Most lenses already have a standard, secure method for attaching accessories to the distal end. So why do we still put up with the infamy of squeeze-style caps that are so easily lost? Below are some iterations of my designs for threaded lens caps, designed with information on the lens filter thread standards from Wikipedia, and printed in various colors of Shapeways basic sintered plastic. They’re durable, can be colorful, and it’s possible to emboss custom text or an image on the front. Oh, and I never worry about them falling off in the bag.
I am testing a squeeze-to-attach Lieberkühn that roughly fits a Canon f/3.5 20mm focal length macro lens (above), and a 58mm threaded version (below), tested with a Canon 35mm f/2.8 manual tilt shift lens. I used a Canon auto macro bellows and a Nikon D5100 with an adapter for all test images.
I haven’t added any reflective material to them yet, so they are essentially “Lieberkühn diffusers” for these tests. I used a domestic desk lamp with a 750 lumen halogen bulb to illuminate the specimens, for slightly off-axis trans illumination.
These are the legs on a cicada molt from last year’s 17-year brood. The photo was taken with the 35mm Canon tilt-shift lens at about the shortest macro-bellows distance possible.
And here is a shot of the same view with the reflector attached. I used a 1/13 second exposures at ISO 1600 and f/5.6 for both shots.
The large claws up front with (below) and without (above) the reflector. Again this was taken at f/5.6, an ISO 0f 1600, and 1/13 second exposure time. I increased the bellows distance slightly for this shot, increasing the magnification.
Although the fill light is definitely better in the shots with the reflector, in some cases a photographer may prefer the image without using it, e.g. to bring out the small details with shadow. The cicada molt is partially transparent, giving a nice effect to the light transmitted through the subject.
I took the two photos of a leaf-cutter bee (Megachile genus, female) below with the same setup. The difference in lighting with and without the reflector is pretty drastic.
I made the next two pairs of photos using the 20mm macro lens and the squeeze Lieberkühn reflector. The photos contain some apparent lens flare resulting from the off-axis light source, manifesting as a slight general brightening (and resulting loss of contrast) in the middle of each image. I am not sure if the aberration is reduced with the addition of the reflector or if it just looks that way due to the rest of the image being brighter. Looks like a job for some quantitative comparisons, for the next post.
The position of the lamp and bellows stand were maintained for each pair of images. The bellows was set at the same distance but displaced between exposures to make room to attach the reflectors without disturbing the subjects, so the comparison images may be focused ever-so-slightly at different depths.
The lighting was definitely improved by the use of reflectors for these (mostly opaque) subjects. The images above were intended as a qualitative investigation, I will be looking into the performance and useability of the designs further.
As a final note, compare the print of the 58mm threaded reflector with the render from the STL file. The consistency is inhomogenous, with some bulges introduced during manufacture that were not part of the design file. Can’t say that I’m impressed with the print quality.
Designing a Lieberkühn Reflectors for macro- and micro-photography
A Lieberkühn Reflector gets its name from one Johann Nathaniel Lieberkühn, who invented the speculum that bears his name which you may recognize from reflective headband decorations for doctor costumes. The name is generally changed from “speculum” to “reflector” when referring to optical reflectors used in photography and microscopy, perhaps because the term has drifted from its original Latin root meaning “mirror” to refer to probing instruments for dilating orifices.
Lieberkühn reflectors were a way to bathe an opaque specimen in fill light. Lieberkühn reflectors and their use have unfortunately fallen by the wayside with the advent of modern conveniences like LEDs and fiber optic illumination. The above example from the collection of the Royal Microscopical Society displays a Lieberkühn on a simple microscope. In use, the reflector would be pointed towards the specimen, and fed light by a second mirror like the one on the rightmost microscope. Both of the microscopes pictured were on display at the Museum of the History of Science in Oxford
The working part of the Lieberkühn reflector is a parabolic mirror, which doesn’t add the spherical aberrations of hyper- or hypo-bolic configurations. As an added benefit, mirrors don’t tend to add chromatic dispersion or other aberrations associated with refraction (though they can effect polarisation). A parabola can be described as a a particular slice through a cone , but for the purposes of my first prototype, the functional description in cartesian coordinates will do.
Where depends on the focal length of the parabola.
To get a functional, 3-dimensional mirror, I describe the parabola in terms of the focal length and a given radius as a 2D trace and spin it with rotate_extrude() in OpenSCAD. Leaving an aperture in the middle leaves room for light to reach the objective. The reflector shown below has a 4mm central aperture for the objective, 16mm focal length and 32mm diameter.
I have sent a few prototypes (matched to particular lenses or objectives) to Shapeways for prototyping. After some characterisation these will appear on theBilder shoppe.