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:
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.
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.
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.
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
Bookmarks