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03-10-2009, 05:41 PM   #1
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Lens compatibility confusion

OK. This must have been explained to death, but I find it extremely odd and somewhat frustrating.

I have an MZ-M and a K20D as well as a couple of A lenses (50mm f2 & 80-200mm f4.7-5.6), a couple of F lenses (35-70mm f3.5-4.5 & 50mm f1.7) and a DA lens. Obviously, they all work on both bodies, but there's something mighty strange going on with the aperture information visible in the viewfinders once I take the aperture ring off the A position.



apperture shown
in viewfinder?
MZ-M (A) no (F) yes
K20D (A) no (F) no

I've had a look at the lens compatability charts, so I understand that the K20D has the crippled KAF2 mount and therefore doesn't have the necessary contacts, but why, oh why, is the MZ-M able to read the aperture information of the F lenses (when it can't make use of their autofocus), but not A lenses? I'd have thought it would make more sense for the MZ-M to be able to read the A lenses - it IS a manual body, after all! I prefer to use aperture priority on the manual body as I can change the aperture without taking my finger from the shutter release button, so it would be nice to be able to see in the viewfinder what aperture I've chosen! Is there any way to get the body to do this, or might I have more luck going to an older body - perhaps that'd give me a better viewfinder as well (the MZ-M's is tiny!) In fact, I've come the conclusion that the MZ-M is a rather odd little camera, but I'm rather attached to it, so I'd like to keep it if possible.

Many thanks,

Philip

03-10-2009, 06:03 PM   #2
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The current batch of Pentax DSLRs has "crippled" KAF2 mount. It does not have a mechanical stop-down coupler/indicator to tell what the aperture setting is. So it can only use stop-down metering on pre-A lenses (or A lenses but aperture not set on A).

The "mechanical stop-down coupler/indicator" is the lever at 1:30 position when you look into the lens mount on the camera.

If you look at the rear end of a lens with aperture ring (including F- and FA-lenses), you'll see a lever at the corresponding position of the one on the camera's lens mount. The lever is reccessed into the body of the lens. The position of this lever will change with the aperture ring setting. If the body has one of K/KF/KA/KA2/KAF/KAF2 mounts, it can tell the aperture based on the position of the lever on the lens.

"Crippled" KAF mount (on MZ-30/ZX-30, MZ-50/ZX-50, MZ-60/ZX-60, *ist series, K100D/K110D) and "crippled" KAF2 (on K10D, K100D Super, K20D, K200D, K2000, KM) don't have the "mechanical stop-down coupler/indicator," thus can not tell the aperture setting on the lens.
03-10-2009, 07:49 PM   #3
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So what's the exact process for "stop-down metering" on my K10D with M series lenses?

I've been shooting with them in Av mode, setting my aperture manually on the lens, then pressing the green button to meter, and hitting the shutter release. This typically seems to work, though sometimes has been disastrous... is there more to the process, I mean, to doing it correctly?
03-10-2009, 08:34 PM   #4
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QuoteOriginally posted by Hannican Quote
So what's the exact process for "stop-down metering" on my K10D with M series lenses?
The K10D can't perform "full-aperture" metering (because it doesn't know the aperture setting on the lens). To meter, you have to set the K10D to M mode, then either (1) hit the green button so that the K10D can stop down the lens, perform the metering, and set the aperture on the body, or (2) pull the lever to do an optical preview, then adjust the aperture and/or the shutter speed until the metering bar shows correct exposure.

QuoteOriginally posted by Hannican Quote
I've been shooting with them in Av mode, setting my aperture manually on the lens, then pressing the green button to meter, and hitting the shutter release. This typically seems to work, though sometimes has been disastrous... is there more to the process, I mean, to doing it correctly?
The K10D, when in Av mode and with an M-lens, will meter correctly. But the photo is taken with the lens at the largest aperture, regardless the setting of the aperture ring on the lens.

03-10-2009, 08:58 PM   #5
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QuoteOriginally posted by pa79 Quote
OK. This must have been explained to death, but I find it extremely odd and somewhat frustrating.

I have an MZ-M and a K20D as well as a couple of A lenses (50mm f2 & 80-200mm f4.7-5.6), a couple of F lenses (35-70mm f3.5-4.5 & 50mm f1.7) and a DA lens. Obviously, they all work on both bodies, but there's something mighty strange going on with the aperture information visible in the viewfinders once I take the aperture ring off the A position.



apperture shown
in viewfinder?
MZ-M (A) no (F) yes
K20D (A) no (F) no

I've had a look at the lens compatability charts, so I understand that the K20D has the crippled KAF2 mount and therefore doesn't have the necessary contacts, but why, oh why, is the MZ-M able to read the aperture information of the F lenses (when it can't make use of their autofocus), but not A lenses? I'd have thought it would make more sense for the MZ-M to be able to read the A lenses - it IS a manual body, after all! I prefer to use aperture priority on the manual body as I can change the aperture without taking my finger from the shutter release button, so it would be nice to be able to see in the viewfinder what aperture I've chosen! Is there any way to get the body to do this, or might I have more luck going to an older body - perhaps that'd give me a better viewfinder as well (the MZ-M's is tiny!) In fact, I've come the conclusion that the MZ-M is a rather odd little camera, but I'm rather attached to it, so I'd like to keep it if possible.

Many thanks,

Philip
For a prime 'A' lens the MZ-M could determine the aperture value but for a variable aperture zoom it cannot since the information about the varying max aperture is not coded in the lens contacts. So Pentax decided to not show any value at all for 'A' lenses.

'F' and later lenses transmit the actual aperture value set on the lens through the digital data contact and the camera can display that information. For variable aperture zooms the transmitted value changes as you zoom and will be displayed correctly.
03-11-2009, 01:15 AM   #6
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Thank you Ole - that was just what I wanted to know. Shame they took that decision (at least to me - I use the prime almost exclusively on the MZ-M), but I can see their logic.
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