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11-09-2011, 08:26 AM   #1
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SMC Pentax A vs. M

Is the metering more accurate when set to Aperture Priority when using an A vs. M. I find the M needs about 1 or so EV compensation. Does this also hold true for A lenses?

Thanks.

11-09-2011, 08:47 AM   #2
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Which body are you using? How are you doing the metering in M mode? With an M lens with an auto-aperture (like Pentax-M series) you have to use the green button to stop down the lens and take a reading (you should set the menu option for the green button to set shutter speed in M mode). If you are not doing that, then the aperture is just staying wide-open all the time and you're getting bad readings.

Nevertheless, it seems like it ought to be *more* accurate with an M lens because it has to be physically stopped down to get a reading and it will therefore be measuring the actual light rather than calculating it, but that often doesn't seem to be the case. You'll probably find that certain particular lenses will need Ev comp as you stop down but others will meter more accurately.
11-09-2011, 09:03 AM   #3
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I have a K20D. I use either Av mode or M and meter with the green button. I was wondering if metering is more accurate with the A series lenses, as metering with the M seems to be off by one or so stops, esp. at wider apertures. If the metering was more accurate, that to me would be a real reason to get an A lens, although there other other advantages conferred by the communication it provides.
11-09-2011, 09:11 AM   #4
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Yes, probably so, but you'll see different behavior with different manual lenses. But I believe also that 'A' lenses can use the full matrix-metering of the body and the M lenses can't, so that is an advantage. (Can someone confirm that?)

11-09-2011, 09:20 AM   #5
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Something that a lot of people overlook when using M or earlier lenses is that the metering is being done stopped down. It is very easy to go outside of the metering range of the camera with manual aperture lenses.
I've found the metering to be overall more accurate with A lenses and later, not that it's that big a deal to do a couple of trial exposures until you've got the exposure correct.
11-09-2011, 09:22 AM   #6
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Metering is no more accurate. The only difference is that you have to press the green button for Ms, but not for As. The metering is the same.
11-09-2011, 09:37 AM   #7
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QuoteOriginally posted by Jonathan Mac Quote
Metering is no more accurate. The only difference is that you have to press the green button for Ms, but not for As. The metering is the same.
This is definitely not the case from my experience, speaking as someone who has owned many Pentax DSLRs. Comparing two similar focal length lenses, the A series lens will always be more accurate in terms of exposure determination than an M series lens.

Wheatfield is correct in this regard.
11-09-2011, 10:44 AM   #8
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I've never liked stop-down metering, but when I have tried using my M lenses in Av mode it seems like if I set the aperture on the lens, the camera selects the correct shutter speed. Maybe I've just gotten lucky, as I tend to only do it in controlled environments. It's all a moot point for me though as I never allow my camera to do the metering, I have a Sekonic 758 for that.

11-09-2011, 10:48 AM   #9
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Metering can vary with lenses and apertures. This has been reported quite a bit here. Some have developed elaborate adjustment methodologies. I just meter, shoot, chimp, adjust as needed and shoot again.

EDIT: I should mention that I have very very many M-type and no-stopdown lenses, and very very few A-type lenses, so I don't really have a basis for prejudice. Maybe I'll assemble an A-kit and see.

Last edited by RioRico; 11-09-2011 at 12:32 PM.
11-09-2011, 10:55 AM   #10
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OK, but I am trying to understand if using an A lens vs. an M in Av mode or M mode with green button generally results in more accurate metering.
11-09-2011, 11:09 AM   #11
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The only times i actually trust the metering on a M lens (a little bit) is in Av mode and wide open. Stopped down i just do not trust the metering and always check and adjust. With A lenses (pentax and third party) it is different with almost every A lens i own but results are more consistent.
11-09-2011, 11:17 AM   #12
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QuoteOriginally posted by Spodeworld Quote
OK, but I am trying to understand if using an A lens vs. an M in Av mode or M mode with green button generally results in more accurate metering.

It probably depends as much on what you are metering. I can only speak for the K20D, but it frequently underexposes when the subject is backlit regardless of the lens that's mounted. Probably the easiest thing to do is shoot in manual and use a $5 gray card. Take a shot of the gray card, and check your histogram. If there is a spike in the middle, then you've got "proper exposure" if it's to the left, open up; if it's to the right, stop down. That method is almost as fast, and every bit as accurate as using a light meter as long as you're not trying to control lighting ratios. I've always found that theory goes out the window in the face of reality.
11-09-2011, 11:18 AM   #13
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A lenses are more accurate. As wheat says when you stop down an m lens to meter is may exceed the meters tolerance and cause an error. Simple enough to chimp a shot to make sure you have the exposure you are looking for when it's not an action oriented thing. For street i will take a reading and shoot until i notice light changing then i'll take another, some need post adjustment but as long as it's 1ev or less I don't have an issue with that, I'd rather capture the moment. Not realy far different than when I shot sunny 16 on film and compensated in the darkroom
11-09-2011, 11:52 AM   #14
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accuracy of metering

Very interesting confirmation that I made the right decision going with A lenses instead of M. I have KX and K2 bodies for film where A or M doesn't matter (the only M lens I own is a 50 f/4 Macro), but I also use the lenses on my K20D. I hope Pentax NEVER goes the way of Canikon and abandons legacy glass.
11-09-2011, 12:22 PM   #15
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Find the A lens to be much more accurate.When using M lens I just go to manual and guess the exposure which I seem to be able to do as well as the camera and much faster.This probably comes from using a Spotmatic for 30 or so years a lot of the time with no battery installed.
Jake
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