# What does it mean for a telecentric lens to be focused to infinity?

I have a question about a telecentric lens Ive been studying (for physics research!) and was wondering if you guys could help clear some cobwebs in my head. For starters, I am simplifying the TC lens as a compound lens. Also, this lens has 2 degrees of freedom: (1) it can be extended and retracted (2) it can rotate (I believe rotating the lens changes its f# e.g. aperature).

So when I fully extend the telecentric lens (specifically http://computarganz.com/file.cfm?id=167) it is focused at infinity. I interpret this as it being being able to image an object at a distance of infinity and produce an image focused at the CCD behind the lens.

• Is this a correct assumption? Will the TC lens focsued at infinity produce a focused image at the CCD? What does this mean for an object not at infinity? For objects close to the TC lens? Are their images still produced at the CCD?

Continuing, if I placed an object at some distance in front of the lens (not equal to infinity), I could create a focused image of the object at the CCD by retracting the lens until I see a focused image at the CCD.

• How did the focus/image of the object change as the TC lens was retracted, going from focused at infinity to focused at the image location? Is it that the object is focused but the resultant image is not at the CCD until you adjust the lens (by retracting it) until the image is placed on the CCD?

Thanks so much for your help in advanced. This stuff is really bending my brain lol!

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You might get more help at physics.stackexchange – MikeW Mar 25 '13 at 21:12

You can't focus an object space telecentric lens to infinity. An object space telecentric lens holds magnification constant regardless of subject distance. If you could focus such a lens to infinity (or thereabouts) then you could just point it at the moon and get closeup images of Neil Armstrong's footprints.

Real object space telecentric lenses have a limited range of focus distances at which the lens can produce focussed images, called the "telecentric depth". As most such lenses are designed for assembly line machine vision applications and can only focus on objects a few tens of centimetres away.

I the datasheet you link to looks like a regular close focus lens. It is possible that it is an image space telecentric lens. This just means the rays exit the lens perpendicular to the sensor and magnification doesn't change if you focus by moving the lens assembly.

To answer your remaining questions (ignoring the spurious issue of telecentricity for now):

• When focussed at infinity objects far enough away for the light rays to reach the lens almost parallel will form a focussed image at a distance of f behind the lens, where f is the focal length. Objects closer than that will form a focussed image behind the sensor (if the sensor wasn't in the way) and will thus form a blurred image on the sensor as each point of light from the object gets spread out.

• Rotating the lens to focus closer simply moves the lens further from the sensor, so the closer object now forms a focussed image on the sensor.

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Infinity focus is an object beyond the infinite focus point for the lens. When the focal point is set at infinity, anything at or beyond that point will be in focus, things which are closer than that point will not be.

There is also a limit to how close a lens can focus. Macro lenses are normally able to focus up closer than other lenses. I am unsure about the final question about if there is a depth at which the image would resolve, but I am fairly sure that the answer is yes, it simply does not line up with the position of the sensor.

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