Page 2 of 11 FirstFirst 1234 ... LastLast
Results 11 to 20 of 104

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

  1. #11
    Nodda Duma's Avatar
    Join Date
    Nov 2014
    Location
    Brookline, NH
    Posts
    1,051

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

    Hi Andrew,

    It was my pleasure! I've got some more to try when I can find the time.

    If I remember correctly, the process went something like this:

    Set up a Cooke triplet with the appropriate image plane size and focal length. Set the positive glass types to N-BK7 ( what most catalog positive lenses are made of). At this point you'll have a really nice Cooke with RMS spot diameter ~20um for 4x5 image plane.

    Constrain the outer lenses to plano-convex. This is a better approximation to the lens shapes of an ideal Cooke than an equi-convex. Also set the outer glass types to N-BK7. Reoptimize at this point. I optimized for P-V Wavefront (this is my preference), but I track spot size. I also set the stop either before or after the middle lens, depending on what gives better merit function value.

    Set the element focal lengths to be equivalent to common values in the catalog. It's fairly straightforward to find a suitable combination.

    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.

    There's no catalog flints really available to pair with catalog crowns to correct color, which never made sense to me (except maybe catalog lenses are for tinkerers, professors, or people who just don't know any better).

    So the real sneaky part that makes this work is that Fused Silica is another choice for "flint" that has low and similar dispersion like BK-7 (Abbé of both are in the 60s), but index is different enough that it's not the same glass. If you use a fused silica lens for the flint, then at least color is reduced enough for acceptable spot sizes. Set it up as an equi-concave lens for design symmetry.

    Now it's just a matter of finding catalog lenses that are close to what you have. Zemax has a tool which allows me to rapidly filter through stock lenses from all the vendors for focal length and diameter, element type. I don't think Oslo has that unless they've incorporated it recently. If not, then just flip through the Edmund and Thorlabs catalogs. Once selected, optimize lens spacings to give you the proper focal length and final aberration correction. Don't constrain focal length too tightly, just keep it roughly normal or it'll go short (smaller focal lengths make smaller spot sizes).

    The diameter of the fused silica lens constrains the f/#. If that weren't the case, then I would instead stop the design down until the RMS spot diameter is about 100um across the field. This allows you really nice contract prints and maybe enlarge 2x or so from a wide open photo. As it is, this design is about there anyways and provides good performance. Not as good as a true Cooke of course, but surprisingly good enough with maybe some pleasing softness.
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  2. #12
    Nodda Duma's Avatar
    Join Date
    Nov 2014
    Location
    Brookline, NH
    Posts
    1,051

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

    Btw a good reference for how to design a Cooke is "Lens Design" by Joseph Geary. Has a great Cooke design example in there, among others.
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  3. #13

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

    Thanks for your answers! I have another question:

    In your design you mention distances to the second lens. Are these distances to the center of the lens or to the edge? Is the convention to always do it one way or the other?

  4. #14

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

    Has anyone made one yet?!!

  5. #15

    Join Date
    Apr 2012
    Posts
    61

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

    This is interesting. I would like to try to do something like this at some point. May be not in the near future though. Anyway I drew the attached diagram to check if I'm following your description correctly. My interpretation is to measure the air caps from the edge of the lens to the edge of the lens. Air cap between lens and aperture is measured from the edge of the lens to the centre of the opening. Is that correct? If that is, there is a clearance issue if the suggested aperture assembly is used (as shown with blue dotted line).


  6. #16
    Nodda Duma's Avatar
    Join Date
    Nov 2014
    Location
    Brookline, NH
    Posts
    1,051

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

    Quote Originally Posted by AndrewButts View Post
    Thanks for your answers! I have another question:

    In your design you mention distances to the second lens. Are these distances to the center of the lens or to the edge? Is the convention to always do it one way or the other?
    Yes, prescription thicknesses are always along the optical axis (airgaps and lens thicknesses are usually vertex to vertex, for example).
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  7. #17
    Nodda Duma's Avatar
    Join Date
    Nov 2014
    Location
    Brookline, NH
    Posts
    1,051

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

    Quote Originally Posted by Henrim View Post
    This is interesting. I would like to try to do something like this at some point. May be not in the near future though. Anyway I drew the attached diagram to check if I'm following your description correctly. My interpretation is to measure the air caps from the edge of the lens to the edge of the lens. Air cap between lens and aperture is measured from the edge of the lens to the centre of the opening. Is that correct? If that is, there is a clearance issue if the suggested aperture assembly is used (as shown with blue dotted line).

    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.
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  8. #18
    Nodda Duma's Avatar
    Join Date
    Nov 2014
    Location
    Brookline, NH
    Posts
    1,051

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

    Quote Originally Posted by richardman View Post
    Has anyone made one yet?!!
    I haven't heard of any yet...I'm still waiting for someone to try it and report the results!!
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  9. #19

    Join Date
    Sep 2003
    Location
    Massachusetts USA
    Posts
    8,485

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

    One thing I noticed in the catalog was that they offer new lens diaphragms with what appear to be many blades. Hmmm...

  10. #20

    Join Date
    Apr 2012
    Posts
    61

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

    Thank you, I get it now.

    By the way, what is the acceptable tolerance for the airgaps?

    I was thinking that the aperture on the D25S is probably offset but that is not indicated in the drawing and none of the photos show the other side either. Would be on the safe side to try with the bigger model.

    And an other question about the aperture. I googled for triplets and in many cases the aperture is drawn behind the second lens element. What difference does it make if it is on front or behind the element?
    Last edited by Henrim; 20-Mar-2015 at 12:52. Reason: Added question

Similar Threads

  1. Taylor Hobson Cooke 30" and 48" process lenses
    By diversey in forum Lenses & Lens Accessories
    Replies: 14
    Last Post: 12-Feb-2015, 09:03

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •