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12-07-2011, 05:52 AM   #1
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Tamron 17 - 50 2.8 flakey zoom data.

Shaking down the K-5 I noticed in the EXIF that this lens was frequently feeding erroneous zoom data to the camera. (seems to favor "32mm" longer than 24mm) Checked it against the K200d and found it was doing the same. Has anyone else had the same problem. Would just turning off the SR be better than chancing incorrect sensor compensation?

12-07-2011, 06:18 AM   #2
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I don't have that lens, but I have their 28-75, which is probably almost identical except for the glass. I have noticed it likes to report incorrect focal lengths. (Haven't tried shaking it or testing properly, but it definitely gets it wrong sometimes.)

SR probably works better than no SR as long as the FL is reported as shorter than actual. But this is just my guess, it's no substitute for hours of boredom testing it.
12-07-2011, 09:18 AM   #3
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Have you tried cleaning the contacts?
12-07-2011, 10:32 AM   #4
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The value has always been more or less an approximation, and since the zoom range is so small I don't find your results surprising at all. 32mm at 24mm might be a bit exaggerated, though.


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12-07-2011, 11:13 AM   #5
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I misphrased that. 24mm is the longest zoom setting that is consistently (fairly so) accurate. After that it may read 32mm all the way to 50mm. Is this analog current measured by the camera or a digital signal created by the lens? I did notice that it got more reliable after several turns of the ring.
12-07-2011, 11:16 AM   #6
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I have noticed less than 50mm in the EXIF with it when the lens has been set to 50mm. My understanding is that the EXIF value is correct and this due to its design where the FL set on the zoom ring is accurate only when the lens is focused to infinity. Also, this sort of thing / design is common with zoom lenses. The lens reporting its actual FL to the body is good IMO, if for no other reason than SR getting the correct data to work with.
12-07-2011, 11:33 AM   #7
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I shoot my Tamron 17-50mm at 17mm quite often and lately it always shows in the exif as 45mm.
12-07-2011, 11:55 AM   #8
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QuoteOriginally posted by krp Quote
I shoot my Tamron 17-50mm at 17mm quite often and lately it always shows in the exif as 45mm.
That sounds like a *problem* (as opposed to the "feature-not-a-bug" of the FL being slightly short of the zoom ring setting due to the lens not having been focued to infinity). With luck, cleaning the contacts, as mentioned might indeed fix it. (I'd put a bit of rubbing alcohol or Eclipse on an old microfiber lens cloth, rub the contacts with the moist spot of the cloth and then repeat with a dry spot the idea being getting whatever crud there might be mostly removed and then mopping up the rest (now hopefully in solution)).

12-07-2011, 12:05 PM   #9
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QuoteOriginally posted by jolepp Quote
I have noticed less than 50mm in the EXIF with it when the lens has been set to 50mm. My understanding is that the EXIF value is correct and this due to its design where the FL set on the zoom ring is accurate only when the lens is focused to infinity. Also, this sort of thing / design is common with zoom lenses. The lens reporting its actual FL to the body is good IMO, if for no other reason than SR getting the correct data to work with.
Last night's test were inside at about range of five feet. I just took the camera out and tried the series at infinity. (actually a tree top about 200 yards away, it's pretty socked in here today) Four shots at the zoom markings on the lens, 17 - 24 - 35 - 50.
Results:
17, 32, 36, 50
17, 32, 36, 32
17, 23, 34, 32.

It seems, with my lens at least, if the chip can't get a definitive reading, it arbitrarily kicks out "32."
12-07-2011, 12:22 PM   #10
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Hmm ... guesswork: 17+50 = 67, 67/2 = 33.5 which is close to 32. Could be the body using a middle of the range FL when it loses communication with the lens (?). I think the body gets the FL info as a stream of bits via the data pin. It could be a simple arrangement where the body periodically asks the lens to send it a block of data including the current FL, if there is a problem with the contact / circuit the body would time out and carry on with a FL in the middle of the zoom range.
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