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08-17-2015, 09:36 AM   #1
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Over exposure problem

Hello all. I am on my second K3, as the first was stolen a couple of months ago. I bought the first when the cameras first came out and at teh same time bought a Sigma 70-200 f/2.8 lens. The 2 functioned perfectly.

Since I got the new K3 body, using the Sigma lens gives me over exposed images by about -3 EV. This happens regardless of the mode I am using. My other lens is a Sigma 18-250 and I have no exposure issues with that lens.

The first image below is wihout any exposure compensation. The second is with -3 EV

My guess is that the problem is with the camera. Any ideas how to solve it? Does the camera need to go in for evaluation?

I appreciate any help.

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08-17-2015, 09:42 AM   #2
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More likely to be a problem with the lens; check the aperture opening of the lens if it is smooth or not. In general, Pentax cameras are more conservative in terms of metering and it seldoms overexpose and often underexpose to preserve blown highlights.

Also, do you have any other lens to try besides the Sigma lenses?
08-17-2015, 09:53 AM   #3
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QuoteOriginally posted by SCKOMS Quote
My other lens is a Sigma 18-250 and I have no exposure issues with that lens.
Don't see how it could be the camera if the other lens works fine. More likely the aperture is sticking on the 70-200. Take the lens off the camera and move the aperture lever on the lens so that the aperture is fully open then let go. The aperture should snap closed very quickly. You might have to repeat multiple times for a good test. Check your other lens for comparison. If there is any delay or sluggishness that is causing your over exposure.

Last edited by jatrax; 08-17-2015 at 10:03 AM.
08-17-2015, 09:55 AM   #4
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check the aperture lever on the lens - is it slightly bent out of proper orientation?

08-17-2015, 10:25 AM   #5
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Thanks for the quick responses everyone. I will check the lens when I get home tonight. I just find it odd that it worked perfectly with my first K3 and not with this one. What could cause the aperture to stick, if that is the problem?
08-17-2015, 10:40 AM   #6
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QuoteOriginally posted by SCKOMS Quote
What could cause the aperture to stick, if that is the problem?
Can be oil on the aperture blades, that is fairly common on vintage glass, not so much on newer. Or even a broken aperture spring, or bent blade.

It could also be a slight bend or misalignment of the aperture lever. This should be fairly obvious as it will drag or rub against something. Might also be a bent lever on the camera but since the other lens works correctly this is less likely. Still something to look for.

Might be a combination of bent lever on the lens with a slight misalignment on the camera lever. Perhaps your old camera's lever was slightly bent, enough to offset a bend in the lens lever but the new camera is correct and thus does not play nice with the lens.

But that is all speculation. Take a good careful look at the lens and compare the aperture mechanism to the known good lens. Check the blades for oil.
08-17-2015, 11:25 AM   #7
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QuoteOriginally posted by jatrax Quote
Can be oil on the aperture blades, that is fairly common on vintage glass, not so much on newer......Check the blades for oil.
The lens was purchased new, this particular one: Amazon.com : Sigma 70-200mm f/2.8 APO EX DG HSM OS FLD Large Aperture Telephoto Zoom Lens for Pentax Digital DSLR Camera : Camera Lenses : Camera & Photo
08-17-2015, 12:17 PM   #8
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Oil is less likely on newer lenses but not impossible. Just something you should check. I'm betting it is the aperture lever.

08-17-2015, 01:12 PM   #9
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You might also try a factory reset on your camera just to make sure there's not some strange setting enabled. What that might be, I don't know - if exposure compensation reads normal.

Worth a shot, doesn't cost anything...
08-17-2015, 01:17 PM   #10
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If you have them still, take a look at the EXIF from a sequential set of images that has both good and bad images. If the aperture/ISO/shutter speed are the same on both a good and a bad image then the only thing I can think of is a sticking aperture.

If it is something in the camera then you should see different settings in the EXIF. Bracketing for example, or if you have exposure compensation set can do this. But nothing I can think of in the camera would do what you are describing.
08-17-2015, 01:24 PM   #11
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QuoteOriginally posted by jatrax Quote
If you have them still, take a look at the EXIF from a sequential set of images that has both good and bad images. If the aperture/ISO/shutter speed are the same on both a good and a bad image then the only thing I can think of is a sticking aperture.

If it is something in the camera then you should see different settings in the EXIF. Bracketing for example, or if you have exposure compensation set can do this. But nothing I can think of in the camera would do what you are describing.
For the above images the over exposed one shows 1/250s at f/11; the properly exposed is 1/1600s at f/10 with the camera set for -3 EV compensation. I don't know how to interpret that, other than the shutter speed is significantly faster.
08-17-2015, 02:32 PM   #12
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QuoteOriginally posted by SCKOMS Quote
For the above images the over exposed one shows 1/250s at f/11; the properly exposed is 1/1600s at f/10 with the camera set for -3 EV compensation. I don't know how to interpret that, other than the shutter speed is significantly faster.
But it looks like the exposure mode changed between images as well. What I was trying to get was two images, one good, one bad, where no settings were changed. Might give us some additional information.
But if I'm reading those correct they are still the same exposure. 1/250 is roughly three stops slower than 1/1600 and with the small aperture change I think they are the same.

What exposure mode are you using for these? The first one says "Creative Program (based towards depth of field)" not sure what that is?
08-17-2015, 03:11 PM   #13
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QuoteOriginally posted by jatrax Quote
But it looks like the exposure mode changed between images as well. What I was trying to get was two images, one good, one bad, where no settings were changed. Might give us some additional information.
But if I'm reading those correct they are still the same exposure. 1/250 is roughly three stops slower than 1/1600 and with the small aperture change I think they are the same.

What exposure mode are you using for these? The first one says "Creative Program (based towards depth of field)" not sure what that is?
I don't recall making any other changes. Both images were shot in "P" mode as far as I recall.
08-17-2015, 06:28 PM   #14
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exif says metering goes from center weight (blown pic) then to pattern (-3).......could the metering be the issue?
08-17-2015, 07:56 PM   #15
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UPDATE: So checking out the lens, the aperture seems to be stuck wide open. Moving the lever does nothing, and it does not seem to be bent. When I mount it to the camera and set a small aperture (f/19), pushing the on/off switch for depth of field preview does not close down the aperture either. After flicking the lever repeatedly, the blades closed slightly and move with the lever, but do not close all the way. So it looks like it is the lens and needs to go back to Sigma.

Has anyone had an issue like this before?
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