The Four Thirds system is a standard created by Olympus and Kodak for digital single-lens reflex camera (DSLR) and mirrorless camera design and development.[1]
The system provides a standard that, with digital cameras and lenses available from multiple manufacturers, allows for the interchange of lenses and bodies from differentmanufacturers. U.S. Patent 6,910,814 seems to cover the standard. Proponents describe it as an open standard, but companies may only use it under a non-disclosure agreement.[2]
Unlike older SLR systems, Four Thirds has been designed from the ground up to be entirely digital. Many lenses are extensively computerized, to the point that Olympus offersfirmware updates for many of them. Lens design has been tailored to the requirements of digital sensors, most notably through telecentric designs. The size of the sensor is significantly smaller than for most DSLRs and this implies that lenses, especially telephoto lenses, can be smaller. For example, a Four Thirds lens with a 300 mm focal lengthwould cover about the same angle of view as a 600 mm focal length lens for the 35 mm film standard, and is correspondingly more compact. Thus, the Four Thirds System hascrop factor (focal length multiplier) of about 2, and while this enables longer focal length for greater magnification, it does not aid the manufacture of wide angle lenses.
The image sensor format, generally intermediate between those of larger SLRs and smaller compact cameras, generally gives levels of cost, performance and convenience intermediate between those two classes.
Image side telecentric implies that the light rays impinging on the sensor arrive at right angles to the sensor surface and therefore are parallel to the optical axis.
It is my opinion that in order for light on the image side of a lens to arrive parallel to the optical axis at the sensor the rear element of the lens must be equal to or (preferably) larger than the imaging surface. If smaller then the light rays will arrive at an angle. This leads to loss of light I think. This can be alleviated to some extent by an optical design that makes the effective aperture larger for off-axis light rays. It can also be alleviated by lenslets on the pixels but these can’t be optimized for all angles of incidence I think. I may be wrong on this … as in anything else here!
Tele-centric is a well known definition of a lens system where the light rays arrive as a parallel bundle at the lens and exit in the same manner. This allows one to reproduce objects located at varying distances to the lens tat the same magnification. This is very useful in machine vision applications where comparisons between objects that are the same but not in same subject plane to be compared. But the field of view of a tele-centric lens is only as large at the lens. Not very practical for general photography.
In any case, on the matter at hand, I would not hesitate in using an old lens on a digital camera. But I am not a lens designer or optical expert … just my observation.
Andy
