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14
May
A Lens Is a Lens is a Lens
posted by
Topics Lenses / Optics
Tags , ,

A lens is a lens is a lens, and it doesn’t know what size sensor or film gate is placed behind it.  A lens projects light and the magnification size of that projection is determined by its focal length.

Nevertheless, there has always been a great deal of confusion associated with the focal length of lenses and their relationship to the size of the image area.

I’ve read many explanations in various books and on the internet over the years and authored some of them myself. But often written explanations confuse as much as they explain. If a picture is worth a 1000 words, I figure this video is equal to a small novella on the subject. Many people seem to use the various optical terms incorrectly or interchangeably, so I figured showing what happens would help demystify the concepts involved.

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  • Kinoeye

    You should have titled this “2 lenses of the same focal length can be completely different”.

  • Tobias Jall

    love your vids!

  • Anonymous

    But a 50mm lens made for ASP-C cameras is not really a 50mm in terms of field of view, it’s more of a 85mm, so why not just call it an 85mm?

  • Anonymous

    But a 50mm lens made for ASP-C cameras is not really a 50mm in terms of field of view, it’s more of a 85mm, so why not just call it an 85mm?

  • http://twitter.com/Mitch_Gross Mitch Gross

    Because it is not an 85mm, it is a 50mm.  That number is based on a magnification factor and an ANGLE OF VIEW.  The fact that the projection is physically not covering any given format has nothing to do with it.

    Think of it this way:  why should anything be defined by a 35mm stills format?  Or any other format?  If you took a 50mm designed only to cover APS-C and then a 50mm designed to cover Full Frame 35mm and put them both onto an APS-C camera, the resulting field of view would be identical.  So then it would be rather curious to have them labeled as different focal lengths.

    A lens has no idea what sensor is placed behind it.  It simply projects light.  The mathmatical relationship of the magnification of light to the size of the aperture defines equal to the distance between the image plane and a pinhole that images distant objects the same size as the lens in question.  To call it anything else would be both a fallacy and lead to vast confusion.  Truly a lens is a lens is a lens.

  • kino

    It’s even more fun to discus the difference between same focal length 16mm and 35mm format lenses.

  • David Siegfried

    Hi Mitch,
    Thanks for the demo! I agree with everything you said except for your comment
    above where you said that the focal length is based on an ANGLE OF VIEW. Focal
    length does not determine angle of view. You also need to know the format.

    The angle of view depends on the ratio of focal length to sensor size. A 50mm
    lens would have a wide angle of view on a 4×5 view camera, a normal angle of
    view on a 35mm camera, and a telephoto angle of view on an APS-C or Micro 4/3
    camera.

    Also, a lens designed specifically for one format, such as APS-C would have an image
    circle which is too small for a full-frame 35mm camera, causing it to vignette
    strongly. In that regard, a lens is NOT a lens. A 50mm view camera lens is a vastly
    different beast than a 50mm SLR lens which is different from a 50mm Micro 5/3
    lens. You can’t use a 50mm SLR lens on a 4×5 view camera even if you had an
    adapter.

     

    I get that “a lens is a lens
    is a lens” in terms of only how much it magnifies light rays but I think it’s
    more important for people to understand “lens equivalence” between different formats,
    because smaller formats are automatically magnified more when the image is output
    to your screen or printed.

  • David Siegfried

    Because the focal length is simply a physical characteristic of the lens. You should stop thinking in terms of mm and start thinking in terms of normal, wide angle and telephoto. On a pocket point and shoot camera, a “normal” lens is 7mm!! This is why point and shoot cameras have almost unlimited depth of field.

  • Scottoconnor

    Thats a really good tutoial, thanks for taking the time to set it up.
    I was just wondering what camera you shot it with?

  • http://twitter.com/Mitch_Gross Mitch Gross

    Thanks for the positive comment.  I’m glad you found the video useful.

    This was done some time ago so I can’t recall, but we generally shoot these with a Sony PMW-EX1.

  • Jake

    why editing your posts? others don’t know now what your original questions were…

  • http://www.linkedin.com/pub/stephen-wood/33/713/5aa Stephen Wood

    Very useful video. Thanks for sharing.

  • Asif

    Thanks for helping and its cool…

  • Stergios

    Thanks for putting all that info in this easy to understand video.

  • http://www.facebook.com/kevin.middleton.7359 Kevin Middleton

    You’re absolutely right that many written articles on sensor size confuse as much as they explain. I really appreciate the comparison you made.