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Lens mm conversion on APS-C camera

Discussion in 'Help Team' started by Chris Bell, May 16, 2017.

  1. Benchista

    Benchista Which Tyler

    So for my 24mm Tilt Shift lens, you would quote a figure based on it's entire image circle? That would be a radically different figure to my 24mm f2.8 even on the same format. I thought the idea was to make things simpler?
  2. Terrywoodenpic

    Terrywoodenpic Well-Known Member

    It is a fixed property of a particular lens, But not all lenses of the same focal length have anything like the same image circle.
    A 300 mm for a 35mm format would nowhere near cover a 5x4 camera.
    The coverage of a lens is a function of its construction more than its focal length.
    All lenses designed for 35 mm, of what ever focal length have much the same coverage as each other, with the exception of tilt shift lenses, which have an image circle large enough to cover the necessary movements.

    That said, a lens designed for a larger format will always cover a smaller format.
  3. Jacques Rebaque

    Jacques Rebaque Active Member

    No I wouldn't quote the properties of a lens by it's image circle, I was questioning your statement that a lens angle of view changes according to format.

    Is a tilt/shift lens a 'fixed' focal length lens though? The very nature of physically moving elements within a lens must alter the focal length.
  4. Jacques Rebaque

    Jacques Rebaque Active Member

    Would that not depend on where the image plane is (Film,Sensor) in relation to the focal length of the lens though. In theory a 300mm lens for 35mm would cover a 5x4 if the film was far enough back.

    Anyway I'm not trying to get in an argument, I'm just questioning my own assumptions of optics.
  5. PeteRob

    PeteRob Well-Known Member

    Yes it is fixed. You don't move the elements wrt each other, you move the centre of the image circle (shift) and the angle of the image plane (tilt) with respect to the recording medium.
  6. Jacques Rebaque

    Jacques Rebaque Active Member

    Just to clarify my understanding.
    I always assumed lens optics (forget image formats) simply to have the light paths of a figure 'X' where light entered from the right and was projected onto a surface on the left. the angle of the 'X' whether it was acute or obtuse governed the properties of the lens and never changed. The positioning of the film (or sensor) plane and therefore size of film (sensor) as it has to fall within the image circle, governed the view.
  7. PeteRob

    PeteRob Well-Known Member

    With a shift lens the image circle is very big - when you shift you effectively move the sensor around to sample different bits if the image. So with a 24 mm shift lens you can see "above the top of the frame" of what you would capture with a fixed 24 mm, at the same time you lose off the bottom of the frame.
  8. Roger Hicks

    Roger Hicks Well-Known Member

    Dear Nick,

    And nominal formats ("6x9cm" = 56x78-88mm) and nominal focal lengths (50mm = 50-52mm).


  9. Terrywoodenpic

    Terrywoodenpic Well-Known Member

    A tilt shif is not achieved by moving the elements. It is achieved by moving the entire lens within a complex mount.
  10. Terrywoodenpic

    Terrywoodenpic Well-Known Member

    The construction of the lens ( arrangement of the elements) defines the coverage of the lens .
    A tessar type and other modified triplet designs. Produce fairly restricted image circles.
    Symetrical type lenses tend to produce wider image circles, and are used for wide angles.
    Telephoto lenses are also based on the basic triplet design, but with quite different spacing of the elements.

    Though modern lenses tend to use far more elements to correct various distortions, they still follow basic designs.
  11. Benchista

    Benchista Which Tyler

    Well the fact is that for all practical purposes the angle of view does change, constrained by the format. You could quote an angle of view based on the image circle, but as image circles are not the same for all lenses, it would be pretty meaningless.

    No, it doesn't, that's not the way a T/S lens works.
  12. John King

    John King Well-Known Member

    How about explaining it a different way in that a T&S lens has a wider coverage than would be normal for a lens of a given focal length at the film plane, for instance say 35mm. A normal 35mm lens for a 35mm camera would probably have a coverage defined by a image circle at the film plane of approx 2.5". This would cover a 24x36mm image comfortably as this lens required covereage is fixed.

    With a T&S lens the projected image circle at the film plane could be as much a 5" which would allow the up and down, side by side and tilting movements and still cover the film frame without cut off in the corners.

    The same applies to a LF Camera, where a lens of say 135mm ( amoderate wide angle for that format) will cover the 5x4 format without undue problems. If you employed a 135 lens designed for, say a 10x8 LF camera the projected circle at the film plane is that much larger and so can be used as a T&S lens on a 5x4 format camera.
  13. PeteRob

    PeteRob Well-Known Member

    On a LF camera the movements are usually on the front panel and sometimes on the back panel too and the lens is fixed. There was a link in another post somewhere (? the buy a new film camera poll?) to a low-cost LF camera manufacturer that in turn had a link to a table of LF lenses listing the image circle sizes and focus distances of LF lenses and suitablility for 4x5, 7x5 and 10x8 use.

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