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10-06-2009, 07:15 AM   #1
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Tamron 28-75 f/2.8 over exposure problem SOLVED w/ new lens

I recently purchased this lens from B&H Photo. It seemsed to be slightly overexposing everything. So, I set up my K-7 on a tripod, and took the same shot at 50m with three lenses - DA18-250, Pentax A 50mm 1.4, and the Tamron 28-75. I put the camera on "P" mode and took a shot of the exact same thing with all three lenses. The DA 18-250 and the Pentax A 50mm 1.4 had overall exposure levels about the same, the tamron looked one ot two stops overexposed. The histograms confirmed this.

Is this typical for this lens? Should I just send it back for a refund or exchange?


Thanks


Last edited by Sew-Classic; 10-23-2009 at 04:54 AM.
10-06-2009, 07:20 AM   #2
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Just to make sure: you have set the diaphragm ring of the Tamon on the "A" position, right?
10-06-2009, 07:26 AM   #3
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QuoteOriginally posted by Asahiflex Quote
Just to make sure: you have set the diaphragm ring of the Tamon on the "A" position, right?
Yes- ring on the "A" position.
10-06-2009, 07:34 AM   #4
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I can't comment about the K7 yet, as I have not mapped out exposure on the K7 with any of my lenses.

All I have checked with the 28-75 F2.8 so far is on the K10D.

It appears that on the K10D using spot metering, the tamron 28-85 exposes correctly at F2.8 and exhibits a generally rising trend to over expose by .5 to .7 stops at F32.

This is based upon a change of 40 in greyscale value on the histogram representing 1 stop in the middle of the histogram. In grey scale values it rises from 105 at F2.8 to 142 at F32, and pentax normally metering to 110 greyscale.

10-06-2009, 07:38 AM   #5
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The test shots were at f2.8 on the Tamron, f2.5 on the Pentax A 50mm, and F4.0 on the DA 18-250. Since I used program mode the camera chose f stop and shutter speed. The metering was set to evaluative.

Since 2 out of the three lenses exposed correctly, I would hesitate to "blame" the camera for the overexposure with the Tamron. I'm jsut trying to decide if I should exchange it or just return it for a refund.

If this is typical for this lens, then there is no point in exchanging it.
10-06-2009, 08:13 AM   #6
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the only way to understand what is happening is to take the lens and do a series of shots against a uniform surface (i like to use paved roads or block walls)

then look at the histogram, it should be a rather sharp peak at the center. your image editing software can then give you the mean value,

I usually look at the central 10% of the frame.

You shoud check this at both extremes of the focal length range.

If your error is constant, you can simply add a exposure compensation, but you should also check that the contacts are all clean. one bad contact might influece the camera readings
10-06-2009, 08:13 AM   #7
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I've never noticed such a problem with that lens on my K10D, but I've never gone looking for it either. Still, it seems to me I would have noticed it sometime, since I have been lazy and relied on camera metering lots of times (especially when I first got the camera). When I'm shooting people I tend to use this lens in the f2.8 to f4 range, but I've shot landscapes at much narrower apertures and not really seen a problem. I'll try to remember to do some test shots the next time I have a daylight opportunity.
10-06-2009, 08:43 AM   #8
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QuoteOriginally posted by CFWhitman Quote
I've never noticed such a problem with that lens on my K10D, but I've never gone looking for it either. Still, it seems to me I would have noticed it sometime, since I have been lazy and relied on camera metering lots of times (especially when I first got the camera). When I'm shooting people I tend to use this lens in the f2.8 to f4 range, but I've shot landscapes at much narrower apertures and not really seen a problem. I'll try to remember to do some test shots the next time I have a daylight opportunity.
I didn't have to go looking for the overexposure problem. It was pretty evident from the get go. In an effort to investgate the issue, I shot the same scene with the same camera, setttings, lighting, distance, etc... This experiment showed the Tamron 28-75 exposing significantly more to the right than the other two lenses.

Based on what you say, I will assume that I just got a defective copy of that lens and exchange it.

Thank you

10-06-2009, 11:38 AM   #9
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I once had the same lens, with similar symptom.

First, test the lens at full aperture (F/2.8) - use Av mode. You should not have the overexposure problem.

Remove the lens from the camera body, and set the aperture ring at F/2.8.

Now feel the aperture lever on the lens mount, can you still wiggle it? I bet you can. (it's difficult for me to describe "wiggle." I don't mean the "linear" movement of the lever, but it feels like the lever is pivoting).

Set the aperture ring at F/8. Try to move the aperture lever very slowly and observe the aperture blades. Just at the point where the aperture blades are about to open up, you can feel the aperture lever pivoting just a bit (but the aperture blades don't move). This pivoting action is the cause of the overexposure, because during exposure, the aperture blades don't close down to the correct aperture setting (but the camera thinks they do).

The fix is to eliminate this pivoting action.

When I dissembled the lens mount, where the pivoting action started became obvious. I jammed a small piece of cardboard to prevent the lever from pivoting. That fixed the problem.

Clear as mud, huh?

Last edited by SOldBear; 10-06-2009 at 02:02 PM.
10-06-2009, 01:48 PM   #10
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QuoteQuote:
I once had the same lens, with similar symptom....The fix is to eliminate this pivoting action.
Just tested that out- no apparent pivoting or play with the ring set at f2.8.

I'm either going to return or exchange it.
10-06-2009, 07:15 PM   #11
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QuoteOriginally posted by Sew-Classic Quote
The test shots were at f2.8 on the Tamron, f2.5 on the Pentax A 50mm, and F4.0 on the DA 18-250. Since I used program mode the camera chose f stop and shutter speed. The metering was set to evaluative.

Since 2 out of the three lenses exposed correctly, I would hesitate to "blame" the camera for the overexposure with the Tamron. I'm jsut trying to decide if I should exchange it or just return it for a refund.

If this is typical for this lens, then there is no point in exchanging it.
You should be checking each lens in manual exposure mode at the same aperture and shutter speed.
10-07-2009, 03:28 AM   #12
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If you are using a tripod, be sure to cover your viewfinder because differences in stray light can effect exposure as well.
10-07-2009, 03:24 PM   #13
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QuoteOriginally posted by mistergee Quote
You should be checking each lens in manual exposure mode at the same aperture and shutter speed.
The tamron seems to overexpose at the wider apertures. I can certainly set the Pentax A 50mm f.14 to the wider f stop, but not the DA 18-250. So, I can't compare all three lenses at 50mm f2.8, etc.. Only two out of three..

IMHO, there is really no point to having a fast auto zoom if it will over expose at wider apertures. -at least not for me. I'm jsut going to send it back.

ETA: With the aperture ring at 2.8, the exposure is spot on. Using the "A" setting on the ring and setting the aperture to f2.8 in camera, it's overexposed. Same shutter speed for both shots. This really makes me think that there is some issue with this copy of the lens. All the more reason to send it back. Unfortunately, B&H is on Holiday until next week. I'm assuming that they will extend the return period by the amount of time they were shut down. Noe of my other lenses behave this way.

Last edited by Sew-Classic; 10-07-2009 at 06:09 PM. Reason: ETA
10-22-2009, 02:49 PM   #14
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All Happy Now

B&H exchanged the lens for me without a hitch. The new copy arrived today, and appears to be working flawlessly.
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