Lens length confusion

Started Jan 14, 2013 | Questions thread
Leonard Migliore
Leonard Migliore Forum Pro • Posts: 14,816
Marketing, I think

corneaboy wrote:

We have mostly been discussing lenses that are telecentric in image space (the world between the lens and the sensor). One typically has a lens telecentric in image space because you want the light to have normal incidence on the sensor; one of the concepts in the 4/3 system was having such telecentricity to eliminate loss of sensitivity in the corners due to the loss of efficiency with angle of incidence that is characteristic of image sensors."

So where do they put the aperture stop in a 4/3 system and why just a 4/3 system? Is it because you have smaller diameter lenses to work with?

The aperture stop is the physical diaphragm. We don't really care where it is. What we are looking for is the location of the exit pupil, which is the image of the aperture stop in image space. These tend to be pretty far from the sensor for SLR lenses because the lens can't get real close to the sensor without hitting the mirror.

Now, I view "telecentric" as a relative term. I would consider a lens telecentric if the ray angle at the edge of the sensor is less than 6 degrees from vertical. Sensors probably don't care if the angle was 20 degrees. But Olympus wanted some fancy conceptual justification for the 4/3 system so they added the telecentric feature. Note that this is not a part of micro 4/3, probably because it's a useless constraint and you can have lenses close to the sensor.

I don't know what Olympus did with their wide angles to move the exit pupil away from the sensor but it's not real difficult. As long as the exit pupil is more than about 60mm from the sensor the lens is as telecentric as it needs to be.

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Leonard Migliore

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