Adapting Short-Throw Lenses for 16mm Projectors

Context

This guide will focus (!) on lenses for Eiki projectors (R, N, SL, SSL) because that’s what I have. They’re reliable, and their simple modular construction makes repairs easy. (They’re also quite fun to modify!) I would consider these to be “late era” 16mm projectors: Built in the 1960’s – 1980’s with large lens diameters (40mm +) to accommodate fast glass (f1.2 – f1.4).

Earlier projectors (1920’s – 1950’s) used smaller, slower lenses. Manufacturers that began in that era (ie Bell & Howell) switched to larger lenses in their later projectors. So there are many different projection lenses floating around the used market, with different diameters, aperture, and focal length. (See lens mount specs chart further down this page.)

The problem

Eiki projectors (and all others I know about) include a 50mm lens by default, designed to fill a typical screen in a theater or lecture hall. If you have a smaller room, the projected image will be tiny. (See screen size chart at end of this page.) How to get a larger image?

Solutions (sort of)

A short-throw lens (AKA wide-angle projection lens) will produce a larger image, although less bright. Projector manufacturers offered “official” short-throw lenses like 25mm (1″) or 38mm (1.5″) 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 f0.95 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 specs at end of this page.)

• 

  Appropriate “flange-back” distance: That’s the distance between the back of the lens and the film plane (when the lens is focused on the screen). On a manual-load or auto-load projector you can get pretty close, but on slot-loading projectors the pressure plate is pulled forward until loading. On Eiki slot-loaders, the back of the lens must stay about 15mm away from the film plane. Some lenses sit much too close, so pay attention to the “OK for Eiki Slot Load” column in the tables below. (Dunno about other manufacturers, but the principle should be similar.)

• 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 f1.6 is acceptable. 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 now (currently 2025), 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 non-standard lenses into the Eiki lens mount. They are very basic, using a simple friction-fit for simplicity. 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	OK for Eiki Slot-Load?	notes
	Computar 25mm f1.3	1″ CCTV sensor	Excellent	Excellent	Yes	Works great! This is one of the sharpest and most punchy lenses I’ve tested. The manual-aperture version of this lens is model “V2513” but they are rare because astro-photography folks like them too! I took apart a cheaper auto-iris “APC TV Lens” version to remove aperture and outer frame. It’s a bit complicated, but worth it in the end.
	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	Good	?	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.

16mm Projector Lenses

Older 16mm projectors used small diameter lenses. They can be easily adapted to fit more recent projectors, but some are quite old and lacking in sharpness and flare control. (I’ll also list a few “native” Eiki 16mm lenses for the sake of completeness.)

Pic	Name	Intended Format	Edge Brightness (Vignette)	Edge Sharpness	OK for Eiki Slot-Load?	notes
	Buhl 16mm f:2 (various options, also known as 0.625″ or 5/8″)	Buhl 103-450 fits Graflex / Kalart / RCA 400 Buhl 103-810 and Advance Products 8286 fits Bell & Howell (29.4mm dia threaded)	Excellent	Good	Yes	Small Buhl lenses tend to be low contrast and a bit soft, but it works. (f2 is pretty dim, though)
(no pic)	Buhl by Kodak 19mm F1.4 (593-430)	Bell & Howell (29.4mm dia threaded)	Excellent*	Good*	?	* I haven’t tested this lens, but this 2024 forum thread claims that it works.
	Buhl Eiki RP 0.75″ (19mm)	Eiki (42.5mm)	Excellent	Excellent	Yes	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 38mm internal lens (not usable on its own due to field curvature), mirror box, and auxiliary wide lens (which also corrects the field curvature so the resulting lens is sharp edge-to-edge. Maybe these parts could be reconfigured to make a 19mm straight lens?
	Super Sankor-16 25mm f1.5	Bell & Howell (29.4mm dia threaded)	Excellent	Excellent	Yes	Works great! This is a small optic that was sold inside several different barrels to fit B&H, Eiki, etc. It’s punchier and higher contrast than the small Buhl lenses in this mount.
	Buhl 1.0″ f:1.4 (803-410) (25mm)	Bell & Howell (29.4mm dia threaded)	Excellent	Good	Yes	Small Buhl lenses tend to be low contrast and a bit soft, but it works.
	ISCO Vario Kiptaron MC 35 – 65mm Zoom F1.3	Eiki (42.5mm)	Excellent	Excellent	Yes	A sharp, multi-coated fast zoom that covers useful focal lengths. Not cheap though!

