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  1. #21  
    Junior Member Greg Lowry's Avatar
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    Quote Originally Posted by Anders Holck View Post
    Yes it's Field Of View.

    The misconception is that there is somehow a multiplier included in the given focal lenght of a lens based on the gate size. Which not true.
    The focal length of a lens is not dependent on the gate size but strictly on the distance from the secondary principal point of a given lens to the imaging plane.

    The FOV of the lens is dependent on both the focal lenght and the gate size. The formula for H FOV at infinity focus is 2 * arctan (Sensor width/(focal length * 2))
    I have never disputed that the focal length remains the same no matter the format. in fact, you'll find dozens of posts on the old forum where I reinforced this fact.

    However, the EFFECTIVE FOV (perhaps the word "effective" is necessary here for clarity) of a 14mm (35mm stills format 36x24mm) still camera lens when used on the RED ONE is not the same as a cine format purpose-built 14mm lens. I stand by my statement that the 14mm 35mm still format lens is about the equivalent of a 20mm cine lens in terms of FOV when used on the RED ONE.

    Indeed, FOV is dependent on focal length and "gate size" as you call it.

    Are we disagreeing or miscommunicating?
    Greg Lowry
    Scopica Inc.
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  2. #22  
    Junior Member Greg Lowry's Avatar
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    Quote Originally Posted by Graeme Nattress View Post
    Yup. 14mm = 14mm = 14mm. You can use a crop factor to think about how the lens might look on a camera with a different size sensor, but, mm always gives the FOV of the lens.

    Graeme
    Yes, 14mm = 14mm = 14mm. But, for example, the RED 300 when used on the S16 format provides a FOV equivalent to between a 400mm and 500mm if recall correctly (I made the computation months ago and would have to look up the exact number). When using lenses designed for a different format it is simply not true that the "mm always gives the FOV of the lens". This is always focal length and format dependent.

    A more extreme example: Does a 150mm camera lens designed to cover the 6cmx6cm medium format (e.g. Hasselblad) provide the same FOV or image magnification when mounted on a 35mm still camera? No, it doesn't.
    Greg Lowry
    Scopica Inc.
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  3. #23  
    Senior Member Martin Drew's Avatar
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    I think a lot of the confusion comes from the the talk of equivalent focal length, which sort of implies the focal length changes when a lens is used for a different format. The only big difference between a lens designed for a larger format and a lens designed for a smaller format will be that the larger format lens has to project a larger image.

    Martin
     

  4. #24  
    Senior Member Martin Drew's Avatar
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    A cine 14mm lens is designed to cover a smaller format so it will not need to project as large an image as a still camera 14mm lens, but they will both have the same magnification. If we use both lenses on the same sized sensor the image they project will be the same. If you used a S16 14mm lens it will project the same magnification image on the sensor too but because it is designed for a smaller format it will vignette.

    Martin
     

  5. #25  
    Senior Member Martin Drew's Avatar
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    Quote Originally Posted by Greg Lowry View Post
    A more extreme example: Does a 150mm camera lens designed to cover the 6cmx6cm Medium still camera format provide the same FOV or image magnification on a 35mm still camera? No, it doesn't.
    It provides the same image magnification but because the sensor is smaller that image is cropped hence FOV is reduced, it is all down to the size of the sensor. If you mounted a 150mm medium format lens on a 35mm camera it would give you the same FOV as a 150mm 35mm still format lens on that same camera.

    Martin
     

  6. #26  
    Junior Member Greg Lowry's Avatar
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    Quote Originally Posted by Martin View Post
    A cine 14mm lens is designed to cover a smaller format so it will not need to project as large an image as a still camera 14mm lens, but they will both have the same magnification. If we use both lenses on the same sized sensor the image they project will be the same.
    Martin
    This is fundamentally incorrect.

    If you use both lenses on the same size sensor, the image circles for each of the lenses are obviously different -- one is larger than the other. The fact that you are cropping an image area from the large image circle effectively alters the FOV of that lens relative to the format for which it was designed.
    Greg Lowry
    Scopica Inc.
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  7. #27  
    Junior Member Greg Lowry's Avatar
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    Quote Originally Posted by Martin View Post
    It provides the same image magnification but because the sensor is smaller that image is cropped hence FOV is reduced, it is all down to the size of the sensor. If you mounted a 150mm medium format lens on a 35mm camera it would give you the same FOV as a 150mm 35mm still format lens on that same camera.

    Martin
    These are contradictory statements.

    Wow. We couldn't disagree more. But rather than burn up more bandwith, I'll defer further discussion on this subject to others and will contribute further if I feel I can be useful.

    It's an important discussion.
    Greg Lowry
    Scopica Inc.
    Vancouver
     

  8. #28  
    Senior Member Anders Holck's Avatar
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    Greg,
    The 18 mm Zeiss Masterprime has a horisontal FOV of 68.6 in DIN S35 (24mm x 18mm)

    The Nikon 18mm 2.8D Prime has a horisontal FOV of 69.2 on a Nikon DX DSLR (23.7 x 15.7)

    Thats pretty close both in imaging format and FOV.

    So a 18mm purpose built cine lens should be pretty damn close to a 18 mm still model.
     

  9. #29  
    Senior Member Anders Holck's Avatar
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    Quote Originally Posted by Greg Lowry View Post
    If you use both lenses on the same size sensor, the image circles for each of the lenses are obviously different -- one is larger than the other. The fact that you are cropping an image area from the large image circle effectively alters the FOV of that lens relative to the format for which it was designed.
    The image circle has nothing to do with magnification or FOV. It only affects the net area that the lens is projecting at the sensor plane.

    A 100mm lens that covers a radius of 12mm, has the exact same magnification as a100mm lens that covers a radius of 24mm.

    In a 20x20mm gate size both lenses would project the same FOV. In 40x40mm the FOV would also be identical, but the first lens would vignette horrobly (project a round image)
     

  10. #30  
    Junior Member Greg Lowry's Avatar
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    Quote Originally Posted by Anders Holck View Post
    The image circle has nothing to do with magnification or FOV. It only affects the net area that the lens is projecting at the sensor plane.

    A 100mm lens that covers a radius of 12mm, has the exact same magnification as a100mm lens that covers a radius of 24mm.

    In a 20x20mm gate size both lenses would project the same FOV. In 40x40mm the FOV would also be identical, but the first lens would vignette horrobly (project a round image)
    You focused of an incidental part of my post.

    Let me use another illustration:

    A 18mm Zeiss Master Prime covering the S35 format (24.9mmx18.7mm) has a horizontal angle of view of 77 degrees.

    The same 18mm Zeiss Master Prime covering the Normal 35mm format (22x16) has a horizontal angle of view of 70.2 degrees.

    Unfortunately, ARRI doesn't specify the vertical angles of view and I'm not inclined to compute them myself right now, but just from these differences in horizontal angle of view one can infer that the field of view between these two formats, using exactly the same focal length lens, will be different. How could it not be?
    Greg Lowry
    Scopica Inc.
    Vancouver
     

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