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09-12-2008, 03:51 PM   #1
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How can this be?

Just got the new FA 28-105 and started some test shots.

Wanted to see how sharp the new lens was contrasted with the 50 f/1.4.Shot flat on the table from the same position (yes, I put down tape to make sure both the lens-body flange positions were the same, then did the same for the lens front just to make sure). Exif data are identical, both in RAW at 6@ f/8 with two-second delay... yet one image is "larger" than the other?

Below is a screen shot of the two unedited images@100% combined into one PS file.


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09-12-2008, 04:05 PM   #2
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I'm assuming you tried your hardest to set the zoom at 50mm, and the eXIF may even report that you succeeded. But it's pretty common that focal lengths on zooms are only approximations. And usually, it's only close to the stated focal length when you're focused at infinity - the closer in you focus, the shorter the focal length becomes.

That's a pretty big difference, though... I can't see the EXIF, but does it *really* report the focal length of the zoom is 50mm?
09-12-2008, 05:40 PM   #3
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-the zoom is totally imprecise
-most lenses very from their specified focal length, for example here the FA40 was tested to be slightly longer Lens Test: Pentax SMCP-FA 50mm f/1.4 AF - - PopPhotoDecember 2007
09-12-2008, 06:18 PM   #4
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Two images with Exif...

QuoteOriginally posted by Marc Sabatella Quote
I'm assuming you tried your hardest to set the zoom at 50mm, and the eXIF may even report that you succeeded. But it's pretty common that focal lengths on zooms are only approximations. And usually, it's only close to the stated focal length when you're focused at infinity - the closer in you focus, the shorter the focal length becomes.

That's a pretty big difference, though... I can't see the EXIF, but does it *really* report the focal length of the zoom is 50mm?
I can't figure out a way to copy the data, so here are "small" jpegs of the two mages that should have the Exif attached. Thanks for double checking, maybe I'm just reading it wrong,
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09-12-2008, 06:33 PM   #5
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Ah hah, I think I see...

QuoteOriginally posted by k100d Quote
-the zoom is totally imprecise
-most lenses very from their specified focal length, for example here the FA40 was tested to be slightly longer Lens Test: Pentax SMCP-FA 50mm f/1.4 AF - - PopPhotoDecember 2007
Well, I'll be damnned. I would have thought 50mm meant 50mm, not something sort of/around/not exactly/kinda like/nearly/almost 50mm.

No wonder I'm getting frustred, I thought this was a science and instead it's an art.
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09-12-2008, 06:47 PM   #6
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I expect the lens design part is a science, then marketing slaps on whatever number sounds reasonably close to the truth and easy to sell.
09-12-2008, 08:23 PM   #7
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The size of the two images are not the same because measurement of focal length is based on the lens focused at infinity to the optical center of the lens, which is somewhere between the front and rear elements. So any measurement from the front element to the object is pointless.
09-12-2008, 08:41 PM   #8
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QuoteOriginally posted by FHPhotographer Quote
Well, I'll be damnned. I would have thought 50mm meant 50mm, not something sort of/around/not exactly/kinda like/nearly/almost 50mm.

No wonder I'm getting frustred, I thought this was a science and instead it's an art.
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09-12-2008, 08:48 PM   #9
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You read the EXIF data correctly. Your experiment shows that at a close distance to the object, the focal length of this particular zoom lens increases above what's marked on the lens. The chip in the lens which reports the focal length obviously only takes the position of the zoom ring into consideration, not the distance to the object.
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