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05-05-2011, 01:13 PM   #1
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A bit of confusion on Sigma 24-70mm F2.8 EX DG MACRO (AF Lens)

I just bought this used from the marketplace, and while the pictures look great and all that, but I am confused about focal lengths read on the computer.

I have a K-x, so there is no focal length indicator on the viewfinder.
This has not been a problem, since I use primes for the most part.

Anyways, after some test shots, I found that the camera can't lock on the focal length indicated on the ring.
On the ring, it has 70, 50, 35, 28 and 24 indicated, but when I tried to take pictures with it, this is only capable focal lengths my camera will read :
70, 65, 60, 53 instead of 55, 48 instead of 50, 36 instead of 35, 33 instead of 28, and 24.

Is it normal for the camera to skip recording focal lengths like that?
I can't remember the last time I used a zoom lens, and I don't recall things like that grabbing my attention, so I'm just very confused.

I mean, this doesn't matter at all when it comes to taking shots.
It's just that on the file, it's giving me the wrong focal length info.

05-05-2011, 01:48 PM   #2
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my copy does the same thing dont know why
05-05-2011, 02:14 PM   #3
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My Pentax 50mm f1.4 shows up in Lightroom as having a 55mm focal length. It's likely just an aberration in the lens, camera firmware, or somewhere else. Personally, when composing photos, I frame as I see fit & don't worry about the reported focal length at the time of shutter release ...
05-05-2011, 02:18 PM   #4
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So, after some testing, I think it's the K-x's fault.
I used the Kit lens 18-55mm to test it, and the results are similar.
Even if I tried to inch the zoom VERY slightly to see if it'll record it by 1mm, it won't.
It went from 55, 50, 47, 42, 40, 37, 35 (kit lens indicates 50mm and 35mm, but not on sigma...odd. must be some third party compatibility issues), 32, 30, 28, 26, 24, 23 (the first 1mm difference!), 21 (aww. 2mm again), 20, then 18.

Like I said, this is just something I just noticed.
It won't effect the quality or framing of a shot in any sort way, but just curious, was all.

05-05-2011, 02:32 PM   #5
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it is most likely lens focal length electronic identification of the camera or pp software. it's not really an issue other than not giving a precise measurement nor identification of which lens is being used. it is most likely approximation at best, especially with zooms. e.g. at 115mm, it will register as 135mm on the exif.
05-08-2011, 07:57 PM   #6
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I see. Thanks for all the responses!
05-09-2011, 10:03 AM   #7
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when you open a lens, you notice that the focal length and focus distance sensors arent continuous, but discrete. so they dont register every movement
05-09-2011, 09:22 PM   #8
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So i don't have a ton of different lens i use but the other day i kind of liked that the sigma posted a different focal length when i looked though a few hundred i snapped off last sunday. I knew i wanted a photo that i shot with my F50 but couldn't find it at first when i looked though the pics on the computer then i found that i could sort the photos by 35mm focal length i did that and boom the 75mm one popped up and thats what i needed.

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