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02-16-2008, 10:08 PM   #1
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Older Lenses in Av Mode?

Hi Folks!

I'm a bit confused about metering in the Av (aperture priority) mode with current Pentax digital cameras and older M, K and screw mount lenses...

I understand the concept: In Av mode, the camera doesn't attempt to do anything with the aperture. It just meters whatever light is coming through the lens (either full open or with a manually stopped down diaphragm) and varies the shutter speed accordingly.

However, the trick doesn't seem to work with all of my older lenses. For example, it does work with my Tamron mirror lens. The shutter speed varies depending on the brightness of the scene. However, it doesn't work with my Takumar 500mm. The shutter speed simply doesn't change.

Any explanation? Suggestions?

Thanks!
Dave

02-16-2008, 10:49 PM   #2
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well, for one, make sure you're not in AF mode...i kicked myself in the shin really hard when i was trying to take pictures with my SMC 55/2 not knowingly in AF...

second, make sure you have your camera's custom functions set to allow the aperture ring.

no-brainer type things here, but i forget them all the time!
02-17-2008, 05:38 PM   #3
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QuoteOriginally posted by dbuffington Quote
Hi Folks!

I'm a bit confused about metering in the Av (aperture priority) mode with current Pentax digital cameras and older M, K and screw mount lenses...

I understand the concept: In Av mode, the camera doesn't attempt to do anything with the aperture. It just meters whatever light is coming through the lens (either full open or with a manually stopped down diaphragm) and varies the shutter speed accordingly.

However, the trick doesn't seem to work with all of my older lenses. For example, it does work with my Tamron mirror lens. The shutter speed varies depending on the brightness of the scene. However, it doesn't work with my Takumar 500mm. The shutter speed simply doesn't change.

Any explanation? Suggestions?

Thanks!
Dave
older pentax K lenses will only work in Av mode at wide open apature

you need to work in manual mode, and use DOF to meter, or with *istD and K10D use the green button
02-17-2008, 06:43 PM   #4
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QuoteOriginally posted by Lowell Goudge Quote
older pentax K lenses will only work in Av mode at wide open apature
I assume that my older Takumar lens -- with its "Manual" stop-down lever -- can be used at all apertures with the aperture stopped down. Correct?

Thanks to all!

02-17-2008, 06:57 PM   #5
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That's correct dbuffington, your camera doesn't have anything to press on that pin so it won't be held wide open.

There is a way to modify your M-Mount and K-Mount lenses to work in AV, but I don't recommend it unless you can afford to replace the lens
I've done it on several of my lenses, so I can also use them with my Sigma camera, and almost buggered up one.
02-17-2008, 07:16 PM   #6
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In short, you can't use K and M series lenses in Av. You must be in M mode, and stop down the lenses manually with the AE-L (or green) button. M42 lenses can be used just fine in Av, because they just stop down when you set the aperture. I find I need to bump the EV's by a stop or two when using screwmount lenses, because they tend to underexpose.
02-17-2008, 07:51 PM   #7
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QuoteOriginally posted by Finn Quote
I find I need to bump the EV's by a stop or two when using screwmount lenses, because they tend to underexpose.
I've noticed that also, but I don't understand why? Thanks!
02-18-2008, 04:41 AM   #8
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Don't understand it myself, but I do know that a piece of aluminum foil taped over the electrical contacts will either greatly reduce or eliminate the problem. A roll of scotch tape and a few square inches of spare aluminum foil are standard items in my camera bag. Give it a try.

02-18-2008, 07:21 AM   #9
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QuoteOriginally posted by Mike Cash Quote
Don't understand it myself, but I do know that a piece of aluminum foil taped over the electrical contacts will either greatly reduce or eliminate the problem. A roll of scotch tape and a few square inches of spare aluminum foil are standard items in my camera bag. Give it a try.
Over ALL the contacts?!?

Thanks!
Dave
02-18-2008, 02:38 PM   #10
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Yep. Just take a small piece of foil (I fold mine over double) and place it so it covers all the contacts at once. Then hold it in place with a couple small pieces of scotch tape.
02-18-2008, 03:02 PM   #11
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QuoteOriginally posted by dbuffington Quote
Hi Folks!
However, the trick doesn't seem to work with all of my older lenses. For example, it does work with my Tamron mirror lens. The shutter speed varies depending on the brightness of the scene. However, it doesn't work with my Takumar 500mm. The shutter speed simply doesn't change.
i use my 500/4.5 in Av with no problem. it's really the way to go as the lens stopsdown when adjusted. there is nothing connecting the mount to the aperture.
have you checked that the aperture is actually stopping down when you adjust it??? look into it when you adj it. the ap opening should get smaller. i've tested mine and the speed/ap does correspond to changes..
02-18-2008, 04:41 PM   #12
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QuoteOriginally posted by Mike Cash Quote
Yep. Just take a small piece of foil (I fold mine over double) and place it so it covers all the contacts at once. Then hold it in place with a couple small pieces of scotch tape.
hm..have you also tried it with tape only? I'd be curious if that would work since the locgic would be to avoid the lens to shortcut the contacts, but wouldn't the aluminium foils shortcut them as well?
02-18-2008, 05:14 PM   #13
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QuoteOriginally posted by kuuan Quote
hm..have you also tried it with tape only? I'd be curious if that would work since the locgic would be to avoid the lens to shortcut the contacts, but wouldn't the aluminium foils shortcut them as well?
Actually the point is to short out the contacts. None of my m42 lenses touch the electrical contacts with the lens mounted. Using the aluminum foil shorts out the contacts, and for some strange reason helps with the metering. Although I usually just dial in a EV adjustment and fire away, the foil works if you need more EV adjustment then the +2 that my k100d has.
02-18-2008, 05:24 PM   #14
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thank's Fritz, now it's clear!
02-20-2008, 04:04 AM   #15
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Same as Fritz here. I could just set the EV at +2, but that doesn't leave me any room for quickly boosting the EV to deal with backlit situations. The foil buys back that wiggle room.
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