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02-27-2010, 02:48 PM   #1
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Question for those who have K-7 and 16-50mm.

Hi,

Recently, I found something wrong in the metering system with my K-7 and 16-50mm.
The photos taken at F2.8 would be brighter than taken at other apertures.

The metering systems seems to have more exposure when at F2.8 (no matter spot, center-weighted, or segment multi-pattern).

For example, I took a photo at different apertures and fixed iso in A-mode, the shutter speed the camera provided was:
++++++++++
F2.8 - 1/60s
F4.0 - 1/40s
F5.6 - 1/20s
F8.0 - 1/10s
++++++++++

However, to achieve the same exposure, the shutter speed at F2.8 should be 1/80s, not 1/60s which makes the photo at F2.8 brighter than others.

I tried this 16-50mm on my K10D but it worked normally without having such an issue. I also tried other lens like 50-135mm and Penta-A 28mm F2.8 on K-7, and the metering system worked good. Only the combination with 16-50mm and K-7 would cause this problem.

Does anyone having 16-50mm and K-7 find the same issue? Or there's something wrong with my 16-50mm or K-7?

Thank you very much.

02-27-2010, 03:08 PM   #2
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How steady was your tripod? Is it possible the scene changed ever so slightly between shots to cause the meter to read differently? Also, did you try this at a variety of focal lengths? It's possible the lens is telling the camera it's not *quite* f/2.8 at some focal lengths.
02-27-2010, 03:23 PM   #3
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Heh, interesting. I'm noticing the same thing :-D

What's funny is that it's also the case even if the shutter speed seems to be correct.

That is, an f2.8 shot @ 1/80s looks brighter (half stop?) than an f4 shot @ 1/40s.


Hmm. Since metering is done wide open, I wonder if the aperture mechanism is closing the blades too far. I.e., the f2.8 shot is the "correct" exposure (since it's shot wide open), but all the other ones are turning out slightly *underexposed*, because the calculated aperture/shutter speed is correct, BUT, the blades are being stopped down about a half stop too far. That's the only mechanism I can think of that'd explain this.


As for Sleepy's example, it's not inconceivable that the calculated shutter speed for f2.8 is 1/60s; it could just be on the border line between 1/60 and 1/80 (I believe there aren't any shutter speeds that would go between those, like 1/70s).


I tried several focal lengths and the problem seems to go away @ 50mm, but any wider... 35mm, 28mm, or 16mm, the same deal.

Very, very interesting.
03-01-2010, 11:33 AM   #4
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Original Poster
QuoteOriginally posted by Marc Sabatella Quote
How steady was your tripod? Is it possible the scene changed ever so slightly between shots to cause the meter to read differently? Also, did you try this at a variety of focal lengths? It's possible the lens is telling the camera it's not *quite* f/2.8 at some focal lengths.
Thanks for your reply.
My tripod is not VERY steady but I've done the testing few times and I don't think it's related to the slight movement of the tripod or ball head.
Besides, this issue happens at different focal lengths.

QuoteOriginally posted by RawheaD Quote
Heh, interesting. I'm noticing the same thing :-D

What's funny is that it's also the case even if the shutter speed seems to be correct.

That is, an f2.8 shot @ 1/80s looks brighter (half stop?) than an f4 shot @ 1/40s.


Hmm. Since metering is done wide open, I wonder if the aperture mechanism is closing the blades too far. I.e., the f2.8 shot is the "correct" exposure (since it's shot wide open), but all the other ones are turning out slightly *underexposed*, because the calculated aperture/shutter speed is correct, BUT, the blades are being stopped down about a half stop too far. That's the only mechanism I can think of that'd explain this.


As for Sleepy's example, it's not inconceivable that the calculated shutter speed for f2.8 is 1/60s; it could just be on the border line between 1/60 and 1/80 (I believe there aren't any shutter speeds that would go between those, like 1/70s).


I tried several focal lengths and the problem seems to go away @ 50mm, but any wider... 35mm, 28mm, or 16mm, the same deal.

Very, very interesting.
Thanks for your reply and testing.
I guess there's something wrong with the K-7 since you have the same issue as mine and I don't see this issue on my K10D.
Hopefully Pentax could get the fixed in the next firmware.

03-02-2010, 03:58 AM   #5
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Ha, I tot I'm seeing stuff!

I remember few months back when I was calibrating focus on my DA16-50, I noticed such behaviour too.

Maybe RawheaD theory is correct?
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