Advice on understanding 'zoom'

Discussion in 'Digital Photography' started by evianplus, Sep 29, 2008.

  1. evianplus macrumors newbie

    Joined:
    Sep 29, 2008
    #1
    Hello

    Apologies up front for what is probably a very stupid query. :eek:

    I've used 'consumer' cameras for some time both stills and video and am looking to get a bit more serious.

    I 'get' white balance and aperture, ISO etc but what i don't get and no book or magazine i've read has yet been able to do this for me is this:

    On consumer cameras the optical 'magnification' achieved via 'zooming in' is described as x3, x5 or x10 etc etc

    But once you move into DSLR territory obviously it isn't described in this manner but how do I compare?

    What focal length on a lens gives me approximatlely what X magnification.
    :eek:

    It's a bit like converting metric to imperial but i don't know the 'how'

    See I said this was a daft question, i'm sure i've even asked it badly!:eek:

    Thanks for any help you can give.:)
     
  2. robbieduncan Moderator emeritus

    robbieduncan

    Joined:
    Jul 24, 2002
    Location:
    London
    #2
    The x3, x5 or whatever means absolutely nothing on it's own. You cannot assume that two consumer P&S cameras with x5 lenses have the same wide/telephoto reach. All the xY (where Y is a number) means is that the telephoto end of the zoom is Y times the wide end. So a P&S camera with a 24-72mm lens would be x3 as would a 30-90mm one.
     
  3. evianplus thread starter macrumors newbie

    Joined:
    Sep 29, 2008
    #3
    Thank you

    Reading your answer it sounds obvious............................ok it was obvious but it just wasn't clicking for me:eek:
    I tend to visualise things and 'numbers' aren't, for me at any rate, 'visual'.

    But your reply makes complete sense to me and gives me the ability to compare, as i do have the details of the existing 'point n shoot' focal range so i now know how i can 'roughly' compare/visualise it in my minds eye - so to speak.

    thank you again.
     
  4. Slovak macrumors regular

    Joined:
    Jul 26, 2008
    #4
    I agree with the previous poster. In addition, the P&S manufacturers often quote the digital zoom combined with optical zoom. Optical zoom is what your lens can physically do, digital zoom happens inside the camera electronics. Simplified, it's basically digital cropping. If using one of these cameras, I always turn digital zoom off.

    Look at http://www.dpreview.com/reviews/canonsd850is/ for example. About 8th item in the table is Lens, and gives a 35-140 equivalent (optical zoom). In addition, it also offers a 4x digital zoom.

    Peter...
     
  5. robbieduncan Moderator emeritus

    robbieduncan

    Joined:
    Jul 24, 2002
    Location:
    London
    #5
    You may need to check if the focal lengths you P&S has written on it are 35mm equivalent. If they are tiny numbers (say starting at <20mm) they almost certainly are not. Also depending on the DSLR you get a crop factor so a 100mm lens will have the effective view of a 160mm lens (on a Canon crop body for example). Note that this is not actually quite the same as a 160mm focal length, but for the purpose of comparison is a sensible way to look at it...
     
  6. ChrisA macrumors G4

    Joined:
    Jan 5, 2006
    Location:
    Redondo Beach, California
    #6
    The 2X, 4X and so on numbers are actually the ones that don't mean anything. The market have just "dumbed down" the the lens specs to one number. What you should really care about is the field of view as measured in degrees. This tells you exactly how wide the lens is.

    To a first order approximation a "4X" zoom should have a field of view at one end that is four times wider at one and then at the other. "4x" tells you nothing about magnification unless you know the image scae at one end you only know the two end are a factor of four apart.

    On an SLR with interchangable lenses they tell you focal length. This number tells you exactly both zoom ratio and image scale. A lens that zooms for (say) 50mm to 100mm would be a "2x zoom" but also you'd know that at the shorter 50mm end the lens would still be a little long, good for head and shoulders shots indoors. I own a 24mm to 50mm zoom that is also "2x" but that lens is a wide angle lens at the short end.

    If you want to do a bit of math. Draw a line as wide as you camera's sensor then draw a dot centered over the line but 100mm away. Then connect the dot withthe ends of the sensor line and you have a triangle. The angle at the tip of the triangle is the width in degrees of the len's field of view. So you can see that if the triagle is shorter the angle gets wider. Basically (for "far away" subjects) a lens acts like a pin hole placed one focal length away from the sensor
     
  7. evianplus thread starter macrumors newbie

    Joined:
    Sep 29, 2008
    #7
    Thanks despite not 'getting' the other bit until you kindly explained, the details about about crop factors I do get and the manual of my old PnS had it's mm quoted with it's 35mm equivalent focal length.

    Thanks again guys:)
     

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