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.)

Solution (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. Lucky for us, there are many other lenses in the world, 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. 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 2023, and test if they can be adapted using easily accessible technology like 3D-printing.

Results

Success! I 3D-printed several lens adapters to fit c-mount lenses and Super8 projection lenses into the Eiki lens mount. (Contact me if you want the files.) 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.

C-mount lenses (for CCTV and film cameras)

There are many cheap lenses for CCTV cameras, but most will only cover a small image sensor (1/4″ CCTV format). The 16mm frame falls between the 2/3″ and 1″ CCTV formats). Modern 1″-format lenses are somewhat 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 25mm f1.4	1″ CCTV sensor	Excellent	Good	Works great! In my copy of this tiny lens, the glass is quite yellowed. A clear one would probably work well.
	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! Needs a custom adapter: 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.

Other Projector Lenses (16mm and super8)

Small 16mm projection lenses are an obvious choice, but wide ones are rare and usually slow. The oldest ones aren’t multi-coated, so internal reflections will limit contrast.

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 these lenses is short, which may interfere with your projector. When focused, the Bell & Howell 1″ lens almost touches the pressure plate on my auto-loading Eiki RT projector. (It protrudes to the left of the projector’s lens barrel by about 5mm.) On a slot-loading projector, the pressure plate is pushed away from the film during loading, so it would probably hit the back of the lens!

Pic	Name	Intended Format	Edge Brightness (Vignette)	Edge Sharpness	notes
	Super Sankor-16 25mm f1.5	16mm film (Bell & Howell 29.4mm dia)	Excellent	Excellent	This is a small optic that was sold inside several different barrels to fit B&H, Eiki, etc. As expected, it works fine with the right adapter, but is a bit slower than I would like.
(no pic)	Bell & Howell “Increlite” 1″ f1.6	Super8 film	?	?	Not tested, just figured it was worth a mention because they are very common.
	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″ (25mm) f1.2	Super8 film	Good, maybe some fall-off on extreme edges	Excellent	Works great! This lens is very easy to adapt and performs well enough for 16mm. (but see warning above about flange-back distance on slot-loading projectors)
	Bell & Howell 19-32mm f1.2 (zoom)	Super8 film	Unusable vignetting at all focal lengths	N/A	Fail

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
29.4mm (1.156″)	Kodak Kodascope and Pageant series, Bell & Howell 100-200-300 series (Kodak and B&H lenses aren’t necessarily interchangeable even though the diameters match. B&H mounts are much farther from the film plane and have a smaller focus thread pitch.)
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