Calculation of focal length for dSLR lenses

Discussion in 'Lenses and Accessories' started by Nicky2007, Mar 20, 2009.

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  1. Nicky2007

    Nicky2007 Well-Known Member

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    I'm an old school photographer, brought up on 35mm SLR cameras. During those days, I knew that when a lens manufacturer specified that a certain lens is a 50mm lens, it's a 50mm lens.

    With lenses for dSLR, the focal length specs seem to have changed. For example a dSLR 50mm lens is equivalent to 75mm in 35mm format. However, when it comes to zoom lenses, everything becomes fuzzy to me. For a Zmm - 200mm may mean Zmm - 300mm for one manufacturer, but may mean Zmm - 320mm for a different manufacturer, in 35mm format.

    As you can see, I can only visualize focal length from a 35mm prospective. Sorry to sound absolutely ignorant, but can someone tell me how focal length for dSLR lenses are calculated? And how I can work out the 35mm format equivalent, so that I can get a handle on this fuzzy problem of mine.

    Many thanks in advance.
     
  2. Andy Stanton

    Andy Stanton Super Moderator Super Moderator

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    DSLR's have a crop factor that you use to determine the 35mm equivalent. Nikon's DSLR's with the APC sensor have a crop factor of 1.5 and Canon's are 1.6. Olympus 4/3 DSLR's have a crop factor of 2. Pentax and Sony DSLR's also have a crop factor of 1.5.

    Thus the 35 mm equivalent of the kit lens that comes with the Nikon D60, 18 - 55mm, is 18 x 1.5, or 27mm, by 55 x 1.5, or 82.5mm.
     
  3. rp2s

    rp2s Active Member

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    Andy is correct about the different crop factors. They are the result of sensors not being the same size as 35mm film negatives. The APC sized sensors used by Sony and Pentax (1.5 crop) are a little larger than what is used by Canon (1.6 crop), which results in the slight discrepancy between lens specs.

    You could also purchase a Full-Frame Digital SLR, where the sensor is the same size as 35mm film and not have to worry about "crop factors", what you see in the lens, is what you have.
     
  4. CalebSchmerge

    CalebSchmerge Super Moderator/Reviewer News/Review Writer

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    I know the question is answered, but I wrote an article that explains the crop factor and will give you a little bit more practical information about it. You can read it here.
     
  5. Nicky2007

    Nicky2007 Well-Known Member

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    Thanks guys. Now that makes a lot of sense. And Caleb, thanks for that article. It was very helpful indeed. I'll let you if I've anymore questions pertaining to the crop factor later.

    And no, I don't think I can afford a full format camera right now. I lost my job a few months ago. Meantime, I'll have to make do with a compact digital camera until the light shines at the end of the tunnel ;)
     
  6. KCook

    KCook Well-Known Member

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    shameless bump
     
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