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Thread: what focal lenght for macro?

  1. #1

    what focal lenght for macro?

    For a project I attached a Nikon DSLR to the back of a Cambo 5x7, which extends to about 60cm. My goal is to get max. enlargment, so what is better, a short or a long lens or it just depends on the bellows extention?
    Regards
    Martin

  2. #2

    Join Date
    Jan 2002
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    Besanšon, France
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    Re: what focal lenght for macro?

    Hello from France !

    You need first to determine first what will be your magnification ratio M = (image size)/(object size).
    Your detector covers either 18x24 mm of 24x36 mm for the full-frame series of DSLR.

    Consider the 24x36 mm size as an example.

    If your object is 1 cm in diameter, the maximum you can enlarge will fill 24mm in height so your magnification ratio will be M=2.4

    From the value of M you'll find the needed bellows draw :

    - additional bellows extension from the focal point : ext = M.f
    - total bellows draw from the lens board, add one focal length ;
    total bellows length approx. ext+f = (1+M).f

    In this condition the multiplying factor for the exposure time will be
    (1+G)^2 ((1+G) squared) but your DSLR will probably keep the shutter automatically open until the proper light dose is recorded !

    Those simple formulae will tell you the amout of bellows draw required for your projet as a function of the focal length.

    At magnifications above 1, since the object will be located at about one focal length in front of the camera, a focal length substantially longer than the standard 50mm is a good choice to keep a minimum comfortable distance to set-up your lights properly and avoid, for example, the shadow cast on the
    object by the lens itself.

    Regarding depth of field, general formulae exist as a function of the magnification factor, focal lengths and f-number, but at 1:1 ratio the depth of field is not directly dependant on the focal length, the value in this particular condition (object siez = image size) is plus or minus N.c where N is the f-number engraved on your lens and c the circle of confusion of acceptable sharpness.

  3. #3

    Re: what focal lenght for macro?

    Thanks for your kind answer; I am not that good in calculations; I have my lenses attached to other lensboards, so I have some work to try different lenses with my Cambo; beside the issue with light and shadow, is there a difference in magnification between say a 90mm lens and 360mm at given bellows extension?
    Thanks
    Martin

  4. #4

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    Re: what focal lenght for macro?

    a shorter lens will require less extension, however your working distance will be less as well.

  5. #5

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    Re: what focal lenght for macro?

    Martin, at the same extension, a 90 will give roughly 4x the magnification a 360 will.

    If you want sensible advice that doesn't require you to calculate (you can't calculate? shame on you, sir!), tell us which lenses you have, including lenses for y'r Nikon and how much clearance you need between subject and lens.

  6. #6

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    Re: what focal lenght for macro?

    For purposes of discussion: If you had a setup with unlimited bellows extension, and a selection of good macro lenses of all reasonable lengths, you could achieve the same magnification with all of them.

    What does change, is the distance from the camera to the subject, and thus the sense of perspective.

    When shot from a greater distance, with a longer lens (or with a short lens but given equivalent enlargement), the foreground and background will appear closer together. That is because from the greater distance, the separation between foreground and background is comparatively small.

    When shot close-up, the foreground and background will appear to be far from one another, because with respect to the actual shooting distance, the separation between foreground and background is now substantial.

    If your subjects are flat, then this isn't an issue. If they are 3-dimensional, then it does becomes an issue.

  7. #7

    Join Date
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    Re: what focal lenght for macro?

    Quote Originally Posted by Martin Miksch View Post
    For a project I attached a Nikon DSLR to the back of a Cambo 5x7, which extends to about 60cm. My goal is to get max. enlargment, so what is better, a short or a long lens or it just depends on the bellows extention?
    Regards
    Martin
    Better in what sense? For your grandma's cheese cake preparation? Before you start to ask imprecise questions why not to take two different lenses and experiment a little bit on the gg? You will then know better what you want to ask...

  8. #8

    Re: what focal lenght for macro?

    Quote Originally Posted by Martin Miksch View Post
    ... My goal is to get max. enlargment, so what is better....
    Hmm, my mistake, I thought defining a goal shows that "better" is meant as "better working to reach my goal" and as mentioned above my lenses are on boards for wooden field cameras and I have to attach them to Cambo boards, what of course were not a problem if I had all the boards with right drills.

    I have no dedicated macro lens but I hope that this is not a problem, I just use a tiny bit from the center of the lense. My lenses are 90/8 SA, 120 Angulon, 135, 165, 180, 210, 270 and 360 Xenars.

    Thanks and Kind Regards
    Martin

    Edit: my F-mount lenses are 24, 35, 50, 85, 135, 180 and 300mm,
    clearance just what I need for light, maybe I try to use some glass-fibres, maybe someone can point me to a place where I can find some information about DIY glass fibre flashes or similar.

  9. #9

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    Re: what focal lenght for macro?

    Quickly, if you want the same dimensions on your neg as the subject, the real meaning of "macro", your lens will be out twice it's stated (infinity) focal length, and you will have to open two stops from the stated iris indicators. Thus, to shoot with a 150mm, your lens is extended 300mm, and you set your iris at 5.6 if your meter suggests f/11.
    s/

  10. #10

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    Re: what focal lenght for macro?

    The 120 Angulon would allow you to get you to about 4:1. The 90/8 SA would get to 5.66:1. For better results and magnifications to 11:1 consider a 50mm macro or enlarging lens (either will perform better if mounted backwards at these magnifications significantly greater than 1:1). If you need bigger magnifications than that, then consider a shorter enlarging lens (backwards) or certain specialized short focal length macro lenses like the Zeiss Luminars. Luminars and the like are designed for magnifications greater than 1:1 and so do not need to be mounted backwards. A 16mm Luminar will let you get to about 36:1.

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