Lens coverage area on ground glass

Discussion in 'Large Format' started by eric_m|4, Aug 22, 2020.

  1. I'm thinking of buying a 150mm or 180mm for my 4x5. I'd like to know how do you know how much area outside of the ground glass is covered by a lens so as to have the most movements. Thanks.
     
  2. Any coverage beyond 152mm is how much extra coverage for 4x5.
     
  3. Hi Bob, Could you please elaborate a bit? I'm not sure what you mean. So does that mean a 180mm's coverage would be 180 - 152 = 28mm?
     
  4. Read spec sheets. A whole lot of them are floating around on the Internet. They tend to list lenses' image circles I suppose
    is what you need to cover a 4x5" neg.
    You talk about focal length. That could mean anything and nothing image circle wise. But yes, in the same lens family / series the longer one should give more image circle.
    Some lenses are marketed for their coverage in °, you 'll want more. For a 150mm maybe check the Super Symmar XL (that would cover 8x10").
     
  5. The coverage is determined by the design of the lens. More is better with a view camera and more tends to cost more. IMO, 210 mm lenses are popular with 4x5 because you get lots of coverage to use your swings and tilts without an expensive exotic design, just a longish lens. As you go shorter you need what's essentially a very wide angle lens for its focal length.
     
  6. The diagonal of a 4x5 film holder's gate is approximately 150 mm. This is why the minimum coverage needed for the format is 150 mm.

    As Jochen said in post #4 above, lens makers have published catalogs and specification sheets that report their lenses' claimed coverage. Finding them is fairly easy. The first post in this https://www.largeformatphotography.info/forum/showthread.php?138978-Where-to-look-for-information-on-LF-(mainly)-lenses discussion has a link to a list of lens catalogs and other bits of information of interest to LF photographers. Use it.
     
    Jochen likes this.
  7. A lens throws a circle of illum8nation. Your film sits within that circle.
    If that circle is smaller then 150mm your lens does not cover that format.
    If that circle is 150mm then it covers your format but without camera movements.
    If that circle is greater then 150mm then that lens allows movements within that larger circle.

    the closer you focus the larger the circle becomes.
    The more you stop down the larger the circle becomes.
     
    petrochemist likes this.
  8. +1 to reading the spec sheet for your particular lens model. Remembering that most image circles are specified for an aperture of f/22, f/16 or thereabouts.

    Better spec sheets will show the maximum horizontal or vertical shift for a particular format.

    The area outside of the 'cutoff' circle might show poor image quality, vignetting or both. It's rarely a sharp and total loss of image.
     
    petrochemist likes this.
  9. Lenses for large cameras with swings, tilts and rising front, must project a rather large circle of good definition. Otherwise the vignette and acuteness fall-off becomes undesirable.

    The coverage power of a lens depends on its focal length and its design. Once four process lenses, made by Jos. Schneider & Co. were evaluated. The focal length for all four was 210mm.

    Xenar @ f/4.5 = 253mm circle of good definition

    Symmar @ f/5.6 = 299mm

    Angulon @ f/6.8 = 353mm

    Super Angulon @ f/8 = 501mm

    As a rule of thumb: The use of smaller diameter apertures slightly increase the size of the circle of good definition. With most large format cameras, one can tilt the lens and back independently. The tilting of the back along with the other movements is useful as this may allow you to accomplish your goal and still maintain position of the lens axis near the center of the film format.

    When the lens is moved and the object is at infinity, the image movement equal lens movement. If the object is closer than infinity, the amount of the image movement follows this formula.

    Formula: I =( u + v) ÷ v

    I = lens motion

    v = object distance

    u = image distance
     
  10. None of those lenses are “process” lenses. Why did you think that they are?
     

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