum aperture is, and if the "A" setting is selected. Also, the aperture stop-down lever controls the aperture diaphragm in a standardized manner, such that the same amount of travel will yield the same number of stops no matter what lens is mounted. Thus, whenever you dial in an aperture, the camera can calculate exactly how far it needs to move the lever in order to get the desired aperture setting.

The "A" setting allows the stop-down lever to move from the minimum aperture to the maximum aperture, much like the F22 setting. This lets the camera select any possible aperture setting when it goes to take the shot.

The aperture mechanism is only operated when the shutter is open. At all other times when the lens is mounted, the camera forces the aperture control level all the way up to the wide-open position.

BTW, modern lenses employ the same principle That's why a DA lens can theoretically be used on an old A-era body such as the Super Program.

So what's the difference between DSLRs and film SLRs, you might ask? Well, you may have noticed that on the back of your lens, there's a flange that moves whenever you turn the aperture ring. Unlike DSLRs, film SLRs also had some hardware that could sense the position of this flange, thus being able to tell what f stop was selected on the aperture ring. That's why on film cameras, you could use the aperture ring instead of / in addition to in-camera menus to select the aperture setting. This isn't possible on digital, which means that if the aperture ring isn't set to "A", you need to stop-down meter. This is only an inconvenience when using M or K lenses, which is why so many people wish Pentax would "de-cripple" the K-mount used on DSLRs.

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08-11-2013, 01:14 PM   #3
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Actually, now that I think about this, I suppose I can answer part of my own question. Obviously, the camera can't rotate the aperture ring, so the default aperture is set by the ring. The camera moves it for the photo only, and then it returns to the aperture ring setting.

But how is the lens communicating with the camera?
08-11-2013, 01:15 PM   #4
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See here for an excellent summary of the evolution of the K mount:

Pentax Technology

08-11-2013, 01:19 PM   #5
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Adam, thanks for the explanation. That makes total sense. It may be low-tech, but it's an elegant solution!

I have some M42 lenses and a Pentax-M. This Pentax-A is really a joy to use in comparison. The auto-aperture makes a huge difference. I already love it.
08-11-2013, 01:19 PM   #6
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QuoteOriginally posted by baro-nite Quote
See here for an excellent summary of the evolution of the K mount:

Pentax Technology

Awesome. Thanks!
08-11-2013, 01:23 PM   #7
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The small lever is controlling the aperture. If you take the lens off the camera, set the aperture ring to 'A' and move the lever while watching the iris you should see it open and close.

In the 'A' position the camera will use either use the aperture setting you chose with the wheels in Av or TAv mode or if it's in an auto-aperture mode it will select the best aperture setting for the shot.

When you press the shutter button the camera will move the lever on the lens to the required position, raise the mirror and then take the shot.

The tiny push button is a lock mechanism so that you can't accidentally take the lens out of (or put it into) the auto-aperture position. You have to press and hold the button while rotating the ring to go in and out of the 'A' setting.
08-11-2013, 01:34 PM   #8
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QuoteOriginally posted by Adam Quote
The way A lenses work is quite low-tech, but consistently gets the job done as you've already observed. The contacts on the lens mount only tell the camera what the maximum aperture is, and if the "A" setting is selected. Also, the aperture stop-down lever controls the aperture diaphragm in a standardized manner, such that the same amount of travel will yield the same number of stops no matter what lens is mounted. Thus, whenever you dial in an aperture, the camera can calculate exactly how far it needs to move the lever in order to get the desired aperture setting.

The "A" setting allows the stop-down lever to move from the minimum aperture to the maximum aperture, much like the F22 setting. This lets the camera select any possible aperture setting when it goes to take the shot.

The aperture mechanism is only operated when the shutter is open. At all other times when the lens is mounted, the camera forces the aperture control level all the way up to the wide-open position.

BTW, modern lenses employ the same principle That's why a DA lens can theoretically be used on an old A-era body such as the Super Program.

So what's the difference between DSLRs and film SLRs, you might ask? Well, you may have noticed that on the back of your lens, there's a flange that moves whenever you turn the aperture ring. Unlike DSLRs, film SLRs also had some hardware that could sense the position of this flange, thus being able to tell what f stop was selected on the aperture ring. That's why on film cameras, you could use the aperture ring instead of / in addition to in-camera menus to select the aperture setting. This isn't possible on digital, which means that if the aperture ring isn't set to "A", you need to stop-down meter. This is only an inconvenience when using M or K lenses, which is why so many people wish Pentax would "de-cripple" the K-mount used on DSLRs.
So if Pentax "de-crippled" the mount, then any legacy glass essentially could be used in auto-aperture mode?

On a tangent, how do people feel about in-camera stabilization vs. in-lens, like Nikon and Canon (I think) do? Seems like in-camera is better for backwards compatibility.

