This isn't a new subject for most digital camera users, and I'll assume you've already seen at least a couple of articles or posts on using them before now. What you may not have seen are a few twists on using an unconventional projector lens, meant for viewing medium format slides, and a way to mount a 50's classic projector lens for 35mm slides, by using it's original mounting hardware as a ready-made starting point. It allows anyone to easily get shooting these lens types without much effort or advanced and/or custom parts construction.
In the 1950's, Leitz created a series of slide projectors that were redesigns of their earlier models that essentially let you mount your Leica camera lens into the front of a projector and then light up your slides on the wall, by using them a different way. These camera lenses were mounted into focus tubes that could be exchanged as needed, and both were screw mounted to a support bracket on the frame of the projector's main body. The later and dedicated type of projector lenses as presented here, is a Leitz Hektor 12cm f/2.5 lens, and inserted into it's original Grey hammer-tone finished focus tube and mounted in the same way.
These came from a late 1950's Leitz Prado, model 500, slide projector. These lenses were designed to be fully in focus for edge-to-edge sharpness, across the entire frame of a glass-mounted 2" x 2" 35mm film slide. Later, Leitz and other makers made adjustments to their designs for curved-field projection (CF) lenses suited for slide mounts that didn't use sandwiched glass plates. The quality of those results are said to be noticeable when projecting slides, but less so when adapting them as 'taking lenses'. I can only vouch for the qualities of the one I have, and it does not disappoint with what I've seen so far!
For a photographic lens of the 1950's, f/2.5 apertures were pretty fast for anything near the 120mm focal length, but obviously it was never designed to be camera mounted. One negative side effect of using this particular lens attached to a camera, is a rather pronounced halo that glows around anything in the frame that is, or is near to, the color of white. The same thing occurs when a highlight or reflection is overly strong and appears as white to the camera sensor as a 'blowout'. The same Blue/White glow is apparent when the lens is used on a full frame, APS-C, or M4/3 sensor, so it isn't going to disappear by switching to another body or brand. More on the remedy in fixing this, comes later.
The Hektor lens is shown mounted in temporary hardware with a little help from some strong packing tape. The underlying adapter below the focus tube is a cheap 'dumb' aluminum version that was designed to attach Canon EOS Mount lenses to Sony's E-Mount; and in this case, a Sony NEX-F3. The trick to getting it to accept the screw-in base from the focus tube, was to completely remove the Canon-fit bayonet mounting plate from the front of the adapter. The throat dimension was then large enough to accept the threaded section of the focus tube. There might be room for a single piece of typing paper to slip into the gap between them, but it's certainly not wide enough to alter the lens's center position and needed no adjustment - just the tape to hold it firmly in place.
What can be done to get rid of the halo effect? A simple Waterhouse stop was created with some 1mm thick Neoprene sheet foam, which was tucked inside the rear barrel of the lens, directly behind the last element in the last optical group. The circular hole made with a punch was fairly large and had about a 2mm wide band around it's edge. I'd guess that it closed the aperture by maybe one full stop when in place. A small price to pay in speed to get razor sharp focus and to boost contrast and eliminate the halos, all at the same time.
At $11 total for the Canon adapter (that I never really could use on apertureless lenses anyway), it was a bargain, and it's also gives the lens the perfect depth to allow for adjusting the Hektor for infinity focusing on any E-Mount body I'd use. At some point I'll probably come up with a more permanent solution to better attach that or another E-Mount adapter, but it's solid the way it is and shouldn't cause any issues until then. The only caveat, is that you can unscrew the lens completely when focusing it in it's original tube, so it pays to remind yourself to hold on to the barrel when it's at the extreme end of travel.
On the other end of the projector lens adaptation spectrum, is an almost feather-light medium format gem, known as the standard 150mm f/3.5 lens included with the Rollei P66-S projector. It was designed for the extra 'throat' size required to illuminate the window in 2.25" x 2.25" frames when they're holding 6cm x 6cm square-format slide positive film stock. Yes, this is the slower sibling to the lauded Heidosmat f/2.8 versions for 35mm slides, but in my humble estimation, it's equally compelling in it's abilities - and with my winning bid at roughly 1/10th the price of it's more fabled brother - it's a major bargain if you're prepared to do the work to get it mounted.
It's larger throat means larger diameter glass, and a larger diameter barrel to hold everything in place. The outside diameter of the barrel is 73.85mm, and it needed a little bit of math, some dumb luck, and some creativity in materials to get it into a stable enough adapter to start testing with. From the E-Mount to lens tip, there are 3 sections I could tape or strap or click/stack together before I could confirm the conversion idea could work with this lens. Starting with an E-Mount to Nikon F adapter, then adding one more for a Nikon F to Bronica ETR Mount makes a pair of stacked adapters that gets me to an approximately correct flange distance between Sony mirrorless bodies and the rear lip of the Rollei's outer barrel.
The ETR lens mount has a throat diameter of 76.5mm, leaving enough clearance inside the lip on it's adapter to rest the lens flat inside it, and keep the image circle close to being centered. A wide strip of the same 1mm thick Neoprene foam takes up a good bit of that extra space, and acts like a soft friction pad to maintain the lens's position at it's focus point when it's pushed/pulled to sharpen it. A cut-up plastic tub for cottage cheese is acting as a makeshift extension barrel for the lens to slide into. Another wide strip of neoprene on the outside of it helps with keeping the taped plastic cylinder more light-tight.
It works well enough for a quick test session, but it requires a far more substantial setup to be truly user friendly and provide the hardware needed to maximize the capabilities of the lens that it holds. A quick look at what's available to make a proper mount assembly suggests that a 75mm x 100mm tube with a male M65x1 attached threaded section can get it most of the way there, with an additional 45-47mm of a 'filler tube' to then get it attached to a Sony E-mount adapter plate to cleanly finish it up.
Unlike the other Heidosmat's in the family, this one is plastic and glass, and nothing else. That makes it very light, and has a total weight of 396.4 grams, or 13.98 ounces - and that's including all the temporary mounting bits and pieces, but without the rear adapter cap and the tripod foot. It doesn't have the heft that it might appear to have from it's size and dimensions - but - it certainly isn't too light or badly made or a cheap feeling product either.
Sooo... stay tuned for 'Part 2', and we'll see how that all works out once the parts are in-hand. For now, there's test shots to show what the early results are on both lenses, or call up the tags for Heidosmat -or- Leitz Hektor to browse each photo set.