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04-02-2018, 10:55 PM   #1
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Exif FL info is wrong for Tamron 28-75mm

Hi,

For Tamron 28-75mm, Exif shows wrong focal length throughout the range.
28mm = 33mm
50mm = 55mm
75mm = 63mm

Then I wanted to compare the area covered at 75mm against the area covered by DFA 70-200mm at 70mm. Latter covered more width than Tamron lens. This means 63mm is probably right.

Since I know the distances, and I thought of computing the FL myself. So I went to a website and entered 'distance to subject'(202cm) & 'width of subject'(96cm),and FL came out as ~75.

As you see in the picture below this lens is sharp at 2.8. So I am keeping this lens anyhow. Is this a known disease?

Please see the attached pictures. Pictures are from K1.

Tamron 28-75

Pentax 70-200

Attached Images
 
04-03-2018, 01:17 AM   #2
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It's normal for there to be some slack in the range, but in this case perhaps the focal length sensor in the lens is malfunctioning.

Adam
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04-03-2018, 05:03 AM   #3
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One known disease ("focus breathing") suffered by many zooms and even some primes is that the focal length shifts with distance. At closer distances, the long end especially will be shorter than the spec.

A second disease is that manufacturers typically cheat a bit.
04-03-2018, 06:21 AM   #4
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QuoteOriginally posted by pentaxfall Quote
Hi,

For Tamron 28-75mm, Exif shows wrong focal length throughout the range.
28mm = 33mm
50mm = 55mm
75mm = 63mm

Then I wanted to compare the area covered at 75mm against the area covered by DFA 70-200mm at 70mm. Latter covered more width than Tamron lens. This means 63mm is probably right.
If 70mm shows wider than the Tamron, then the Tamron is giving you a longer effective focal length, not shorter. Your calculated 75mm seems accurate. The one caveat is focus breathing. Take another shot at a long distance and compare those as well, if t he 70mm is still wider, 75 would seem accurate.

The SR system may be slightly under correcting at the long end and overreacting at the short end due to the misreading.

04-03-2018, 08:26 AM   #5
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When this sort of thing happens, the usual culprit is the focal length "feeler" that's attached to the zoom barrel. It gets adjusted at the factory to line up with the circuit track glued onto the inner barrel. Sounds like yours is jiggling loose.
04-03-2018, 10:09 AM   #6
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QuoteOriginally posted by pentaxfall Quote
Hi,

For Tamron 28-75mm, Exif shows wrong focal length throughout the range.
28mm = 33mm
50mm = 55mm
75mm = 63mm
I suspect that the reason is that the Tamron is not geared for full frame (as in K1). Can you do that with another Pentax DSLR?
04-03-2018, 10:36 AM   #7
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Thanks for all the info. In couple of days I will compare by taking few shots out door. I had heard of 'focus breathing' but had not expected it to happen to my lens


QuoteOriginally posted by UncleVanya Quote
If 70mm shows wider than the Tamron, then the Tamron is giving you a longer effective focal length, not shorter.
The SR system may be slightly under correcting at the long end and overreacting at the short end due to the misreading.
I would like some clarification.
In this case, EFL is 63mm or 75mm?
When you say SR, are you referring to shake-reduction?. I had turned off SR because camera was on tripod.


QuoteOriginally posted by Ontarian50 Quote
When this sort of thing happens, the usual culprit is the focal length "feeler" that's attached to the zoom barrel. It gets adjusted at the factory to line up with the circuit track glued onto the inner barrel. Sounds like yours is jiggling loose.
I hear minor sound. Not louder when compared to other lenses. Recently I had sent the lens for AF tuning.

---------- Post added 04-03-18 at 10:46 AM ----------

QuoteOriginally posted by danielchtong Quote
I suspect that the reason is that the Tamron is not geared for full frame (as in K1). Can you do that with another Pentax DSLR?
Even on K3, Exif says 63mm @75mm. I have not verified it by measuring the area of the subject.

I believe this lens uses an older design, made for FF. This is the second copy. First one I purchased at around 2004 and sold it to PDML member. Then purchased the 2nd copy(brand new) at 2007.
04-03-2018, 11:26 AM   #8
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I don't know the effective focal length but in your post you asserted that since the view on the 70mm was wider than your lens; the lens claiming to be 63mm made sense to you. I am saying no, the opposite is true. If your lens was 63mm actually and you are using the existing 70mm as a benchmark - then your lens is narrower than this and must have a focal length longer than 70mm. However it is true that focus breathing could make the 70mm much wider than normal and it's effective focal length could be shorter than 70mm. However we haven't established that yet. At this point I'm only pointing out that the logical conclusion that your lens is 63mm based on it being narrower than a 70mm lens field of view is flawed. That's the reverse of what I would conclude.

04-03-2018, 09:02 PM   #9
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Here are two outdoor shots and in both Tamron covers narrower area. Top picture is from Pentax DFA 70-200mm @75, and lower one is from Tamron 28-75 @70mm
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04-03-2018, 09:07 PM   #10
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QuoteOriginally posted by pentaxfall Quote
Here are two outdoor shots and in both Tamron covers narrower area. Top picture is from Pentax DFA 70-200mm @75, and lower one is from Tamron 28-75 @70mm
Distance to the subjects? On my DA* 60-250 focus breathing makes it shorter than advertised until about 10 meters, but to be safe I'd test at 30 meters.
04-03-2018, 09:55 PM   #11
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QuoteOriginally posted by UncleVanya Quote
Distance to the subjects? On my DA* 60-250 focus breathing makes it shorter than advertised until about 10 meters, but to be safe I'd test at 30 meters.
For the 2nd image distance is more than 100ft. I went back to original image and counted number of parking slots. Between the camera and the wall, there are 20 parking slots, spot for tree lane, one-lane 2-way road and a pavement . It all should add to more than 100ft

Articles that I read also recommended to maintain a distance of at least 30m. But the distance scale on DFA 70 - 200mm goes until 10m/30ft, and after that it becomes infinity.

Last edited by pentaxfall; 04-03-2018 at 10:01 PM.
04-04-2018, 05:36 AM   #12
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QuoteOriginally posted by pentaxfall Quote
For the 2nd image distance is more than 100ft. I went back to original image and counted number of parking slots. Between the camera and the wall, there are 20 parking slots, spot for tree lane, one-lane 2-way road and a pavement . It all should add to more than 100ft

Articles that I read also recommended to maintain a distance of at least 30m. But the distance scale on DFA 70 - 200mm goes until 10m/30ft, and after that it becomes infinity.
Wow! Ok. That seems to suggest that either the DFA 70 is wider than expected or the Tamron is narrower. I'm stumped.
04-04-2018, 01:04 PM   #13
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QuoteOriginally posted by UncleVanya Quote
Wow! Ok. That seems to suggest that either the DFA 70 is wider than expected or the Tamron is narrower. I'm stumped.
I think Tamron may be narrower because Pentax reputation is higher than Tamron's
04-04-2018, 01:07 PM   #14
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QuoteOriginally posted by pentaxfall Quote
i think tamron may be narrower because pentax reputation is higher than tamron's
lol
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