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01-02-2011, 03:30 PM   #1
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K-r and M series lenses in aperature mode

I am new to DSLR, but a longtime Pentax user from my college years in the 70's. I have a number of M series lenses and a K-r on order. My understanding is that the camera body will assume the lens is set at the largest aperture if shooting in aperture control mode. Wil you get proper exposures if you go up an f/stop or 2 and use EV +1 or +2?

01-02-2011, 03:51 PM   #2
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Welcome to the forum!

If the k-r works the same as the k-x, then no: the exposure is with fully open aperture no matter what is set with the aperture ring. However, in M-mode the green button can be used to briefly close the aperture to the ring setting, take a meter reading and set the shutter time to match for you. In M-mode the lens is also stopped-down to the aperture set on the ring for exposure so with this things work pretty nicely with M-series lenses.

Last edited by jolepp; 01-02-2011 at 05:06 PM. Reason: typo fix
01-02-2011, 05:00 PM   #3
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Aperture Priority (Av) with K/M lenses will not stop the aperture down. If you want to shoot wide open however, it will do that (no EC needed). Anything later than that (A, F, FA, DA, DA*, DFA) however all work fine in Av. To use your manual lens (to get started) discover where the stop down metering occurs. On all of my cameras, it has been with the green button. From what I understand however, that button on your camera can be mapped for several things (as it can on the K5 but it's mapped for different priority and still is used for stop down metering in M mode).

Enjoy your new camera and welcome aboard!

01-02-2011, 05:21 PM   #4
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I am so new I am still trying to figure out which camera the k2000 is.

But with a k2000 today, tripod mounted and manually focused, using an M 1.7 lens and Aperture Priority mode, I took still life pictures inside the house from f1.7 to 11 and could see no difference between any of them when shot at the same ISO setting.

Changing the ISO made minor difference between the groups.

Just sign me, a young frog.

01-02-2011, 05:28 PM   #5
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QuoteOriginally posted by Colbyt Quote
I am so new I am still trying to figure out which camera the k2000 is.

But with a k2000 today, tripod mounted and manually focused, using an M 1.7 lens and Aperture Priority mode, I took still life pictures inside the house from f1.7 to 11 and could see no difference between any of them when shot at the same ISO setting.

Changing the ISO made minor difference between the groups.

Just sign me, a young frog.
You don't see any difference because they were all shot wide open at f1.7 regardless of where you set the aperture ring.. Look at the Exifs for your f1.7 and f11 shots and you'll see that the shutter speeds are the same (or nearly so). If you change the ISO, all that will change as a result is the shutter speed, and it will meter with the lens wide open. To verify, take some flash shots at f1.7 and at f16. They will be identical. Completely blown out because the flash is firing at full output..

01-02-2011, 05:46 PM - 1 Like   #6
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noledoc welcome to the world of digital slr cameras, the forum and congratulations on the wise choice of continuing with the Pentax legacy with a K-r and classic glass.
As mentioned, in any program mode, you have no control in the aperture of a non-coupled (manual) lens. The camera body holds the aperture lever, and thus the aperture, in a fully open position. The camera will meter exposure times well enough with any M lens in program modes but your shots will be with the lens at it's maximum aperture setting. Switch to Manual (M) mode and you have a few choices when using manual aperture lenses. This will be shown in the user manual when you get your K-r. If not search the forum for topics like "Using Manual Lenses With Pentax DSLR. I'm sure it will come to you easily considering your previous experience.
01-02-2011, 06:25 PM   #7
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Thanks all, I appreciate the clarification
01-02-2011, 06:31 PM   #8
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This should be worth checking out: https://www.pentaxforums.com/forums/pentax-beginners-corner-q/110658-using-ma...ml#post1140022

01-03-2011, 05:12 AM   #9
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QuoteOriginally posted by Nowhere Matt Quote
The camera body holds the aperture lever, and thus the aperture, in a fully open position.


Thank you very much. An explanation of the physical side of things.

I respected all the statements that said it happened but without an explanation acceptance is hard when I am physically moving that darn ring and if I do it off cam I can see it change from a pin hole to a half dollar.
01-03-2011, 07:38 AM   #10
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QuoteOriginally posted by Colbyt Quote


Thank you very much. An explanation of the physical side of things.

I respected all the statements that said it happened but without an explanation acceptance is hard when I am physically moving that darn ring and if I do it off cam I can see it change from a pin hole to a half dollar.
On the lens there are 2 aperture couplings, one that opens and closes the aperture, and one that on film bodies told the camera the selected aperture so that the camera could meter without stopping down.

On the K&M series bodies, any time the shutter opened, the lever in the camera body holding the aperture activation lever on the lens open, moved and stopped the lens down to the preselected aperture.

On digital cameras, there is no aperture coupling to tell the camera what aperture is set by the ring. THis is why you need to meter with the green button in manual mode, where the camera stopps the lens down momentairly, so it can get a snap reading of exposure. It then sets the shutter to match this exposure reading.

In Av mode, the metering is active all the time but since the camera does not know with an K or M series lens the maximum aperture OR the selected aperture, the camera body does not let the aperture close during exposure. It holds the aperture at maximum all the time.

When the lens is off the camera, there is nothing holding the spring loaded aperture lever from allowing the aperture to close, and therefore, the aperture will close down as you move the ring.
01-03-2011, 07:42 AM   #11
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QuoteOriginally posted by Colbyt Quote


Thank you very much. An explanation of the physical side of things.

I respected all the statements that said it happened but without an explanation acceptance is hard when I am physically moving that darn ring and if I do it off cam I can see it change from a pin hole to a half dollar.
Yeah grab any unmounted lens with a manual aperture, set the aperture to say f22 and look through the lens. Now push that little aperture lever in the opposite direction while looking through the lens. In all automatic program modes and probably even in manual mode, the aperture lever is being held by the camera body so that the aperture is always open.
The exception must be, and anyone can correct me if I am wrong of course, that even in manual modes the lever is still being held open
but allowed to close very briefly to the point you have set with the aperture ring during the "stop down" metering and during the actual exposure.
01-03-2011, 08:18 AM   #12
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QuoteOriginally posted by Nowhere Matt Quote
Yeah grab any unmounted lens with a manual aperture, set the aperture to say f22 and look through the lens. Now push that little aperture lever in the opposite direction while looking through the lens. In all automatic program modes and probably even in manual mode, the aperture lever is being held by the camera body so that the aperture is always open.
The exception must be, and anyone can correct me if I am wrong of course, that even in manual modes the lever is still being held open
but allowed to close very briefly to the point you have set with the aperture ring during the "stop down" metering and during the actual exposure.
That is exactly what happens.

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