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Thread: A DIY 123mm f/8 Cooke split triplet for 4x5

  1. #1

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    A DIY 123mm f/8 Cooke split triplet for 4x5

    Hi Everyone,

    I was inspired by Nodda Duma's popular thread A DIY 177mm f/8 Cooke triplet for 4x5 from off-the shelf lenses that YOU can build and thought I'd take a crack at improving the performance by splitting the front and rear positive lenses of his Cooke triplet design.

    Here's what I got:

    Click image for larger version. 

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    Focal length: 123 mm.
    Pretty sharp on- and off-axis at f/8. The RMS spot size in the far corner of a 4x5 format is 108 um.
    Biggest drawback is vignetting, which causes ~50% light loss at the far corner.

    Here's the lens data, all lenses can be purchased from Thorlabs for a total cost of $258. Air gaps are measured from the center of the lens.

    Lens 1: LA1979-A
    0.10 mm air gap
    Lens 2: LA1399-A
    8.56 mm air gap
    Lens 3: LD4293
    2.40 mm air gap
    STOP (aperture diameter 14.66mm at f/8)
    1.27 mm air gap
    Lens 4: LA1399-A
    2.71 mm air gap
    Lens 5: LA1050-A
    Back focal length: 109.55 mm

    I ran an image simulation to give an idea of its performance. If you click on the photo to enlarge and then hit next/prev you can pick out some of the differences.

    Click image for larger version. 

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    This is the stock image (640x480 pixel resolution) that's used as the input, i.e. this is the object that we'd be taking a photo of.

    Click image for larger version. 

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    This is the resulting image. As you can see, the field darkens as you go out toward the edges, but stays pretty sharp. There's also a small amount of distortion.

    Click image for larger version. 

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    This is the resulting image from Nodda Duma's OTS Cooke design at f/8 for comparison.

    In the next few days I'll put together a mechanical design and post it for the DIY'ers who'd like to build it. My mounting preference is usually Thorlabs lens tubes with spacers cut to length using a lathe.

    In the meantime, I'm not a LF photographer and am curious what the community thinks of trading off field uniformity for resolution at the edges of the field. If anyone has suggestions or comments for further improvements, I'd love to hear them. Unfortunately, it's difficult to have it all with stock lenses.

    Matt

  2. #2

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    Re: A DIY 123mm f/8 Cooke split triplet for 4x5

    The vignetting is pretty much as expected for a 123 mm lens on 4x5.

  3. #3

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    Re: A DIY 123mm f/8 Cooke split triplet for 4x5

    My 121.3mm lens in a Copal #1 has coverage of 82 degree at f22 or 211mm according to Schneider, and it at can be shifted 34-39mm on a 4x5" sheet of film. How will this lens behave when it comes to shift?
    Could you also do a similar thing using Nikon 5t, 6t, Canon 250d or 500d achromatic diopter lenses in combination with a minus-element?

    Sent fra min SM-G975F via Tapatalk

  4. #4
    Nodda Duma's Avatar
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    Re: A DIY 123mm f/8 Cooke split triplet for 4x5

    Nice work. The 0.1mm airgap between the two front lenses is tight. Can you reverse the order of those two front positives and pull them apart slightly?
    Newly made large format dry plates available! Look:
    https://www.pictoriographica.com

  5. #5

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    Re: A DIY 123mm f/8 Cooke split triplet for 4x5

    Quote Originally Posted by Nodda Duma View Post
    Nice work. The 0.1mm airgap between the two front lenses is tight. Can you reverse the order of those two front positives and pull them apart slightly?
    Thanks! I did a quick check and increasing the air gap between the two front lenses to 1 mm results in no appreciable difference in performance. In this case, the reoptimized air gaps are:

    LA1979-A to LA1399-A: 1.00mm
    LA1399-A to LD4293: 8.38mm
    LD4293 to STOP: 2.54mm
    STOP to LA1399-A: 1.32mm
    LA1399-A to LA1050-A: 2.35mm
    Back focal length: 109.43mm

    I've generally found that I can cut spacers to length with a tolerance of 0.1mm, so this separation will be a bit safer in terms of ensuring that positive lenses 1 and 2 don't touch.

    Switching the order of the two front positives yields similar results in terms of spot sizes. With the LA1399-A leading, the field curvature is nearly completely corrected, but at the expense of larger uncorrected astigmatism.

