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05-29-2013, 06:33 PM   #1
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Determining Focal Length

Hello all. I have been browsing the Pentax forums for a long time and for once they've come up short in answering my questions so hopefully someone can directly.
I have a Pentax Kx and absolutely love it as my first dslr. I still own, and occasionally use, my original kit lenses but about a year ago I got a great deal on my Vega 12b 2.8/90. As far as I know the adapter that I got with it allows infinity focus but what I'm really wondering is what the actual focal length of the lens is. I've read several different articles on how to convert the focal length from medium format to my sensor size and come out with two totally different results. Then through some trial and error work of my own I came to my own conclusion.
From a fixed position I shot a photo of a book (with the vega) and aligned the brackets on the finder to the height of the book, then from the same position using my zoom lens I zoomed until the book matched the same point in the bracket. Oddly enough the focal length that the zoom reads is roughly 90mm.
What I really want to know is does the lens remain 90mm, or was my own test somehow flawed and if so how do I actually determine the focal length?

Perhaps a petty question but this has irked me for a year every time I turn on the camera with that lens and it asks me to input the focal length lol.
Any info would be greatly appreciated.


Also a couple pics taken with said Vega 12b, constructive criticism always welcome.

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05-29-2013, 07:23 PM   #2
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Focal length doesn't change unless your adapter is also a teleconverter (which it isn't). However, on APS-C, a 90mm will not have as wide of a field of view as on medium format.

https://www.pentaxforums.com/forums/pentax-lens-articles/93714-field-view-tab...d-645-6x7.html

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05-30-2013, 09:19 AM   #3
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Yep, focal length stays the same regardless of whatever the lens is attached, crop factor changes, FOV too. OK, maybe itīs nice to know what is equivalent to what, but seriously, whatīs the point... Really nice shots, btw!
05-30-2013, 10:42 AM   #4
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Focal length is a physical property of the lens itself, and as such is independent of the format of sensor. The confusion comes from marketing types who try to use the focal length multiplier, to address the different field of view a lens will have on a cropped sensor, when compared to a 35mm film frame.

Inwish it would all go away, but.....

05-30-2013, 03:09 PM   #5
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Thank you everyone for clearing that up for me. After my little experiment I kind of figured that the focal length stayed the same across formats but I really wasn't sure. Regardless it's still probably my favorite lens to shoot with next to my Helios 44. I find using older manual lenses much more rewarding than my kit lenses for some reason, probably because I have to put more effort into composing the picture.
06-09-2013, 05:47 PM   #6
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QuoteOriginally posted by camwill87 Quote
Thank you everyone for clearing that up for me. After my little experiment I kind of figured that the focal length stayed the same across formats but I really wasn't sure. Regardless it's still probably my favorite lens to shoot with next to my Helios 44. I find using older manual lenses much more rewarding than my kit lenses for some reason, probably because I have to put more effort into composing the picture.
And you shot slower and hold your breathe before taking a shot.
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