08-11-2013, 02:01 PM   #9
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QuoteOriginally posted by Takumar55 Quote
So if Pentax "de-crippled" the mount, then any legacy glass essentially could be used in auto-aperture mode?
Only if it has an 'A' setting on the aperture ring otherwise it's still a manual aperture lens. The point Adam was making was that a DSLR cannot 'sense' where you have set the aperture ring so the software doesn't know what to do. When you press the 'green button' in manual mode to stop-down meter the camera moves the lever on the lens to the set aperture position so that you can take a correct light meter reading and then you can adjust the aperture/shutter speed/ISO value to get the correct exposure. It can't do the preceding process automatically as it doesn't know what setting you chose on the lens, it can only move the lever until it stops at the f-stop position set on the aperture ring.

QuoteOriginally posted by Takumar55 Quote
On a tangent, how do people feel about in-camera stabilization vs. in-lens, like Nikon and Canon (I think) do? Seems like in-camera is better for backwards compatibility.
It is as it means legacy glass can then take advantage of the shake reduction. The other advantage is that you only have to pay for the technology once instead of having to pay for it again and again on every lens you buy. In-camera stabilization means cheaper lenses.....bonus!
08-11-2013, 02:46 PM   #10
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QuoteOriginally posted by Tako Kichi Quote
otherwise it's still a manual aperture lens.
Technically, it is still an auto-aperture lens.

Pentax calls its body-controlled aperture feature (found on lenses with "A" contacts) "automatic aperture control" as opposed to auto(matic)-aperture. Auto-aperture is a long-standing general photographic term that traditionally refers to automatic aperture actuation at exposure time and has nothing to do with exposure automation. Vintage lenses are often prominently labeled "Auto XXX" to differentiate them from lenses with fully manual or preset aperture mechanisms. Pentax could have done us all a great favor if they had used "P" or "*" or anything other than "A" on the aperture ring and lens names.

Rant over!


Steve

(...other pet peeve is calling a lens "manual" as in "Why can't I get a manual lens to work work while photographing mosquitoes?" rather than "why can't I manually focus" or "why doesn't M-mode work".)
08-11-2013, 03:15 PM   #11
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QuoteOriginally posted by stevebrot Quote
Technically, it is still an auto-aperture lens.

Pentax calls its body-controlled aperture feature (found on lenses with "A" contacts) "automatic aperture control" as opposed to auto(matic)-aperture. Auto-aperture is a long-standing general photographic term that traditionally refers to automatic aperture actuation at exposure time and has nothing to do with exposure automation. Vintage lenses are often prominently labeled "Auto XXX" to differentiate them from lenses with fully manual or preset aperture mechanisms. Pentax could have done us all a great favor if they had used "P" or "*" or anything other than "A" on the aperture ring and lens names.
This is true and thanks for the correction. I guess I messed my terminology up in my original post. I blame the fact that I'm still a bit rusty having come back to the hobby after several (10+) years away.
08-12-2013, 08:45 AM   #12
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From what I thought/understood the aperture detection is nothing more than a potentiometer (variable resistor) which is operated by a flange lever (vs. a typical tiny screw). Pretty simple but very effective!
08-12-2013, 01:01 PM   #13
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QuoteOriginally posted by Takumar55 Quote
So if Pentax "de-crippled" the mount, then any legacy glass essentially could be used in auto-aperture mode?
It might work, but keep in mind that Pentax decided to come up with the A position instead of what they already had. In other words, they did not really think they could get aperture information very reliably from the M or K lenses. So they invented a new system for electronic aperture control instead. I am not sure they could ever decripple the mount and make it work as well as people expect.
08-12-2013, 01:14 PM   #14
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QuoteOriginally posted by Just1MoreDave Quote
It might work, but keep in mind that Pentax decided to come up with the A position instead of what they already had. In other words, they did not really think they could get aperture information very reliably from the M or K lenses. So they invented a new system for electronic aperture control instead. I am not sure they could ever decripple the mount and make it work as well as people expect.
The A series was developed so Pentax could add Shutter Priority and Program mode to the Super Program. Previous Pentax cameras were Manual or Aperture Priority only. If Pentax were to 'decripple' the mount, M lenses would work in Aperture Priority mode and manual mode without the need for the Green Button just like all Pentax cameras worked before the mount was 'crippled'.

You would still need A lenses for TAv, Tv, and flash automation.
08-12-2013, 02:15 PM   #15
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QuoteOriginally posted by 6BQ5 Quote
aperture detection is nothing more than a potentiometer (variable resistor) which is operated by a flange lever (vs. a typical tiny screw)
Even simpler than that. It is a simple on/off based on the position/pattern of the contacts. The logic is essentially the same as a bit array in computer programming. See the table 2/3 down at the linked page below:

Features and Operation of the Ka Mount


Steve
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