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Thread: A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can build

  1. #101

    Re: A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can bui

    Hello.

    How would a Rapid Rectilinear design compare to this Cooke Triplet?
    I'm planning to build a very simple compact long focus RR. I don't have lenses longer than 300mm. I'm wondering how would be to order two identical achromatic doublets and put it on both sides of a copal #3. This size is for focusing aid (and because I have this shutter available at home).
    The barrel would be easier to prepare and fewer degree of freedom mean less possibilities of go wrong.
    How would a lens like that perform at f/22 to f/45?
    Mi thoughts begin with two of this doublets (D54 f786):

    https://a.aliexpress.com/_uj4p5X

    Any thoughts?

    ____________

    I forgot to ask...
    How would this diy lens compare with my symmar 210mm converted to 370mm?
    Last edited by afotandolaciudad; 23-May-2021 at 05:11.

  2. #102
    Nodda Duma's Avatar
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    Re: A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can bui

    You can get it to work, but it won’t be a rapid rectilinear as off the shelf achromat are the wrong lens shape to correct spherical. You will be able to correct distortion, lateral color and, with proper distancing from the stop, coma.
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  3. #103

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    Re: A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can bui

    Quote Originally Posted by afotandolaciudad View Post
    Hello.

    How would a Rapid Rectilinear design compare to this Cooke Triplet?
    I'm planning to build a very simple compact long focus RR. I don't have lenses longer than 300mm. I'm wondering how would be to order two identical achromatic doublets and put it on both sides of a copal #3. This size is for focusing aid (and because I have this shutter available at home).
    The barrel would be easier to prepare and fewer degree of freedom mean less possibilities of go wrong.
    How would a lens like that perform at f/22 to f/45?
    Mi thoughts begin with two of this doublets (D54 f786):

    https://a.aliexpress.com/_uj4p5X

    Any thoughts?

    ____________

    I forgot to ask...
    How would this diy lens compare with my symmar 210mm converted to 370mm?
    Assuming the numbers on those lenses mean diameter 54mm, focal length 786mm, then if you use two of the doublets separated by 50 mm you will get roughly a 405mm focal length lens. This is from the formula for a combination of thin lenses separated by distance d:
    1/f_total = 1/f1 + 1/f2 - d/(f1*f2)
    see for example https://courses.lumenlearning.com/bo...hapter/lenses/ - it is idealized but close enough for lenses like these.

    Symmetrical lens systems make some of the aberrations very small, the ones Nodda Duma mentioned. If you use small stops that could help reduce some of the uncorrected aberrations. I don't have any real experience with the 370mm single group of the Symmar, but have often read that the single groups are not very well corrected for color. So the symmetrical pair-of-achromats lens might have an advantage if avoiding lateral color is important.

  4. #104

    Re: A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can bui

    Thank you both for your replies.

    I forgot to mention that I'm going to use the lens on 4X5. That means I'll only use the central part of the coverage. Considering that RR are used as normal lenses and my RR approximation would be about 400mm, that rules out the periphery. Spherical aberration is more important in short focals and also decreases stopping down.

    Focus shift would be a problem if I focus wide open I guess. Symmar rear cell too suffer focus shift that I think is related to spherical. When you stop down the lens spherical aberration minimizes and focus shift appears. I thought maybe it's better to use a copal #1 instead of #3. For the barrel I could use pvc pipe gluing natural sheet cork bands to slightly expand the diameter and facilitate threading on the shutter. This is something I've already done in the past (using cork like this).

    With all this in consideration I think this pseudo RR can work pretty well when stopped down to f/22-32. In any case, the monetary loss will have been very small.

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