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

  1. #21
    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

    Ken: yes, it's very important for the type of lab setups those are typically used in.


    Henrim: I didn't do a tolerance analysis but if I had to guess I'd say keep the airgaps (lens seats) to within at least +/- 0.010" and even better to be within +/- 0.005". That's very easy to achieve when machining the barrel on a lathe.


    Stop before or after the second lens: stop position impacts some of the Seidel aberrations such as coma, astigmatism, etc, and was selected based on what gave better performance.

    Generally speaking, the ideal position for the general positive-negative-positive design form is centered, maintaining design symmetry. Physical necessity, however, requires offsetting the stop position to one side or the other of the negative lens.
    Newly made large format dry plates available! Look:
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  2. #22

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

    Thanks again. Yes, it would be hard not to keep it within +/- 0.005"

    I'm tempted to make the lens and I have all the material needed for the barrel. Only thing I don't have is time...

  3. #23
    Nodda Duma's Avatar
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    A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can...

    Awesome please do try to find the time and let me know how it turns out. Feel free to ask questions along the way. First light will be very rewarding.

    I'll try to do a tolerance analysis this weekend and verify my educated guess.
    Newly made large format dry plates available! Look:
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  4. #24

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

    Quote Originally Posted by Nodda Duma View Post
    Now I've basically reduced the variables to the center negative flint lens. In a Cooke, the choice of glass types corrects color. So I have to do this now. The outer glass is set to N-BK7. The most common flint for catalog negative lenses is SF-11, which doesn't work well with N-BK7 to correct color. If you were to optimize for a flint to pair with BK7 youd find the design settles on N-F2, but that isn't an option for catalog optics.
    How about using a cemented achromat and a meniscus, like the Aldis Uno, instead?

  5. #25
    Nodda Duma's Avatar
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    A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can...

    That would make it not a Cooke.

    I mean it could be done, but you don't need a lens designer to tell you how to slap together a meniscus and achromatic. There's no real challenge to it from a "design using only catalog lenses" standpoint. That and the available meniscus are the wrong glass type for color correction.
    Newly made large format dry plates available! Look:
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  6. #26

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

    Quote Originally Posted by Nodda Duma View Post
    That would make it not a Cooke.

    I mean it could be done, but you don't need a lens designer to tell you how to slap together a meniscus and achromatic. There's no real challenge to it from a "design using only catalog lenses" standpoint. That and the available meniscus are the wrong glass type for color correction.
    True true, and that would be a topic for another thread. I'm not an expert on lens design, but I'm interested in old classic lenses and love reading about them, even if I don't understand the calculations and such. Finding a lens designer like you on a forum like this really inspires me. A photograph is not just a picture to me, but something that was seen through a lens of a certain design.

  7. #27
    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

    J. Patric: I'll take a look at that design and add it to my "to do" list. It is a lot like one of the steps taught in the design process from the landscape lens to the double gauss.

    Greg: Unfortunately no. For several reasons including the long focal lengths and the index/dispersion properties of the polycarb material, those lenses aren't conducive to designing better performing camera objectives, even when combined with available catalog lenses.
    Newly made large format dry plates available! Look:
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  8. #28

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

    Quote Originally Posted by Nodda Duma View Post
    J. Patric: I'll take a look at that design and add it to my "to do" list. It is a lot like one of the steps taught in the design process from the landscape lens to the double gauss.
    Please report back with your findings. The Aldis Uno is interesting. For example, it should be cheaper to make than the Tessar, but does it have any advantages over the Cooke triplet except maybe contrast?

  9. #29
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    A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can...

    No, the Cooke is inherently a better design because there are enough variables to correct for all the Seidel Aberrations. The doublet / meniscus combination is short 2 needed to correct them all, as well as not being a symmetric design. In this day and age, the number of air-glass interfaces are inconsequential thanks to coatings...there will be no contrast advantage.
    Newly made large format dry plates available! Look:
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  10. #30
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    Re: A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can...

    Quote Originally Posted by Nodda Duma View Post
    Nope, center to center (technically along the optical axis). This is convention.


    Airgaps and thicknesses are never measured edge-to-edge because lenses are almost always of different diameter. Doing that would lead to over-complication of the callouts.

    The D25S is just an example of an aperture you could use. I make no claim to having rigorously analyze the setup. it's hard to tell from the drawing but I think the aperture is offset to one side. You'd have to look at the solid model to know for sure. I thought I had checked it but it's been a while. Now you know why engineers insist on design reviews to check their work. You can try the D36S too. Or just use Waterhouse stops.
    Would you be able to draw a diagram of this? I'm a simpleton and work best with visuals. Thanks! Yes, I'm tempted to give this a try depending on the cost of materials involved.

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