Super8 Projector Lenses

Purists will scoff at these puny optics, but Super8 projection lenses already meet a lot of our criterion (small, cheap, plentiful). They were designed to cover a much smaller image circle, but it turns out that a few of them “accidentally” cover 16mm too!

WARNING: The “flange-back” distance on most Super8 lenses is usually too short for slot-loading projectors!

Pic	Name	Intended Format	Edge Brightness (Vignette)	Edge Sharpness	OK for Eiki Slot-Load?	notes
	Bell & Howell “Increlite” 1″ f1.6 (25mm)	Bell & Howell 22.2mm (7/8″) dia threaded)	Good	Bad	No	A very common lens, included with B&H 8mm projectors. Mine is all silver but some are silver/black without threads. It sort of covers 16mm, but edge focus is bad.
	Bell & Howell 1″ f1.2 (25mm)	Bell & Howell 24.5mm (1″) dia non-threaded	Good, maybe some fall-off on extreme edges	Excellent	No	Works great (but not for slot-load projectors). This lens is punchy and high contrast, but the rear element rests VERY close to the projector’s pressure plate.
	Schneider Xenovar 15.5mm to 28mm F1.2  (zoom)	Bauer and Silma	Excellent*	Excellent*	?	* I haven’t tested this lens, but a forum thread from from 2018 confirms that it works great.
	ISCO-Gottingen Vario-Kiptagon 15-30mm Zoom f1.3	Bauer and Silma	Unusable vignetting	N/A	No	Fail. A sharp, fast lens with nice contrast… but sadly doesn’t cover 16mm frame.
	LOMO PF-6A 18-30mm Zoom f1.2	Lomo	Unusable vignetting	N/A	No	Fail. Nice lens, and easy to find on Ebay… but sadly doesn’t cover 16mm frame.
	Elmo 15-25mm Super Zoom f1.3	Elmo	Unusable vignetting	N/A	N/A	Fail (also tried similar Etalon lens from Copal projector)
	Bell & Howell 19-32mm f1.2 (zoom)	Bell & Howell 22.5mm (1″) dia non-threaded	Unusable vignetting	N/A	N/A	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 Mount Specs

source: Australian Council of Film Societies technical pages and my own observations. Please get in touch if you have corrections or additional info!

	Diameter	Brands and Models
	25.4mm (1″)	Keystone projectors (and others?)
	29.4mm (1-5/32″) with wide pitch focus threads	Kodak Pageant, Analyst, and Kodascope series. Note: B&H lenses have the same threaded outer diameter, but the thread pitch and position are different
	29.4mm (1-5/32″) with 3.18mm (1/8″) pitch focus threads and narrower un-threaded rear (25.4mm AKA 1″)	Bell & Howell 100-200-300 series. Note: Kodak lenses have the same threaded outer diameter, but the thread pitch and position are different
	30.1mm (1-3/16″) with 3.18mm (1/8″) pitch focus threads	Ampro, Graflex-Singer, Kalart Victor, RCA 400 series
	33mm (1.3″) with focus threads	Eumig projectors (unconfirmed but I think this is correct!)
	40mm (1.57″) with 1 focus groove	Elmo (all series) & Kodak CT1000 (re-badged Elmo)
	41.27mm (1-5/8″) with focus threads	RCA, DeVry, Ampro, Movie Mite, Natco, Victor, etc
	42.5mm (1.67″) with fine grooves	Eiki (all series), B&H 35xx (re-badged Eiki), Hokushin
	42.5mm (1.67″) smooth barrel	Bauer P6, P7, P8 (These will physically fit into an Eiki projector, but the lack of grooves makes focusing impossible.)
	45mm (smooth with 1 focusing “notch” cut into side)	Hokushin?
	52.5mm (2.062″) with fine grooves	Bell & Howell 500-1500-2500 series, Graflex-Singer 1100 series (large barrel), Devry, B&H, RCA, JAN, Viewlex

 

16mm Film Projection Screen Size Chart (meters)

source: adapted from an Eiki projector owner’s manual