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06-02-2008, 07:37 AM   #1
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K100D registers wrong aperture

Is there any reason why a K100D should show an aperture of f/1.7 for a lens with a maximum aperture of f/1.8?

The lens is a Sigma 24mm f/1.8.

I don't know if the other apertures are registering accurately or not. (I have no way to tell.)

06-02-2008, 07:45 AM   #2
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QuoteOriginally posted by Timarete Quote
Is there any reason why a K100D should show an aperture of f/1.7 for a lens with a maximum aperture of f/1.8?

The lens is a Sigma 24mm f/1.8.

I don't know if the other apertures are registering accurately or not. (I have no way to tell.)
Probabaly because the lens tells wrong aperture to the camera.
06-02-2008, 07:56 AM   #3
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Make sure the contacts of the lens and body are clean.
06-02-2008, 09:44 AM   #4
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QuoteOriginally posted by Timarete Quote
Is there any reason why a K100D should show an aperture of f/1.7 for a lens with a maximum aperture of f/1.8?

The lens is a Sigma 24mm f/1.8.

I don't know if the other apertures are registering accurately or not. (I have no way to tell.)
Is it a KPR MF lens?

06-02-2008, 10:10 AM   #5
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QuoteOriginally posted by RiceHigh Quote
Is it a KPR MF lens?
The lens is a brand new Sigma 24mm f/1.8 EX DG Macro AF.
06-02-2008, 10:34 AM   #6
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I've had that happen to me before. I think it was my FA50 when set to f/1.8, the EXIF reported f/1.9. I just chalked it up to it being something like f/1.849, and the lens rounded down, and the camera rounded up. No biggie for me.
06-02-2008, 10:41 AM   #7
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Timarete,
I did a little research and the lens interface codes don't seem the support the F1.8 stop. This would explain why it goes to 1.7, which is the closest stop to f1.8. Here is the link, with pinout and codes.

Dave

Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography
06-02-2008, 12:48 PM   #8
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QuoteOriginally posted by Big Dave Quote
Timarete,
I did a little research and the lens interface codes don't seem the support the F1.8 stop. This would explain why it goes to 1.7, which is the closest stop to f1.8. Here is the link, with pinout and codes.

Dave

Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography
At the bottom of that page, it says there will be no effect for Pentax dSLRs. Maybe they read the aperture differently? And I must have been mis-remembering, because I just took a shot with my K10D/FA50 at f/1.8, and the lens, camera, PEF EXIF, and JPEG EXIF all listed f/1.8.

06-02-2008, 02:45 PM   #9
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QuoteOriginally posted by joefru Quote
At the bottom of that page, it says there will be no effect for Pentax dSLRs. Maybe they read the aperture differently? And I must have been mis-remembering, because I just took a shot with my K10D/FA50 at f/1.8, and the lens, camera, PEF EXIF, and JPEG EXIF all listed f/1.8.
Hmm. So then why is my K100D reading this Sigma lens incorrectly? Could it be that only Pentax lenses read the f/1.8 aperture?
06-02-2008, 03:02 PM   #10
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QuoteOriginally posted by Big Dave Quote
Timarete,
I did a little research and the lens interface codes don't seem the support the F1.8 stop. This would explain why it goes to 1.7, which is the closest stop to f1.8. Here is the link, with pinout and codes.

Dave

Pentax Lens Modifications for Matrix Metering - Mark Roberts Photography
How can they not support f/1.8? Does the body report another aperture when used with a 31 ltd attached?
06-05-2008, 08:43 AM   #11
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I think that for known own Pentax lenses they have data base of Open aperture numbers (or lens itslef is trasfering in by data contact).
WHile sigma lenses don't transfer this and camera guesses it by other "aperture" contacts on bayonet
06-06-2008, 12:08 AM   #12
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I think there was a thread regarding either the very same sigma lens, or Sigma 28 which was registering as 1.7 as opposed 1.8 that it should. I don't know what was the consensus as for why, but you may try to search the forum. ...
06-06-2008, 02:46 AM   #13
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QuoteOriginally posted by Timarete Quote
The lens is a brand new Sigma 24mm f/1.8 EX DG Macro AF.
So, it is just an incompatibility problem where the lens told the camera a not-so-exact reading.
06-06-2008, 06:11 AM   #14
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Not trying to be a wise guy, but does it really matter between 1.7 & 1.8? I'm a newbie, and just curious.
06-06-2008, 06:33 AM   #15
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QuoteOriginally posted by cardinal43 Quote
Not trying to be a wise guy, but does it really matter between 1.7 & 1.8? I'm a newbie, and just curious.
Technically, it's 1/4 a stop. You decide if that's important to you or not.
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