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Thread: Rear lens component used as long lens

  1. #11

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    Re: Rear lens component used as long lens

    Quote Originally Posted by Dan Fromm View Post
    Kanga, there's a rule of thumb to the effect that half of a convertible lens covers roughly the same circle as the complete lens. Check carefully before sacrificing a 16x20 Cibachrome sheet. Arne Croell believes that the RF- series of six elements in four groups double Gauss type process lenses cover 45 degrees. If he's right, and he usually is, don't even bother.

    I'm sorry, but there just aren't that many lenses that cover 16x20. If the trick you want to try worked well, it would be well-known.
    Hi Dan,
    I tried the rear group of my RF3 300mm f/10 on my 4x5in and it required 575mm of extension. On the ground glass at least there appears to be no aberration at a marked aperture of f/32 (effective f/64). If it does not cover 16x20in then it appears I've got a portable 600mm lens (or 800mm if I use the rear group from the RF5) for my 4x5in or an 8x10in. I will let you know how the actual negative looks after I give it a go on the 4x5in.
    Cheers Kanga

  2. #12

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    Re: Rear lens component used as long lens

    Quote Originally Posted by Mark Sawyer View Post
    Even though the focal length roughly doubles and the barrel is shorter? How can that be? (Not disagreeing, as Cooke's specifications for the XVA Triple Convertible agree with you.) Is it that the outer image area degrades?
    That was my thinking Mark- I am not an optical engineer but I thought stopping the lens right down might help. I am aware of diffraction issues but that is always a trade off for me as I generally do macro work and I sacrifice resolution for depth of field.
    Cheers Kanga

  3. #13
    Thalmees's Avatar
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    Re: Rear lens component used as long lens

    Hope the issue of this thread will also go with Ronars.
    Will try my self when I return from my current travel.
    Wondering, why most LF photographers do not think the same way?
    Testing on paper negative or X-ray will not charge much.
    Thanks peterkinchington for the subject.

    The generosity of spirit in this forum is great, its warmly appreciated.
    ------------------------------

  4. #14

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    Re: Rear lens component used as long lens

    Quote Originally Posted by Thalmees View Post
    Wondering, why most LF photographers do not think the same way?
    Separable double anastigmats, symmetrical and not, have a long and glorious history. They were largely abandoned for several reasons. Manufacturing cost, for those with three or more cemented elements. The fact that single cells are, in general, not quite well corrected enough. The fact that lenses not intended to be convertible can have better corrections than lenses intended to be convertible.

    Questions about using single cells come up because many of us are hopeful and don't want to spend more money if that can be avoided.

  5. #15
    Jac@stafford.net's Avatar
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    Re: Rear lens component used as long lens

    Quote Originally Posted by Mark Sawyer View Post
    Even though the focal length roughly doubles and the barrel is shorter? How can that be?
    That is a question that perhaps Dan Fromm can answer. Where does the nodal point go when the front element is removed. I sure don't know.

  6. #16

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    Re: Rear lens component used as long lens

    I don't know either.

  7. #17

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    Re: Rear lens component used as long lens

    Quote Originally Posted by Jac@stafford.net View Post
    Where does the nodal point go when the front element is removed. I sure don't know.
    When the front element is removed you end up with a single doublet or single meniscus. The nodal point is somewhere on the optical axis of the lens in the glass, one and other function of the curvature of the lens.

    Cheers,

  8. #18

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    Re: Rear lens component used as long lens

    Rudi, I have some Boyer Beryls that are convertible. A single cell is a cemented triplet. I've had an f/2.8 6/4 double Gauss type Boyer Saphir that was convertible. A single cell has a singlet outside and a cemented doublet inside. I have some separable plasmat types. A single cell has a cemented doublet outside and a singlet inside. And then there are the old double anastigmats with single cells that have four or five cemented elements. I've never had any of them.

    The definition of focal length tells us where the rear node of a lens whose focal length is known is located. When the lens is focused at infinity, the rear node is one focal length from the film plane.

  9. #19

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    Re: Rear lens component used as long lens

    Quote Originally Posted by Amedeus View Post
    When the front element is removed you end up with a single doublet or single meniscus. The nodal point is somewhere on the optical axis of the lens in the glass, one and other function of the curvature of the lens.

    Cheers,
    Hi Rudi,
    This explains why I needed 575mm of extension for the rear component of the RF3 (300mm lens) the nodal point is probably 25mm in from the surface of the rear element. That is 575+25mm =600mm which is what you would expect the rear components focal length to be in a symmetrical 300mm lens.
    Cheers Kanga

  10. #20

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    Re: Rear lens component used as long lens

    Quote Originally Posted by Dan Fromm View Post
    Rudi, I have some Boyer Beryls that are convertible. A single cell is a cemented triplet. I've had an f/2.8 6/4 double Gauss type Boyer Saphir that was convertible. A single cell has a singlet outside and a cemented doublet inside. I have some separable plasmat types. A single cell has a cemented doublet outside and a singlet inside. And then there are the old double anastigmats with single cells that have four or five cemented elements. I've never had any of them.

    The definition of focal length tells us where the rear node of a lens whose focal length is known is located. When the lens is focused at infinity, the rear node is one focal length from the film plane.
    Dan, totally agreed on the rear node of the lens as you describe.

    My confusion stems from the use of the "nodal point" expression, typically defined as the no-parallax point of a lens system. In compound photographic lens designs, this point lies between front and rear element, in single lens designs, it lies somewhere in the immediate vicinity of the actual lens, depends on the actual shape of the lens. Critical to know in panoramic photography.

    Thanks for catching this.

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