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01-26-2013, 02:22 PM   #1
Pentaxian




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actual focal length of 55-300DA

It seems like I read a review/test of the 55-300DA that indicated that the measured focal length at infinity focus of the sample tested was somewhat longer than 300mm. Of course I can't find that review now and am wondering if anyone has a link, or knows of an actual measurement (at both ends) of the focal length range of this lens? Thanks

Paul

01-26-2013, 03:13 PM   #2
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Lowell Goudge's Avatar

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Why not measure it yourself

Take your camera and look at the sensor size, get this exactly, don't use 16x24 but the actual one,

Take a shot of something you can measure at. A reasonable butnknown distance, and then measure the pixels of thenobject. Work out the magnification from that
01-26-2013, 03:39 PM   #3
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Although the lens has no IF in its name, I am pretty sure the focal length will change as function of the focus distance, so beware at what object distance you measure the focal length, best do it at the infinity setting.
01-27-2013, 10:27 AM   #4
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Original Poster
QuoteOriginally posted by Lowell Goudge Quote
Why not measure it yourself

Take your camera and look at the sensor size, get this exactly, don't use 16x24 but the actual one,

Take a shot of something you can measure at. A reasonable butnknown distance, and then measure the pixels of thenobject. Work out the magnification from that
I don't know how lens test identify the focal length, but I know they can seemingly do it to a fraction of a mm... and I don't think measuring pixels of objects at infinity would be precise enough. I'm pretty sure the infinity part is required, and wouldn't that require testing with very, very large objects of precisely known dimensions?

Paul

01-27-2013, 01:27 PM   #5
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older not wiser's Avatar

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Amateur Photographer in the UK used to have a chap who did technical measurements of lenses as part of their reviews. As I remember very few lenses actually had the focal length stated. I think manufactures put a focal length on the lens that is near to a logical number such as 55-300 mm as opposed to 53.9-306.2 mm, it just looks neater.
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