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08-31-2010, 08:38 PM   #1
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What's wrong with my M200mm/4 ?

Hey all, I just got a pretty decent deal on a pair of Pentax-M lenses, the 200/4 and the 50/1.7 - the two of them for $100. However, I just did some testing and the 200/4 has some kind of metering problem - in manual mode (cause with the k-x, on ap priority, it only shoots wide open..right?), the metering is going crazy on anything besides wide open (F4). Basically, when I stop it down and greenbutton it, it recommends the same shutter speed for any aperture beyond 4 (5.6 and up).

If I take the lens off and use the aperture ring, the lens stops down as it should. And, looking at the lens from the front while pressing the green button, setting different F-stops does cause it to have different opening sizes, as it should, which leads me to think...well, I don't know what to think. What's wrong with it, though?

08-31-2010, 08:40 PM   #2
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what is your green button action set to in the menus? it can do other things as well! (at least on the other pentax DSLR's, no personal experience with K-x)
08-31-2010, 08:56 PM   #3
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It's set to act as a pseudo aperture priority whenever I press it, which, as I understand it, is how it's supposed to be set for usage with manual-aperture lenses.
08-31-2010, 09:10 PM   #4
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might look at apeture coupling lever on end of mount, should move freely, with
no binding, be able to see apeture blades open/close as you do.

08-31-2010, 09:28 PM   #5
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QuoteOriginally posted by BillM Quote
might look at apeture coupling lever on end of mount, should move freely, with
no binding, be able to see apeture blades open/close as you do.
Yep, it moves very slowly with quite low friction and a bit of an elastic/spring-y feel
08-31-2010, 09:43 PM   #6
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change the green button to Tv Shift if that is an option, thats what i use on my K200D. This means it changes the shutter speed to suit the stopped down metering
08-31-2010, 09:51 PM   #7
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QuoteOriginally posted by jezza323 Quote
change the green button to Tv Shift if that is an option, thats what i use on my K200D. This means it changes the shutter speed to suit the stopped down metering
Sorry, that was my mistake in my earlier post; that's what the green button is set to.
08-31-2010, 10:16 PM   #8
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Hmm. Upon further testing, it's apparent that this is a low-light-only issue. When I shot towards a wall illuminated only by a single lamp, apertures 4 to 8 all metered accurately. When I shot to a darker region, it was only 4. A slightly lighter part in between, and 4-5.6. Shooting directly at the lamp resulted in all apertures metering accurately...Interesting. Potentially annoying in the future, but interesting.

08-31-2010, 10:20 PM   #9
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interesting outcome

enjoy shooting with it!
08-31-2010, 10:48 PM   #10
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QuoteQuote:
change the green button to Tv Shift
that was the big one for me jez, once I understood it, nothing but fun.
09-01-2010, 06:27 AM   #11
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the correct way to use an M lens or any lens without an A aperture setting is in manual mode.

there, regardless of green button setting the camera will know that the only possible shift is Tv shift since it knows the lens is not an A lens.

In Av mode the camera always shoots wide open, so trying to meter using green button in Av mode is useless.
09-01-2010, 09:31 AM   #12
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The meter needs a certain amount of light to work. So if you're in a very dark environment, then rob the meter of what little light there is by stopping down the lens, it's not uncommon at all for metering to basically fail. We're talking situations in which there is no way in the world you'd consider shooting shooting stopped down because the light is already so bad, so it isn't normally an issue in practice. But the aperture blades should still have closed just as they do in better light.
09-01-2010, 03:49 PM   #13
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QuoteOriginally posted by Marc Sabatella Quote
The meter needs a certain amount of light to work. So if you're in a very dark environment, then rob the meter of what little light there is by stopping down the lens, it's not uncommon at all for metering to basically fail. We're talking situations in which there is no way in the world you'd consider shooting shooting stopped down because the light is already so bad, so it isn't normally an issue in practice. But the aperture blades should still have closed just as they do in better light.
Thanks, yeah. That's more or less what's going on, I think.
09-01-2010, 03:59 PM   #14
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QuoteOriginally posted by Marc Sabatella Quote
The meter needs a certain amount of light to work. So if you're in a very dark environment, then rob the meter of what little light there is by stopping down the lens, it's not uncommon at all for metering to basically fail. We're talking situations in which there is no way in the world you'd consider shooting shooting stopped down because the light is already so bad, so it isn't normally an issue in practice. But the aperture blades should still have closed just as they do in better light.
What Marc said...

The lack of stop-down meter linearity is because the amount of light at the meter is near or below the meter's low-end sensitivity when stopped down in dim light. This happens REGARDLESS of the ISO setting.


Steve
09-04-2010, 10:42 AM   #15
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It is possible that the meter is simply beyond it's range, but I'm not sure what it does when it reaches that point.

I do remember the relatively insensitive meter in my older Pentax actually shutting off when the light reached a (still fairly high) level. It was designed to do that, to save battery power. Even using open-aperture metering at f1.4, there were many occasions when the subject was still fairly well illuminated, but couldn't be metered. It was especially annoying because after it shut off, it would always indicate correct exposure.

Paul
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