Lenses

This page and the information on it is related ONLY to the Elphel camera, NOT Axiom.

Elphel 353 cameras have a C/CS mount which is basically just a female thread (nominally 1 inch/25 mm in diameter, with 32 threads per inch). The flange focal distance is 0.6900 in/17.526 mm for a C-mount.

C-Mount (The letter "C" is said to stand for "cine") is common for 16mm lenses as well as machine vision, automation and specialised television applications. Which has lead to a very wide range of lenses to choose from: List of almost 300 C/CS-mount lenses with sample images

Image from wikipedia

CS-Mount

CS-mount has a flange focal distance of 0.4931 in/12.526 mm and is otherwise identical to the C-mount. Elphel 353 cameras have a CS-mount by default and a spacer ring (that ships with every Elphel kit) can be used to connect all C-mount lenses to the camera as well.

Field of View

The following illustration shows the viewing angle for a set of different focal lengths at Full HD resolution (with approximate 35mm equivalents & Horizontal FOV).

The following illustrations show a person standing 1 meter from the lens at different focal lengths


Focal Length: 1.3mm | Subject Distance: 1m


Focal Length: 1.7mm | Subject Distance: 1m


Focal Length: 3mm | Subject Distance: 1m


Focal Length: 5mm | Subject Distance: 1m


Focal Length: 8mm | Subject Distance: 1m


Focal Length: 12.5mm | Subject Distance: 1m

SLR lenses

Because of the rather small sensor area (1/2.5") compared to 35mm film in current Elphel 353 cameras the crop factor for using lenses that were designed for SLR cameras is rather high (~6x) which makes these lenses currently unfit for our applications. Future bigger sensor front ends might change this situation with Elphel 373 (see Roadmap)

Optical design for certain sensor area

The mount name alone does not specify if a particular lens is able to cover a certain sensor area. So this technical specifications of a lens needs some extra attention. Typical optical designs are (1/4", 1/3", 1/2", 1/2.5" (Elphel 353), 2/3", 1", 4/3", APS-C, etc.). If your lens is designed for a smaller sensor than the size of the sensor you are using it is possible that the image circle will not be able to cover the whole sensor area leading to vignetting or in extreme cases even complete darkness on the outer sensor regions. In general the quality of a lens (sharpness, amount of distortion, aberration, etc.) degrades with the distance from the image centre, so it is in general better to use only the inner regions of the image circle for the sensor area. Most lenses already account for this and cover a bigger area than the sensor size they are designed for. The opposite case is that the lens is made for a bigger image circle than the dimensions of your sensor, normally this is less of a problem but in extreme cases it could result in stray light which is reflected by parts of the lens mount or sensor PCB reaching the sensor.

B4 Lenses

Lenses designed for 3-chip-cameras like Canon or Fujinon (B4-Mount) broadcast optics have a higher flange focal distance because the light has to pass a prism before hitting the 3 sensors and a so called "lateral dispersion" (to offset colour separation caused by dichroic prisms). This makes them incompatible with any single sensor camera. Though there are adapters (rather expensive, several thousand $) available that correct the colour convergence of broadcast lenses to work with single-chip designs.

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