large format lenses; focal length vs. angle of view?

Discussion in 'Large Format Cameras and Accessories' started by bicycletricycle, Nov 8, 2005.

  1. bicycletricycle

    bicycletricycle Subscriber

    Jan 28, 2005
    i am looking into getting a wide angle lense for 4x5 use and i am coming fron the world of 35mm. I assumed that the shorter focal length a lense had the wider the angle of view. While comparing 2 used lenses i ran into something unexpected, The two lenses were a rodenstock grandagon n 65mm f4.5 and a schneider super angulon 90mm f5.6. The rodenstock specs show that thi slense has an image circle of 170mm at f22 and an angle of view of 105 deg. at f22. the schneider specs show the 90mm f5.6 super angulon(not XL) to have an image circle of 235mm at f22 and an angle of view of 105 deg at f22.
    I dont understand, wouldn't the 65mm rodenstock need a wider angle of view to cover the same format, isee that the image circle is a bit bigger on the schneider so that could explain a little bit of the difference. A quick look at the schneider specs for their super angulon series of lenses shows that the 47mm and 38mm super angulon XL's have the same angle of view (120) and that the 72mm has only 5 degrees less view at 115. I thought that the term wide angle reffered to the increased angle of view. Why would someone choose a shorter focal length lense with the same angle of view as a slightly longer lense, especially since the shorter lense should exhibit more drop off near the edges? Thanks in advance, ifeel so stupid when it comes to this stuff and you guys are always great. Any comments about the two lenses mentioned first as far as which one might be a better choice for architecture kind of stuff would be great, ease of focusing is a concern and it seems that the 90mm schneider should have the advantage, less need for a center filter and a smaller hot spot on the ground glass should make up for the one stop the rodenstock f4.5 has as an advantage?
  2. Paul Sorensen

    Paul Sorensen Member

    Nov 15, 2004
    Saint Paul, MN
    Multi Format
    This comes up occssionally as it is one of the more confusing differences between large format and other formats. When the angle of view of a medium format or 35mm lens is discussed, to the extent that it is, it relates to the angle of view across the film plane itself. Any excess coverage that the lens might provide is not discussed as it is not relevant. Another major difference is that the lens is just set up to focus to infinity. With a short LF lens, you have to change the basic shape of the camera by bringing the front and rear standards closer together to actually focus a shorter lens.

    When you look at large format lenses, that excess coverage is very important. An angle of coverage measurement for LF is independent of the film size and relates to the image circle size only. A larger image circle gives you more movements or allows for a larger format of film to be used with that same lens. If you measure the angle of view of that part of the image circle that is intersected by the film, you will find that the 65mm has a larger angle of view by that measure.

    Visualizing this is kind of tough, but remember that the focal length of the lens relates to how far the film plane is back from the lens plane. 65mm lenses are closer to the film plane than 90mm lenses. Since the light coming through the lens forms a cone that gets wider the further back it goes from the lens, the 90mm lens with the same angle of coverage is going to provide a larger image circle. Since your film takes up a smaller portion of this circle, the angle on the film itself is smaller. This fact means that you can get more movements out of the 90mm lens in question than the 65mm lens. It also means that the 90mm image is less of a "wide angle" image than the 65mm image.

    I hope this makes sense, it is kind of strange when one is coming from other formats, but actually pretty interesting. LF cameras really help one understand some of these optical principles much better than you would just from smaller formats.

  3. Ole

    Ole Moderator Staff Member Moderator

    Sep 9, 2002
    Bergen, Norw
    Large Format
    That's it - they don't cover the same format. The 65mm lens has an image circle of 170mm, enough for 4x5 with a little movements. The 90mm has a 235mm image circle, which gives just about as much movements (and the same angle of view on film if used on 5x7"!

    Whether an LF lens is wide angle or not depends both on the construction (angle of coverage) and on the film format it's used on. The same 210mm lens can be a long normal on 4x5" film, and an ultra-wide on 12x16". Another 210mm might only cover the 4x5", so just giving a small circular image in the middle of the 12x16" film.
  4. MichaelBriggs

    MichaelBriggs Member

    Apr 27, 2003
    Your assumption is correct: shorter focal length lenses have a wider angle of view (for the same size film). In the LF world, there is a distinction between "angle of view" and "angle of coverage". The confusion is because the 105 degree figures that you have quoted are not the angles of view but the angles of coverage (this may be seen by trignometry from the focal lengths of diameter of circle of coverage that you gave).

    The angle of coverage is a property of a particular lens -- the angle of the cone of quality image that it projects. The angle of view also depends on the size of the film or sensor that you put behind the lens (as long as the film is smaller than the diameter of coverage), in addition to the focal length of the lens.

    Here are two previous rounds of this discussion: