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12-27-2011, 03:22 AM   #1
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Aperture linkage (body to lens) question

Hi all, I have never been 100% happy with my k7 and so have started to play with my old z1p again.

I have recently come into a couple of old manual lenses and so have been thinking more about how the mechanics work.

Specifically, how does an old manual lens tell the body what the aperture is? I know that the aperture sensing ring is there to do that, but is the mechanical coupling that precise? And how does the body know if the open or closed settIngs on the ring relate to a 1.4 to 22 lense or to a 6 to 32 lens? Does the ring have a given setting/position for each f stop? If the body knows the aperture why does it not show it on the LCD?

It's fun thinking about all this stuff again!!!!

Thanks in advance!

Brett


Last edited by bmf; 01-03-2012 at 08:27 AM.
12-27-2011, 04:05 AM   #2
Ole
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A manual lens (M, K) cannot tell the camera what aperture is set. It can only indicate how much it is stopped down.
The actual aperture is not indicated and not required for the light meter to work.
An A lens on the other hand additionally tells the camera its smallest aperture and aperture range through the electrical contacts on the lend mount.
12-27-2011, 05:17 AM   #3
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Actually it does. Z-1p has the aperture coupling ring on the body to determine which aperture has been chosen even if the aperture ring is not set to A. I found this out when I masked the M lens mount to simulate A lens and the flash mounted on the camera could actually display the exact aperture set by the lens.

I believe it did that by reading the max/min aperture from A lens mount, and then determine how much has been stopped down through the aperture resistor underneath the coupling ring.
12-27-2011, 05:22 AM   #4
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Ole is right. If it's an A lens with the aperture ring moved the camera will be able to calculate the actual aperture, whereas if it's a M lens then the camera can still meter without stopping down (since it knows by how much to divide the EV), but it doesn't know the exact f stop value.

Modern DSLRs can't sense the position of the aperture position lever, so they have to stop down to meter with M lenses.

12-28-2011, 07:54 AM   #5
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Hi all, thanks for the replies,

I would like to clarify the terminology you used, and a confusion in my head regarding specifically M type lenses on my z1p

When you say the camera knows "how much the lens is stopped down" how does this differ from tellIng the body the aperture?

If I understand correctly, the body stops down the lens only at the point that you actually fire the shutter. How does the body know how much light will enter at that point if it does not know where the aperture is set to while you are composing the shot and setting T and A BEFORE the shot!

Thanks a million!

Brett
12-28-2011, 08:22 AM   #6
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With 'A' lenses and later, the body knows what the aperture of a fully open lens is so it is easy for it to figure out how much light it would meter if / when such a lens is stopped down for exposure. For example, if the meter indicates that, say, t=1/1000s would result in the 'correct' exposure and the body knows that the lens is at f=1.4 fully open it is easy to compute t=1/500s for f=2, t=1/250s for f=2.8, t=1/125s for f=4,...

The important bit is that a K-mount lens with an aperture lever remains fully open until exposure regardless of what is set on the ring.
12-29-2011, 01:01 AM   #7
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Hi Jolepp,

Thanks for the info! It seems to me that M-lenses on the z1p work the same way though? It seems as though the body knows the aperture of this manual lens even though it has no A setting! If i change the aperture manually (the only way on these old lenses) then the body immediately gives me feedback that it is over/under exposed (while i am in Hyp M mode).

It seems to me that the body must "know" the aperture to calculate this? Even though it does not indicate it to the user! How does the body know? Or maybe it doesn't and I will end up with a spool of terrible exposures?

Regards
Brett
12-29-2011, 06:53 AM   #8
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All K mount bodies detect the aperture via the mechanical linkage on the K/M series lens and that is how the lens "communicates" the current aperture setting to the body so that it can show the correct meter exposure for it. This too is communicated to the user in the bodies with the aperture window.
This mechanical coupling is precise enough to allow the user to properly expose even the narrowest latitude film provided the user understands this metering mode. Of course most b&w and color negatives have so much latitude you can be off by 3 or more stops and still get perfectly good results to work with.
The Pentax LX uses this same mechanical coupling and adds Integrated Direct Metering (off the film metering) to constantly monitor the scene for light changes - cutting off or extending the shutter as needed. Coupled with the widest metering range of any camera to date, this body using K/M series lenses can make very accurate and very long aperture priority autoexposures. I am not aware of any camera - film or digital, that matches this ability.

12-29-2011, 07:24 AM   #9
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A camera body cannot know the aperture of a K or M lens. Only A and later lenses communicate this information via the bit pattern of the pins (max and min f-stop) and via the digital interface in modern lenses.

The aperture coupler for all Pentax lenses has the same position when the lens is wide open, no matter what the open f-stop is. That tells the cameraís metering system that the correction factor will be zero (open lens). As you rotate the aperture ring, the coupler moves along telling the body how many stops from the open position is closed down. It is a relative value, but thatís what needed for the meter to compensate.

