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Thread: Magnification factor for Contax lenses on Leica S - please explain

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    Magnification factor for Contax lenses on Leica S - please explain

    Friends, I bought a Contax to Leica adapter to mate the 645 lenses to my S 006. As I researched the options I did so on the assumption that the field of view extrapolation would be the same for both product lines. In other words, that I could use the 0.8 magnification factor to estimate the field of view in full frame terms. I think of my 70mm Summarit S as the rough equivalent of a 56mm in full frame terms. And I assumed that I could do the same math for the Contax lenses.
    But I read the core thread on this site that discussed the merits of the Contax lenses mated to the S. In the course of the discussion, at least two posters suggested that the reverse was actually true. That the stated focal length of the Contax lenses are 1.25x longer than the lens length on the S body given the fact that the S sensor is smaller than the 645 format.
    As posted:

    Well, how the edges are doesn't really matter. The Leica S chip is 30x45mm, so it will never see the edges of any of these lenses.
    The 35 becomes a 43 (x1.25)


    If this is so, then the 80mm would be a 100mm, the 120mm would be the equivalent of a 150mm.
    I have not seen this discussed directly in any thread and would be obliged if someone could help lift the veil. I need to know if one of these lenses is going to have a field of view that, in full frame terms, would be 20% shorter or 25% longer than the stated focal length.
    On another note, I am committed to the 120mm makro and 80mm for its unique signature, but am dithering on the short end - either the 45mm or the 55mm. Any thoughts would be most helpful.
    Obliged for the help.
    David

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    Re: Magnification factor for Contax lenses on Leica S - please explain

    Hi David,

    a Contax lens with a focal length of X mm is identical to its Leica sibling with the same focal length!

    And as such both have the same crop factor compared to FF (135 format) terms. Your approach with the multiplication factor of 0.8 is correct. This crop factor is the quotient of the FF diagonal (43mm) and S006 diagonal (54mm). Due to the sensor's larger diagonal (compared to FF) the field of view is becoming wider which 'shortens' the effective focal length.

    Regards, Udo
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    Senior Member DougDolde's Avatar
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    Re: Magnification factor for Contax lenses on Leica S - please explain

    I'd certainly buy the Contax 35mm then get the Vario 45-90. I found the Vario is better in the corners than the 45mm prime, at least for the copies I had. Of course the 120mm is a no brainer it's incredible
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    Re: Magnification factor for Contax lenses on Leica S - please explain

    Quote Originally Posted by Udo View Post
    Hi David,

    Due to the sensor's larger diagonal (compared to FF) the field of view is becoming wider which 'shortens' the effective focal length.

    Regards, Udo
    Udo, I like that explanation and thank you kindly for it. Now I understand why my 150mm in 4x5 "sees" like a 50mm in FF.
    Best to you.
    David

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    Re: Magnification factor for Contax lenses on Leica S - please explain

    Quote Originally Posted by Deliberate1 View Post
    Udo, I like that explanation and thank you kindly for it. Now I understand why my 150mm in 4x5 "sees" like a 50mm in FF.
    Best to you.
    David
    Actually, a 150mm in 4x5 is like a 42mm in FF. That 8mm difference between 42 and 50 does matter...

    A good approach to this ever raising question of comparing focal lengths between different formats follows:

    First we shall consider the three dimensions every format has: short side (a), long side (b), diagonal (c)

    In the case of FF, a = 24mm, b = 36mm, c = 42mm

    In the case of Leica S, a = 30mm, b = 45mm, c = 55mm

    In the case of 4x5 sheet film, a = 102mm, b = 125mm, c = 150mm

    When a focal length equals 'c', we have the 'normal' lens for that format.
    When a focal length is shorter than 'c', we have a wide angle.
    When a focal length is longer than 'c', we have a 'tele'.

    Therefore, a 35mm is a moderate wide angle in FF, and a wider-wide-angle with a Leica S (like a 28mm in FF).
    [With 4x5 film, we achieve with a 120mm the same viewing angle as with a 35mm on FF.]

    And a 24mm is a real wide angle in FF, and a very wide wide angle with a Leica S.

    [Actually, the 24mm on a Leica S is like a 19mm on FF. We don't need to know the conversion factor (which is 0.8x) to arrive to this: 24 is aprox 45/2 (long side of S format / 2), and 19 is aprox 36/2 (long side of FF /2).]

    If we could adapt a 24mm FF lens (for instance, the Canon TS 24) to a Leica S and make it focus at infinity, it would give us exactly the same view as the Leica S 24mm lens. It's the focal length what decides how wide or narrow the viewing angle is.
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    Senior Member DougDolde's Avatar
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    Re: Magnification factor for Contax lenses on Leica S - please explain

    The Alpa focal length calculator is very useful for these kind of questions

    https://www.google.com/url?sa=t&rct=...tET75ATb0vC22g

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