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Do lenses of the same focal length behave the same regardless of format?

Discussion in 'General Equipment Chat & Advice' started by tamos, Oct 3, 2000.

  1. tamos

    tamos Member

    I fancy an XPan and this has caused me to wonder about the behaviour of lenses for different formats. Now in the case of the XPan there's a single lens for two formats (24*36 and 24*65) and the angle of view is different depending on format but does depth of field and such behave the same or does it take on the characteristics of a wide-angle lens (25mm) to match the coverage? Would the 90mm lens used in 24*65 produce the same picture characteristics as the 45mm for a 24*36?
    This isn't just limited to the XPan, I wonder what the characteristics of medium format lenses are compared to 35mm. Do they correspond? Are they equal at the focal length or the angle of view?
     
  2. Col. Hogan

    Col. Hogan Well-Known Member

    I know that Mamiya has the following lenses for their 7II camera and they are:
    43mm [21mm], 50mm [25mm], 65mm [32mm], 80mm [39mm], 150mm [71mm] and a 210mm, but I don't know what the 35mm conversion is for that one. The numbers in brackets above are the 35mm equivalents for these particular lenses. I don't know how they calculate this.
     
  3. Steve Lewis

    Steve Lewis Member

    Tamos

    I may be mistaken but as far as I am aware, there is a difference between angle of view and angle of coverage. Also the angle of view is not the same for lenses of the same focal length. Certainly in large format photography (5 x4 etc) two lenses can have the same focal length but different angles of view and coverage.

    I suspect that all the lenses for the Xpan have the angle of coverage for 24 x 65 and therefore only use the centre of the coverage when you switch to 24 x 36. This angle of coverage is probably the same for all the lenses, regardless of focal length or angle of view.

    Also, as far as I am aware and all other things being equal, the depth of field on a lens is the same regardless of the film format. This is why users of medium or large format cameras struggle with depth of field. The 90mm lens on my RB67 may have approximately the equivalent angle of view as a 50mm on 35mm format, but it still has the depth of field characteristics of a 90mm optic.

    Hope this helped.

    Steve
     
  4. tonymidd

    tonymidd Well-Known Member

    Steve is about right with his reply except that those of us who use large format do not stuggle with depth of field, we've camera movements so can get sharp focus from immediadtly in front of the camera to infinity at full aperature, depending up on the angle of coverage of the lens. Your profile does not give any info so I'm not sure of your location but if you are in the UK get a copy of the latest Calumet (KJP) catalogue there is a lot of info on this subject in there. Other wise look the subject up in a good book!
     
  5. 0

    0 Guest

    Your advice to 'look it up in a good book' is about the soundest advice I've read in these postings of late...........
     
  6. Steve Lewis

    Steve Lewis Member

    Tony

    Would I be right in thinking that by employing the cameras movements you are bringing the plane of focus more parallel with the plane of the subject rather than increasing the actual depth of field provided by the lens alone ?

    Steve
     
  7. tonymidd

    tonymidd Well-Known Member

    Steve,
    Yes; you are right but the effect is an apparent increse in depth of field which is what matters. I did not want to complicate matters but being specific as to how the effect is achived. Any one interested in movements on large format cameras have a look at the KJP ( Calumet) catalouge page 31; these are the best explanation drawings I've seen, and page 33 for angle of view data.
     

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