Adapting Short-Throw Lenses for 16mm Projectors

The problem

16mm film projectors usually include 50mm lenses, designed to fill a typical screen in a theater or lecture hall. If you have a smaller room, you probably won’t be able to fill the screen. (See handy screen size chart at end of this page.)

Solutions (sort of)

A short-throw lens (AKA wide-angle projection lens) will produce a larger image, although less bright. Projector manufacturers made short-throw lens options (like 25mm), but these are exceedingly rare in the used market. They also sold add-on lenses to widen or shrink the projected image. The Eiki Zoom Converter-16  and Sankor Zoom Converter are pretty common and offer a 0.75x – 1.25x enlargement of the stock 50mm lens.  That’s still only 38mm on the wide end, and they aren’t cheap. Lucky for us, the world is full of lenses, and some of them might do what we need.

What do we need?

In general, a projection lens should fit these criterion:

• Can be adapted to projector’s lens mount diameter: This is tough. Your super-fast SLR lens is not gonna work because the whole thing needs to fit inside the projector’s lens mount. You usually can’t modify the projector because there is a chassis wall right next to the lens, so max diameter is non-negotiable. (See handy chart of projector lens diameters at end of this page.)
• Appropriate “flange-back” distance: (distance between the lens’s mounting flange and the film plane) We might take apart the lens, so this measurement seems irrelevant… BUT the projector’s pressure plate will force a minimum distance between the rear lens element and the film plane, so effectively it still matters.
• Appropriate focal length: 25mm is a good option for short-throw projection. You can go lower, but there are very few < 25mm options that fit our other criteria anyway.
• Fast aperture: Good projection lenses are FAST (often f1.2). I’d say f1.4 is acceptable for an adapted camera lens. Anything slower is eating a lot of valuable light.
• Image Circle: Lenses are designed to project a light cone that covers a specific target (film gauge, video camera sensor size…) Ours needs to cover at least the 16mm film area (10.26 mm × 7.49 mm), which is between 2/3″ and 1″ video camera sensor size. Larger is fine, but smaller = vignetting.
• Sharp and low flare: This is hard. Since we’re working at the widest aperture, no lens will be performing its best. Results will vary wildly.
• Flat focal plane: Many inexpensive camera lenses have pronounced “field curvature” meaning that their focal plane is more spherical than flat. This might be acceptable for photography (since most people don’t shoot perfectly flat subjects parallel to the film plane) but in projection it results in soft corners: unusable!
• Cheap and available: We’re trying to avoid buying a rare expensive lens!

History

Re-housing lenses for projection isn’t new. (They are usually called “sleeved” lenses or lenses in “bushings”.) I have come across several adapted lenses, like this unbranded 25mm in a nicely machined Eiki adapter sleeve.

My goal here is to survey inexpensive lenses that are available in 2024, and test if they can be adapted using easily accessible technology like 3D-printing. (This too, has been done. Example: “Bell & Howell Lens adapter to ELMO 16mm projector” available on Shapeways.

Results

Success! I 3D-printed several lens adapters to fit c-mount lenses and Super8 projection lenses into the Eiki lens mount. They are very basic, using a simple friction-fit because I don’t trust my printer to make precise c-mount threads. For long-term use you’ll need something more reliable. It’s hard to get the tolerances just right to achieve reliable focus in the Eiki focusing mechanism. I didn’t have any problems with the heat deforming the plastic (PLA), but I expect that focus could wander over time as the materials expand. There is probably more optimization to be done.

Download the STL files and OpenSCAD source code (updated 2024-01)

C-mount lenses (for CCTV and film cameras)

There are many cheap lenses for CCTV cameras, but most will only cover a smaller image sensor (1/4″ CCTV format). The 16mm frame falls between the 2/3″ and 1″ CCTV formats). Modern 1″-format lenses are pricey (25mm Kowa LM25HC is about $230) so I looked for used lenses instead.  The popularity of mirrorless cameras has reinvigorated the market for small c-mount lenses, so you can find a lot of info on forums.

Pic	Name	Intended Format	Edge Brightness (Vignette)	Edge Sharpness	notes
	Computar 25mm f1.3	1″ CCTV sensor	Excellent	Excellent	Works great! I took apart an auto-iris “APC” version to remove aperture and make it tiny. It’s a bit complicated, but it works. The manual-aperture version is model “V2513” but they are rare because astrophotography folks use them too!
	Cosmicar/Pentax TV Lens 25mm f1.4	1″ CCTV sensor	Excellent	Good	Works great! My copy has a yellow tint, but otherwise excellent. (There seems to be several versions. You want the small one: just a few mm wider then the c-mount threads.)
	Pixco (Fujian) 25mm 1.4 (“PL2514”)	1″ CCTV sensor	Excellent	Terrible.	Fail! Excessive field curvature makes it impossible to keep center and edges sharp. (This modern lens is cheap and easy to get. Too bad it’s terrible.)
	Lytar Som Berthiot 25mm f1.8	16mm film	Excellent	Excellent	As expected, it works fine, but it’s slow at f1.8.
	Cosmicar/Pentax 12.5mm f1.4 (“C21211”)	1″ CCTV sensor	Excellent	Good	Very wide! Too big for my c-mount adapter but if you remove focus grip rubber band it will fit into Eiki lens barrel with a few mm to spare.

