[This a descrition of the 900mm lens that Dave Boyd has made himself. The original location of these files was: ftp://butler.hpl.hp.com/jacobson/photo/Boyd_900mm_Lens
In there the images are in TIFF format which makes them _very_ large (1.6 MB to 2.5 MB). Here they are in JPEG format which makes them much more compact, without significant loss of quality. For each image there are 2 different JPEG files (quality 75 and quality 95). The 95 files are 3 to 4 times as large but the difference in quality is hard to notice. ]
This file describes the four image files and two other text files that are also found in this directory. The images are 24-bit TIFF format (Motorola) images of three photos taken with my homebuilt 900mm mirror lens, as well as one photo of the lens and me.
The TIFF files have been compressed using the unix compress command, so they will have to be uncompressed before viewing. The viewing program xv handles the files just fine, but it's the only one that's been tested. The files do not include any gamma correction. Try a gamma of 1.8 for viewing on a CRT.
I have not retouched the scans any, so there will be a few dust specks and such visible. However, I did apply a little digital sharpening so that the images displayed on the screen will approximately visually match the prints.
On three of the images, I scanned essentially all of a 3.5 by 5 print. The moon picture is cropped more as explained below.
Here we go:
This is just a picture of me with the lens. (Obviously NOT taken with the homemade lens!) The lens is resting on a beanbag chair which I often use as a support when using the lens. The large white piece in the foreground is the lens cap. The focus knob is just behind my hand, and the camera mount is about a foot in front of my hand to the viewer's left. There is no camera attached in the picture, and a "rear" lens cap is in place.
This is a Canada goose. I picked this shot partly because it shows some good detail in the bird's feathers. But it also has one artifact in it that I described in my writeup. Near the top center of the picture, (actually a little to the right of center) is an out-of-focus highlight. Apparently it's a glint off of a wave in the background. In the picture, it shows up as a blurry white circle with a blurry rectangule cut out of it. This is an unavoidable artifact of any lens which has an obstruction in the optical path, as do mirror lenses. Commercial mirror lenses show out-of-focus highlights as donut-shaped. My lens has a rectangular "donut hole" because the 45-degree turning mirror in my lens is rectangular.
I realize this shot is not particularly well composed (YOU try maneuvering this thing!) but it does show some interesting stuff. Notice the good detail in the fur and grass. And notice the shallow depth of field, even at f/11. Also, the concrete sidewalk in the foreground shows the out-of-focus effect described above at every tiny shiny point. But in this case, I kind of like the texture it gives. So that effect isn't always bad.
This is (obviously) the full moon. I shot this on Ektar 25 at roughly 1/30 second. (I know there's a school of thought that says you have to use a very fast shutter speed to stop the moon's motion. But actually, the moon only traverses about 1/5400 of its diameter in 1/30 second. I suspect that my sharpness is more lens-limited and grain-limited than motion-limited.) I had this custom printed so that the moon is about four inches across, and then scanned that. The moon in the print is not tack-sharp. It's possible that I just didn't focus very well, but probably this is just the best my lens can do. The custom enlargement has enough magnification that some grain is visible in the print.
This is a copy of the original posting I made to rec.photo.advanced describing my homebuilt 900mm lens. Please note that I don't plan to make any more prints available through the mail. If you weren't one of the first batch of people to request them, please try to make do with the electronic versions online. Thanks.
This is a copy of a file I've been mailing to people who've been expressing an interest in the project and/or asking questions. It answers some of the common questions I get.
Dave Boyd Hewlett-Packard, Greeley, Colorado firstname.lastname@example.org