  6. #6

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    Re: A DIY 123mm f/8 Cooke split triplet for 4x5

    Quote Originally Posted by Oslolens View Post
    My 121.3mm lens in a Copal #1 has coverage of 82 degree at f22 or 211mm according to Schneider, and it at can be shifted 34-39mm on a 4x5" sheet of film. How will this lens behave when it comes to shift?
    Could you also do a similar thing using Nikon 5t, 6t, Canon 250d or 500d achromatic diopter lenses in combination with a minus-element?

    Sent fra min SM-G975F via Tapatalk
    To make this sort of lens work well, the surface curvatures, glass choice, and spacing are all critical. I haven't seen this sort of detail published for Nikon or Canon lenses, which means that adding a negative element would unfortunately be pretty unlikely to improve image quality. If anyone knows the lens data for these lenses, I'd be happy to give it a try.

    Regarding the field of view, this split triplet has a 62.6 degree field of view at the diagonal. I'm not sure what you mean by shift. Lateral or axial shift, and for what purpose?

  7. #7

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    Re: A DIY 123mm f/8 Cooke split triplet for 4x5

    Quote Originally Posted by Dan Fromm View Post
    The vignetting is pretty much as expected for a 123 mm lens on 4x5.
    Good to know, thanks. I might play around a little with a stronger negative element to increase the focal length and reduce the vignetting then before moving over to the mechanical design.

  8. #8

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    Re: A DIY 123mm f/8 Cooke split triplet for 4x5

    Quote Originally Posted by OTS Matt View Post
    Good to know, thanks. I might play around a little with a stronger negative element to increase the focal length and reduce the vignetting then before moving over to the mechanical design.
    Vignetting is given by cos^4(theta) where theta is the angle off axis. Increasing the focal length will reduce theta and vignetting, otherwise there's little you can do.

    The shift referred to in post #3 above is movement of the lens' axis off the gate's axis. Direction doesn't matter with decentering movements. Right, left, up, down are all the same.

    To answer the shift question in post #3, a 123 mm lens that covers 62.6 degrees will just cover 4x5 with no room for decentering movements. If you scale the prescription up to make a 150 mm lens, it will cover 182 mm and will allow some movements on 4x5. You haven't designed a wide angle lens.

  9. #9

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    Re: A DIY 123mm f/8 Cooke split triplet for 4x5

    Quote Originally Posted by Dan Fromm View Post
    Vignetting is given by cos^4(theta) where theta is the angle off axis. Increasing the focal length will reduce theta and vignetting, otherwise there's little you can do.

    The shift referred to in post #3 above is movement of the lens' axis off the gate's axis. Direction doesn't matter with decentering movements. Right, left, up, down are all the same.

    To answer the shift question in post #3, a 123 mm lens that covers 62.6 degrees will just cover 4x5 with no room for decentering movements. If you scale the prescription up to make a 150 mm lens, it will cover 182 mm and will allow some movements on 4x5. You haven't designed a wide angle lens.
    That's right. This lens system was designed for imaging on a 4x5 format and I didn't consider a larger field of view. While light will get through the system at a wider angle, the performance drops off pretty rapidly. I'm not an LF photographer and wasn't aware that decentering the lens was a common technique. What minimum diagonal field size would you recommend for designing for a 4x5 format?

    Unfortunately, scaling the prescription is tricky with stock optics. I was able to achieve an improved design with a 165mm focal length, which I'll be posting soon, but it's not what I'd consider to be wide angle and is unfortunately quite a bit more expensive due to the lenses needed.

  10. #10

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    Re: A DIY 123mm f/8 Cooke split triplet for 4x5

    Quote Originally Posted by OTS Matt View Post
    What minimum diagonal field size would you recommend for designing for a 4x5 format?
    The minimum is 150 mm. Every 4x5 photographer has ideas about how much more is absolutely necessary. They got along for quite a while with 150/4.5 Tessars and similar, which cover around 170 mm. Late 1950s - early 1960s plasmat types cover around 210 mm. More modern plasmat types cover around 220 mm.

    If you're thinking about wide angle lenses, a 4/4 double Gauss type might be possible with stock meniscii. Might. This one of four classic (= pre-WW II) w/a forms. The others are wide angle Protars, "dagor" types (symmetrical 6/2) and the Hypergon (two deep thin meniscii). All very slow, I think the fastest really wide ones are f/9 or so and most are f/14 or slower. I doubt that any of them can be cobbled up from stock optics, could be mistaken.

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