Smaller Projector Lenses (16mm and super8)

Small 16mm projection lenses can be easily adapted to fit more recent projectors, but most are quite old. the oldest ones lack multi-coated glass, and wide ones are relatively rare.

Purists will scoff at the puny optics, but Super8 projection lenses already meet a lot of our criterion, so maybe we’ll get lucky and some of them will cover 16mm too?

WARNING: The “flange-back” distance on Super8 lenses is short, which will interfere with a slot-loading projector. The Bell & Howell 1″ f1.2 Super8 lens works fine in an auto-loading Eiki RT projector, but the rear element almost touches the pressure plate. On the slot-loading Eiki SSL, the pressure plate is pushed away from the film by about 15mm during loading, so this lens will not fit that projector!

Pic	Name	Intended Format	Edge Brightness (Vignette)	Edge Sharpness	notes
	Super Sankor-16 25mm f1.5	16mm film (Bell & Howell 29.4mm dia threaded)	Excellent	Excellent	Works great! This is a small optic that was sold inside several different barrels to fit B&H, Eiki, etc.
	Schneider Xenovar 15.5mm to 28mm F1.2  (zoom)	Super8 film	Excellent*	Excellent*	Intended for Bauer and Silma Super8 projectors. (+ others?) * I haven’t tested this lens, but a forum thread from from 2018 confirms that it works great.
	Buhl 5/8″ f:2 (103-810) (17mm)	16mm film (Bell & Howell 29.4mm dia threaded)	Excellent	Excellent	Works great! (Pretty slow at f 2.0 though)
	Buhl 1.0″ f:1.4 (803-410) (25mm)	16mm film (Bell & Howell 29.4mm dia threaded)	Excellent	Excellent	Works great!
(no pic)	Buhl by Kodak 19mm F1.4 (593-430)	16mm film (Bell & Howell 29.4mm dia threaded)	Excellent*	Excellent*	* I haven’t tested this lens, but this 2024 forum thread claims that it works.
	Buhl Eiki RP 0.75″ (19mm)	Eiki 16mm	Excellent	Excellent	A rear-projection lens (short throw, reversed image). Worthy of mention because A) it’s cheap & weird B) It comes apart easily to reveal a 40mm(ish) imported lens, mirror box, and auxiliary wide lens. Maybe these parts could be reconfigured?
	Bell & Howell “Increlite” 1″ f1.6 (25mm)	Super8 film (Bell & Howell 22.2mm (7/8″) dia threaded)	Excellent	Edges are a little soft, but it will work if you’re desperate!	A very common lens, included with B&H 8mm projectors. Mine is all silver but some are silver/black without threads. (The flange-back distance is similar to 16mm lenses so there’s no danger of hitting the pressure plate.)
	Elmo 15-25mm Super Zoom f1.3	Super8 film	Unusable vignetting	N/A	Fail (also tried similar Etalon lens from Copal projector)
	Bell & Howell 1″ f1.2 (25mm)	Super8 film (Bell & Howell 22.5mm (1″) dia non-threaded)	Good, maybe some fall-off on extreme edges	Excellent	Works great but won’t fit slot-loading projectors. (see warning above about flange-back distance)
	Bell & Howell 19-32mm f1.2 (zoom)	Super8 film (Bell & Howell 22.5mm (1″) dia non-threaded)	Vignetting at all focal lengths	Edge sharpness is mediocre	Fail. Too much vignette.

Mirrorless / SLR lenses

I considered re-housing one of the < $100 modern 25mm lenses (Pergear, Meike, TTartisans, 7artisans) because their lens elements are small enough to fit into the Eiki if I remove the original mount, aperture, and focusing helicoid. It would require a lot of effort and they are slow (f1.8), so I didn’t bother.

16mm Projector Lens Barrel Diameters

Diameter	Brands and Models
25.4mm (1″)	Keystone projectors (and others?)
29.4mm (1-5/32″)	Kodak Pageant, Analyst, and Kodascope series, Bell & Howell 100-200-300 series Kodak and B&H lenses aren’t necessarily interchangeable even though the threaded diameters match. (B&H focus threads are farther from the film plane and have a narrower pitch. The narrower unthreaded part is 25.4mm AKA 1″)
30mm (1.184″)	Graflex-Singer, Kalart, RCA 400 series, Ampro
33mm (1.3″)	Eumig projectors (unconfirmed but I think this is correct!)
40mm (1.57″)	Elmo (all series) & Kodak CT1000 (re-badged Elmo)
41.27mm (1-5/8″)	RCA, DeVry, Ampro, Movie Mite, Natco, Victor, etc, etc
42.5mm (1.67″)	Eiki (all series), B&H 35xx (re-badged Eiki), Hokushin
52.5mm (2.062″)	Bell & Howell 500-1500-2500 series, Graflex-Singer 1100 series (large barrel), Devry, B&H, RCA, JAN, Viewlex

source: Australian Council of Film Societies technical pages and my own observations

16mm Film Projection Screen Size Chart (meters)

source: adapted from an Eiki projector owner’s manual