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11-17-2012, 10:19 PM   #1
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Metering/Needle/Aperture Settings

I have a KM, ME Super, and LX. Haven't checked this on the LX and the ME Super obviously doesn't have DOF preview, but when using any of my lenses (K55/1.8, K28/3.5, A50/1.4, etc) on the KM, after getting the exposure needle right, whenever I hit the DOF preview, the needle drops indicating under exposure. I'm assuming I don't need to set the exposure based on the needle's reaction to the DOF preview, right? Can't find anything in the manual that says to do so.

Also, whenever I use an aperture setting other than wide open, the viewfinder brightness does not change. I'm under the impression this is normal, otherwise there wouldn't be a need for DOF preview as one would see the result of the chosen aperture in the viewfinder. Seems this is referred to as open aperture metering. Just heard recently that the A lenses would behave this way but non-A lenses would stop down and darken the viewfinder.

11-17-2012, 10:48 PM   #2
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All that is part of uncrippled K-mount behavior. When a K-mount lens is mounted, the camera's metering circuit can tell where the aperture ring is set, without closing the blades. The needle reacts to that information, where the shutter speed dial is set and where the ISO dial is set. It's actually an analog computer with those inputs plus the light sensor. Using DOF preview closes the blades, reducing the light going to the sensor, and altering the metering circuit voltage, so the needle reads incorrectly in DOF preview.

Open aperture metering is part of the original K-mount so it doesn't need an A position to work. Basic K-mount: When you mount the lens, the lens's aperture lever is moved so the blades are fully open. Then when you shoot, the lever is allowed to fall back to wherever it wants to. The aperture ring tells the blades where they should stop. (This is where DA lenses have trouble.)
11-18-2012, 02:51 AM   #3
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If we are talking about how these lenses perform on a FILM camera then - The lens has a level on the left as you look at the rear of the lens, that tells the camera what aperture it is currently set at. The lens remains wide open, open aperture metering, until the photo is taken when the lever on the right as you look at the lens is moved which stops down the lens to the taking aperture as the photo is taken.

On a film camera the exposure is always worked out at open aperture, and because the lens tells the camera what aperatue it is currently set to, the camera makes the necssary adjustment to exposure. When you preview the depth of field, the iris closes down to the selected aperture, and the metering needle drops, but the exposure is still worked out assuming that the lens is wide open.

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Chris Stone
11-18-2012, 06:53 AM   #4
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Interesting. I'll check the lenses out in more detail. I wasn't sure how the camera was able to know the aperture setting without the electrical contacts like you see on modern lenses.

Thanks for the clarification. I figured all was well, just wanted to confirm as using the older cameras, like the KM with a needle is new to me.

11-18-2012, 09:58 AM   #5
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QuoteOriginally posted by jtkratzer Quote
Interesting. I'll check the lenses out in more detail. I wasn't sure how the camera was able to know the aperture setting without the electrical contacts like you see on modern lenses.

Thanks for the clarification. I figured all was well, just wanted to confirm as using the older cameras, like the KM with a needle is new to me.
The manuals for your cameras are your friends. All three are available at the Butkus site:
Pentax camera instruction manuals, Pentax professional instruction manuals
As for the non-crippled lens mount, the aperture coupling mechanism is the core magic of how the mount works. It is what makes open-aperture metering and aperture priority automation possible on those bodies that rely on the aperture ring to set the iris opening. The cool part is that stop-down metering is still possible as well and allows for use of adapted M42 screw mount lenses. Your observation with the KM on stop-down is a good example. The M42 adapter engages the coupling mechanism in the body and moves it to the full extent of its travel, meaning no additional stop-down at exposure time.

When you get this all figured out, you will have a better understanding of how your older cameras work and how legacy glass behaves on your dSLR.


Steve

P.S. The lens does not really communicate the aperture setting. What it does communicate is the offset from wide-open in the form of voltage. This is all accomplished with a strip of resistor material and feeler connection in the meter circuit. The coupler moves the strip to vary the resistance and the voltage to the meter varies as a result. It is all very old school...No digital logic involved.

Last edited by stevebrot; 11-18-2012 at 10:05 AM.
11-18-2012, 10:11 AM   #6
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QuoteOriginally posted by stevebrot Quote
The manuals for your cameras are your friends. All three are available at the Butkus site:
Pentax camera instruction manuals, Pentax professional instruction manuals
As for the non-crippled lens mount, the aperture coupling mechanism is the core magic of how the mount works. It is what makes open-aperture metering and aperture priority automation possible on those bodies that rely on the aperture ring to set the iris opening. The cool part is that stop-down metering is still possible as well and allows for use of adapted M42 screw mount lenses. Your observation with the KM on stop-down is a good example. The M42 adapter engages the coupling mechanism in the body and moves it to the full extent of its travel, meaning no additional stop-down at exposure time.

When you get this all figured out, you will have a better understanding of how your older cameras work and how legacy glass behaves on your dSLR.


Steve

P.S. The lens does not really communicate the aperture setting. What it does communicate is the offset from wide-open in the form of voltage. This is all accomplished with a strip of resistor material and feeler connection in the meter circuit. The coupler moves the strip to vary the resistance and the voltage to the meter varies as a result. It is all very old school...No digital logic involved.
I have the physical paper copies of the ME Super and LX manual (three of these I believe) and digital on all three bodies, but I don't believe any of them get into that kind of technical detail about how those things function. Is it truly voltage? I would think that the measurement would be in the form of resistance since the voltage shouldn't change much over a short length based on the adjustment of the aperture ring. Just a thought, and again, I haven't read in the specific details of how the K mount functions.

I'm sure it's a simple reason I've never really looked into, but it doesn't make sense that all of these "old" cameras can open aperture meter, determine an exposure setting, then stop the lens down when the shutter is released and accurately expose film but my K-5 cannot get an accurate meter reading without using the green button in manual mode. I understand how the M42 and K mount lenses function differently for metering and aperture settings, but the K-5 metering with lenses that don't have the A setting isn't immediately clear.
11-18-2012, 01:52 PM   #7
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QuoteOriginally posted by jtkratzer Quote
Is it truly voltage?
increased resistance...decreased voltage...decreased current...however you want to look at it. The meter itself is an ammeter and measures current.


Steve
11-18-2012, 02:02 PM   #8
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QuoteOriginally posted by jtkratzer Quote
I'm sure it's a simple reason I've never really looked into, but it doesn't make sense that all of these "old" cameras can open aperture meter, determine an exposure setting, then stop the lens down when the shutter is released and accurately expose film but my K-5 cannot get an accurate meter reading without using the green button in manual mode. I understand how the M42 and K mount lenses function differently for metering and aperture settings, but the K-5 metering with lenses that don't have the A setting isn't immediately clear.
Ahhh...the mystery of the "crippled" mount. If the body is setting the aperture, there is no need for a coupling to the aperture ring. At least that was Pentax reasoning when they removed the coupler from many of its AF bodies back in the mid-1990s and its digital bodies from the *ist-D on. As a result, your K-5 lacks that capability. It compensates by providing "green button"/stop-down metering for non-A lenses in the same manner as a vintage Spotmatic.

That is why there is a petition on this site pleading with Ricoh to "heal" the crippled mount on future bodies.

Here is a link to a page that provides documents having more information on the subject than you might ever want to know...

http://kmp.bdimitrov.de/technology/K-mount/index.html

and much less...

http://kmp.bdimitrov.de/technology/summary/index.html


Steve


Last edited by stevebrot; 11-18-2012 at 02:12 PM.
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