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08-15-2012, 12:57 PM   #1
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K20D and Tamron Lens

I purchased my K20D about a year and a half ago on eBay and I love it. Everything functions as it should no matter what compatible lens I put on it except for one problem. When attach my Tamron, AF70-300mm f4-5.6 LD Di lens and run it out to 300mm and take the shot the camera has recorded it as being taken at 148mm no matter if in regular or macro. Makes no difference what I do all I get is 148mm. Tamron has yet to give me an answer as to what might be happening. All they want is to have me send it in. I want to have some idea what's goin' on before I send it to them as the last time I sent a lens in to be adjusted nothing was done to it. It came back in the same condition as when I sent it in. I don't know anyone else that shoots Pentax so I can't try it out on another camera to see the results. Could I be missing a setting within the camera itself? I've looked at everything and can up with nothin!
I've had real good results with all of my non digital Tamron lenses and is big reason why I got the 70-300 but I'm losing my confidence with them. Someone please come up with some reasonable explanation as to what may be happening. Thanks, everyone, for your time, DMAmaral

08-15-2012, 02:52 PM   #2
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No explanation but I do have a question. If you zoom to say 290mm what is the reading? And if you creep out a bit at a time what are the readings as you go out? Here is a theory: the zoom ring actually goes past 300mm and the mechanism that sends the focal length to the camera gets confused past that point so sends something that is either wrong or just confuses the camera. Again, just a theory, try testing it and see.

There is nothing in the camera that would affect this; the lens is going to send the EXIF info to the camera and the camera records it within the image file.
08-15-2012, 02:54 PM   #3
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How about when you have the camera at something like 250 mm? Does it do the same thing?

I don't have much of an answer for you although I had the same lens on my K10d and never had any problems with it as in your case. However, out of all my Tamron lenses (4 total), it is the only one I no longer use.
08-15-2012, 03:42 PM   #4
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Here's a few numbers I've come up with. Lens set @ 100mm camera says 200mm
135mm " 148mm
180mm " 200mm
230mm " 200mm
280mm " 160mm
Again, lens set at in the first column and camera info in second column. I don't know how this slipped by me for so long. The photos have seemed to come out ok but how am I to know how exact they might be at this late date. Oh well

08-15-2012, 03:57 PM   #5
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Hmmm... not sure why they might go down. I can't say I ever worried too much about the numbers as the images always came out right.

I do know that it isn't unusual for the numbers you have towards the top there to be off. The actual focal length for most lenses vary a bit depending on how the lens is focused. Just play a bit with that lens in manual focus mode and look at how it adjusts as you adjust focus. You can set the zoom to a fixed 200 mm but the actual reported zoom in EXIF will vary according to where you are focusing on. I thought I had seen (in these forums somewhere) that most zoom lenses that zoom to 300 mm rarely actually get you 300 mm due to the focus. I seem to recall suggestions that 300 mm could be as low as a real 240 mm. You might search a bit.

This could be worse with Macro, but I'm not certain on it, and I'm not sure your issue is at all related to what I am talking about above. The above issues are due to the specific mechanics and physics of your lens and the focus process. What you are seeing may in fact be related to the lens electronics or even your camera's electronics.

What you could probably do is take a few shots with the lens at full zoom (or perhaps at 280). Just keep the zoom fixed while you run the test. Set the focus to manual mode.

Try a shot at the minimum focus setting, one at the maximum focus setting, and perhaps some at the extremes for the macro mode and see how the camera reports them. Manual mode means you don't have to worry about items actually being in focus, you just want to test the lens out. See whether the numbers vary and how much they vary by.

You could probably do the same thing at 70 mm (excepting the Macro mode, which i remember doesn't work that wide out). In that case, I doubt you'll have as much variation as the lens moves less during focusing.
08-15-2012, 05:45 PM   #6
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Thanks. I'm gonna try everything I can. I may have found someone in my neighborhood that has Pentax equipment so that could be a big help. I'll know in while. Thanks again.
08-16-2012, 06:24 PM   #7
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I have similar problems with my Tamron 17 - 50 2.8 on a K-5 and K200d. (seems to default to 32mm) It wouldn't be that much of a problem, except the that the Pentax body based shake reduction needs accurate focal length info to work properly.Much of the time I just leave it off when using that lens. After all, I muddled through for 35 years without it.
01-18-2013, 11:37 AM   #8
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Upon getting fed up with the lens I finally sent it in to Tamron over the Xmas holidays. They say that they tore it apart, lubed everything and adjusted all that needed to be taken care of. All works well so far, all readings on the lens match the camera. I have had several Tamron lenses, for many years, with no problems whatsoever but this is the second issue that I have had with this lens in two years (only lens I've needed to send back). At this rate I wonder what will go wrong with the damn thing in the years to come. Thanks you to all that responded to my inquires. Go Pentax!!!

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