gBeam lenses

gBeam is designed for 35mm film format objectives.
This includes objectives of cameras, slide projectors and cinema projectors – basically most of all interchangeable lenses ever made. For brevity we’ll often refer to those objectives as 35mm lenses.

  • SLR camera lenses
  • Slide projector lenses
  • Cinema lenses
SLR lenses can easily be repurposed for projection. While they are in principle more expensive in the making, they are much more common than projector lenses and therefore often more affordable.

SLR lenses have a rather short Flange focal distance (FFD) allowing for shorter projectors.

These lenses are built for projection, what’s more to say? ;)

They are usually much more lightweight than SLR lenses, but have a rather long FFD, requiring larger 3d printed adapters. Most slide projector lenses are rather cheap and simple constructions. 85mm f/2.8 is a very usable and common type. However it’s hard to find faster affordable  lenses.

These lenses have rather gigantic apertures, allowing for extremely bright projections. However they are quite big and have very long Flange focal distances, requiring rather voluminous adapters.

Camera lenses for projection

Just like projector objectives, which are basically camera objectives that work in reverse, camera objectives can be used for projection.
The only real problem is mechanical compatibility – which guerillaBeam solves.

Camera versus projector lenses

In theory, projector objectives are better suited for projection, since they are originally made and optimized for projection. However, camera lenses have the great advantage of being much available in greater variety, better qualities and also lower prices.

To see a more detailed list of differences worth noting, please click below:

  • Show camera and projector lens comparison

Camera lenses Projector lenses

Pros

  • Built-in focusing mechanism with distance indicator
  • Large variety of focal widths available at often quite low prices, powerful lenses are available for lower prices
  • More ruggedized design

Pros

  • Designed and optimized for projection
  • More lightweight and compact due to simplicity, no moving parts
  • 85mm f/2.8 objectives (42.5mm diameter) are dirt-cheap, and the most economic projection lenses

Cons

  • Not originally intended for projectors
  • Larger size and weight, unneccessary aperture mechanism
  • The cheapest reasonably fast camera lenses are a bit more expensive

Cons:

  • No focusing mechanism (focused by moving the objective).
  • Objectives faster than f/2.8 and not-so-common focal widths are rare and expensive
  • Build quality is usually not on par with camera lenses
  • Longe flange focal distance compared to camera lenses, requiring larger build sizes.

 

How to choose a lens

Focal length

The focal length of a lens determines its field of view.
For general projection purposes, a light long-focus lens (for example 85 or 135mm focal length) is most practical. Shorter focal lengths are useful for wide-angle projections, while longer lengths are good for long range.

Throw ratio

Any focal length has specific throw ratio.
The projection width equals the projection distance divided by the throw ratio .
In other words, you need to stand throw number meters away from a surface to project an image 1m wide. For convenience, I’ve calculated and listed the throw ratio of the most common objectives and listed them in the table below.

F-number

The f-number is the measure for the aperture size of an objective in relation to its focal length (denoted f/2.8, f/4, f/5.6 and so on). As larger apertures transmit more light (proportional to the apertures area), smaller f-numbers (‘fast lenses’) are desirable.

For the more common focal lengths, an f-number of at most 2.8 (denoted as f/2.8) is recommended.
A f/1.4 lens will, in theory, be 4 times brighter than a f/2.8 lens, and whopping 16 times brighter than a f/5.6 lens. The f-number is therefore most important when assembling a projector.

The disadvantage of fast Lenses is the their higher size, weight and price.

Lens comparison table

For your convenience, I’ve assembled this list of different typical focal widths, together with their recommended f-numbers and their throw ratio.

Focal width Recommended f-numbers Lens angle (horizontal) Throw ratio T Throw ratio reverse R
This number is most important for the brightness of the image. Lower ist better (yet more expensive). Projection distance for given projection width Projection width for a given projection distance
18mm f/3.5 90° 0.5 2
28mm f/2.8, f/3.5 65.5° 0.773 1.3
35mm f/2.8 54.4° 0.79 1.27
50mm f/1.4, f/1.7, f/2 39.6° 1.39 0.72
85mm f/1.8, f/2.8 23.9° 2.3 0.43
135mm f/2.8 15° 3.73 0.27
200mm f/2.8, f/3.5 10.3° 5.57 0.18
300mm f/4, f/4.5,f/5.6 6.9° 8.3 0.12
400mm f/4.5, f/5,f/5.6 5.2° 11 0.09
600mm f/5.6 3.4° 17 0.06
800mm f/8 2.6° 23 0.043
2.8 is ideal. higher f-numbers result in darker images, are cheaper and will work, however their performance will be significantly worse. e.g. projection distance needed is T times the given image width e.g. image width is R times the given projection distance

Zoom lenses

It is possible to use zoom lenses, however you should be aware that zoom lenses are either much less bright or much more expensive.

Lens compatibility

Lens mount / brand Notes Positively locking Download ready
M42, T2 screw mount No (screw mount) Yes
Pentax PK adapter operates the aperture lever to keep the automatic lenses open Yes Yes
Canon EF also fits EF-S Yes Yes
Nikon F fits virtually any Nikon 35mm lens ever made Yes Yes
Minolta SR Yes Yes
Rollei/Voigtländer QBM Yes No
42.5mm diameter slide projector objectives No No

Flange focal distances

Every objective has a specific flange focal distance (FFD) which must be met in order to get working distance indicators and proper focus to infinity. gBeam realizes this by using specific adapter lengths which add to 37mm FFD of gBeam itself.

Since the minimum adapter length is around 6 mm, the 37mm end of gBeam is only able to adapt lenses with an FFD of at least 43mm. However, shorter FFD can be realized by reversing the slide holder and attaching the lens to its shorter end (19mm).

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