When you take the picture, the aperture lever is disengaged allowing the aperture blades to close down to whatever value was set. I am not certain on this, but the old manual cameras without support for A lenses, simply disengaged the level all the way allowing the set aperture to actually stop the lens as needed.

With A (and later) lenses, the body uses the movement of the lever to set the aperture on the lens if it is in the A position. Again it is a relative motion from the open f-stop, but now the body knows the max and min (therefore the range) so it can calculate the displacement. At the same time, Pentax changed the way the level relates to the aperture opening. On K and M lenses the level movement is proportional to the diameter of the aperture making it non-linear. On A and later lenses it is proportional to the area of the aperture which is linear for the f-stop scale. That simplified the calculation of how much to move the lever to achieve a given f-stop.

Thatís the reason we cannot simply fake the contacts on a K or M lens and expect correct exposure. It will work on both extremes (wide open or closed down all the way with 0% or 100% displacement), but it will be off by more than one stop on certain f-stops depending on the f-stop range of the lens.
12-29-2011, 08:33 AM   #10
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QuoteOriginally posted by bmf Quote
Hi Jolepp,

Thanks for the info! It seems to me that M-lenses on the z1p work the same way though? It seems as though the body knows the aperture of this manual lens even though it has no A setting! If i change the aperture manually (the only way on these old lenses) then the body immediately gives me feedback that it is over/under exposed (while i am in Hyp M mode).

It seems to me that the body must "know" the aperture to calculate this? Even though it does not indicate it to the user! How does the body know? Or maybe it doesn't and I will end up with a spool of terrible exposures?

Regards
Brett
Older bodies make use of a second mechanical linkage which indicates the aperture position set on the ring. However, the newer bodies do not have the counterpart of this second linkage, they implement a 'crippled mount'. I don't know when exactly the second linkage was dropped from the bodies, the current digital bodies do not have it though, instead they depend on knowing the min/max aperture and being able to control the aperture with an A-type aperture lever, which makes the indicator linkage redundant.
12-29-2011, 10:19 AM   #11
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Thanks Demp10, that concept of *proportional* control of the aperture from the wide open setting makes a lot of sense. The lens is always wide open while composing etc. so the camera knows the wide open exposure and the mechanical linkage tells it *proportionally* how many f-stops it WILL be closed down when the shutter is released.

Excellent!
Bmf
12-29-2011, 12:17 PM   #12
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In my statement all "K mount bodies" above, I meant in the context of metered K mount manual camera bodies. BTW BMF, what specific body are you talking about?

There is a Pentax National Camera K1000 service manual and it explains the electro mechanical arrangement that allows accurate open aperture TTL metering. It is the easiest to follow since the K1000 is the simplest. It explains that there is a diaphragm resistor that is moved by the aperture ring to balance the meter with the ISO as well as shutter speeds - also through variable resistors. This is how all K/M series lenses communicates their positions in all K, M & LX camera bodies are.

So when you say "Does the ring have a given setting/position for each f stop?" these mechanical parts do activate their corresponding variable resistors to provide proportional electrical levels corresponding to their designed position and output.
12-29-2011, 09:05 PM   #13
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Hi here I was talking about the z1p - so it all makes perfect sense!
Thanks for the tip on the K 1000 manual, I have downloaded it, very informative!
Regards
Bmf
12-29-2011, 09:17 PM   #14
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I agree with demp10 that a camera body cannot know the aperture of a K or M lens. The coupler ring under the lens mount has 9 gold plated contact points; the camera only knows how many steps down from the maximum opening. On my MZ-6, if I move the aperture ring away from A position, it only displays - - and no aperture value. Therefore, on a crippled lens mount of a DSLR, if the aperture ring of a A (or FA) lens is not set at "A" position, then it works just like a K or M lens and would need to use green button for metering (am I correct?).

Last edited by violini; 12-30-2011 at 10:14 AM.
12-31-2011, 02:33 PM   #15
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QuoteOriginally posted by demp10 Quote
...The aperture coupler for all Pentax lenses has the same position when the lens is wide open, no matter what the open f-stop is. That tells the camera’s metering system that the correction factor will be zero (open lens). As you rotate the aperture ring, the coupler moves along telling the body how many stops from the open position is closed down. It is a relative value, but that’s what needed for the meter to compensate...

I couldn't have said it better myself, though it is important to restate (forcefully) that even with an A-contact lens, the lens does not tell the camera what the current aperture setting is, only the max and min. As a result, when the aperture ring is off the A setting, the camera no longer controls the aperture and has nothing to display to the user except the shutter speed.

The lens NEVER communicates the set aperture to the camera except as the offset from maximum and that only on bodies without the crippled mount and by the mechanical linkage and not by the A contacts.


Steve

Last edited by stevebrot; 12-31-2011 at 02:46 PM.
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