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 have a rather short Flange focal distance (FFD) allowing for shorter projectors.
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.
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In theory, projector objectives are better suited for projection, since they are originally made and optimized for projection.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|
How to choose a lens
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.
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.
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|
|50mm||f/1.4, f/1.7, f/2||39.6°||1.39||0.72|
|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|
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 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|
|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).