Making a big telecentric lens (a though experiment)

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enricosavazzi
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Making a big telecentric lens (a though experiment)

Post by enricosavazzi »

We know that D, the usable diameter of the area of view of a telecentric lens, is

D = dl - da

where dl is the diameter of the front lens element and da is the diameter of the aperture, which is located at the rear of the lens and at a distance from the lens equal to its focal length (as discussed in several threads on this forum). Outside D, the lens is no longer telecentric. If da is small, D ≈ dl. This assumes a reasonably normal lens design (e.g. not an extreme wideangle).

Experimenting with a home-made telecentric setup that uses a camera lens + rear aperture either forces us, in practice, to restrict the subject size to photomacrography (> 1x) with most lenses, or alternatively, to use a lens with a fairly large front element. So what are the non-wideangle lenses with large front element that a photographer is likely to have? That's an easy one: telephoto lenses :idea:

Before starting to mess up with my 300 mm f/2.8, I sat down to make a few simple calculations. Let's assume that I can put the lens diaphragm as close as 50 mm from the sensor plane of a DSLR (a reasonable distance, slightly higher than the registration distance). I cannot put the diaphragm any closer because of the mirror well of the camera (I would have a better luck with a Micro Four Thirds body). The lens would also have to be moved outward from its normal position on the camera body by the same distance. This produces a minimum subject magnification of 0.14x. Assuming that my camera has an APS-C sensor, the needed image circle has a diameter of 30 mm. The telecentric subject area, on the other hand, has a maximum diameter of 107 mm, equal to the front lens element and disregarding the aperture size. Since 107 * 0.14 = 15 mm, the telecentric image circle is much smaller than the sensor. So, in practice, I cannot use this telecentric setup unless I move the lens and aperture further away from the camera, for instance to obtain a magnification of 0.28x, which does cover the sensor.

I don't think that things would get any better by using a second lens instead of a simple rear diaphragm. The limiting factor on the subject size is always the diameter of the front element. I could not use a lens of much shorter focal length, either (even assuming I could find one of similar front diameter), because of the limitation of distance between aperture and sensor. A lens designer might be able to overcome this limitation by adding a divergent group at the rear of the aperture.

Does the above make sense, or did I overlook anything :?:
--ES

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Post by rjlittlefield »

I have not carefully checked your calculations, but I definitely agree with the main thrust of your thinking. Adding an aperture behind an ordinary lens only works at relatively high magnification.

At lower magnifications, it is better to add a large but weak achromat far in front of your main lens, placed so that the entrance pupil of the main lens lies at the rear focal point of the added weak achromat. Then for subjects in front of the added achromat, the limiting aperture gets pushed to infinity and you have achieved telecentricity again.

See HERE for an example at macro distances.

--Rik

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Post by ChrisR »

I assume you've googled and found the Schneider pages. The Bilateral construction is intriguing. Perhaps you could make one with two 300mm f2.8s back to back, and a camera moving laterally, behind, though I'm not sure I understand how it works. Maybe say a 200mm behind the 300?

There's a bit here, with some pages missing:
http://tinyurl.com/yyoh49n

enricosavazzi
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Post by enricosavazzi »

ChrisR wrote:I assume you've googled and found the Schneider pages. The Bilateral construction is intriguing. Perhaps you could make one with two 300mm f2.8s back to back, and a camera moving laterally, behind, though I'm not sure I understand how it works. Maybe say a 200mm behind the 300?

There's a bit here, with some pages missing:
http://tinyurl.com/yyoh49n
Thanks for the reference, this will be useful.

As far as I understand, a telecentric lens on both subject and image side would not be very useful - basically, since light rays are parallel on both sides, the lens should be limited to a fixed magnification unless one changes the reciprocal distance between the two lenses (but in this case, at least one of them is no longer telecentric).
--ES

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Post by rjlittlefield »

See HERE, "Brightfield Illumination of Large Field Sizes" by Theodore M. Clarke. Fig 17 illustrates the basic scheme.

Clarke uses the method to make both the illumination and the imaging telecentric at the same time, in order to photograph mirrored surfaces.

The same technique, applied to imaging alone, is what I used at the link provided earlier.

Can you give us an example of what you would like to accomplish?

--Rik

enricosavazzi
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Post by enricosavazzi »

rjlittlefield wrote:See HERE, "Brightfield Illumination of Large Field Sizes" by Theodore M. Clarke. Fig 17 illustrates the basic scheme.

Clarke uses the method to make both the illumination and the imaging telecentric at the same time, in order to photograph mirrored surfaces.

The same technique, applied to imaging alone, is what I used at the link provided earlier.

Can you give us an example of what you would like to accomplish?

--Rik
Hi Rik,
as mentioned in the title of the thread, I am mostly concerned with a thought experiment for now (actually, I am writing a book on scientific photography and sooner or later I will make a simple demonstration of a telecentric setup, but I don't have a practical application for it). I am making this thought experiment also as a test of whether I really understand the principles involved.
--ES

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Post by rjlittlefield »

It was recently reported on another forum (PanotoolsNG) that adding a 5 diopter closeup lens to a Canon EF 100mm f/2.8 IS USM macro lens set at closest focus coincidentally makes a telecentric combination.

See HERE and HERE for further discussion.

Note that the author says:
Erik Krause wrote:The entrance pupil of the EF lens moves back a lot if you turn the focus ring. This wasn't the case for the old (non-USM) 100mm macro...
--Rik

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