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  1. #11
    Photo Engineer's Avatar
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    A 70 mm lens for 645 and one for 35mm cover different negative areas evenly at the focal plane due to design, not actual theory involved in optics. It is a matter of economics.

    This also involves the plane of object focus of a lens which is nominally curved and must be designed for the given film size. Again, this is a matter of economics rather than theory. So, a 35mm lens is designed with a smaller area of coverage and a smaler area of focus compared to a similar lens for 645.

    A lens for a large camera can more easily be adapted to a smaller frame camera than the other way around.

    At least, this is a practical rule of thumb I was given when I was learning these things.

    So, it is a matter of practice and econimics vs what is theoretically and really possible given an unlimited budget.

    It can be demonstrated by focusing a scene from two 70 mm lenses on a sheet of paper outside of a camera and observing the circle covered by both lenses on the paper. This is how it was demonstrated to me at one time.

    I'm not an optics person, so I'm only reporting on observation and what I was taught pursuant to other elements of system design.

    PE

  2. #12
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    Quote Originally Posted by Photo Engineer View Post
    A 70 mm lens for 645 and one for 35mm cover different negative areas evenly at the focal plane due to design, not actual theory involved in optics. It is a matter of economics.

    This also involves the plane of object focus of a lens which is nominally curved and must be designed for the given film size. Again, this is a matter of economics rather than theory. So, a 35mm lens is designed with a smaller area of coverage and a smaler area of focus compared to a similar lens for 645.

    A lens for a large camera can more easily be adapted to a smaller frame camera than the other way around.

    At least, this is a practical rule of thumb I was given when I was learning these things.

    So, it is a matter of practice and econimics vs what is theoretically and really possible given an unlimited budget.

    It can be demonstrated by focusing a scene from two 70 mm lenses on a sheet of paper outside of a camera and observing the circle covered by both lenses on the paper. This is how it was demonstrated to me at one time.

    I'm not an optics person, so I'm only reporting on observation and what I was taught pursuant to other elements of system design.

    PE
    You're thinking too hard. I don't need to know a thing about lens design to know that it's entirely separate from what determines that one camera is shooting 6x4.5 and another is shooting 6x6. The lens is not in any way part of that equation, if you want to call the size and shape of the mask at the film plane an equation. The masking at the film plane is the only thing that determines the answer to the original question.

  3. #13
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    You are right if it is between 6x6 and 645, but if it is 6x6 and 4x5 or 6x6 and 35mm, then it wont work for reasons I stated.

    I only wanted to make sure that people did not expect to put lenses from widely different camera formats on the wrong camera and expect to get good results.

    PE

  4. #14
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    Quote Originally Posted by Photo Engineer View Post
    You are right if it is between 6x6 and 645, but if it is 6x6 and 4x5 or 6x6 and 35mm, then it wont work for reasons I stated.

    I only wanted to make sure that people did not expect to put lenses from widely different camera formats on the wrong camera and expect to get good results.

    PE
    I only wanted to make sure that the guy understands that this concept that you've introduced; mixing lenses from different formats, has nothing to do with answering his very simple question:


    "What determines the negative size/aspect ratio with 120 film? In other words, what determines if it's 6x4.5 or 6x6?"

  5. #15

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    There are also different formats for 135 film. The Olympus Pen cameras are 1/2 frame. And the old Zeiss Tenax is 24x24.

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