Making a large-ish FOV telecentric lens from a 300mm f4.0?

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viktor j nilsson
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Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by viktor j nilsson »

I've started thinking about building a setup for tracking the movements of small, fluorescently-labeled aquatic organisms.

The ideal optics for this project would be a telecentric objective with a large FOV around 120mm-200mm. But before shelling out that kind of money, I'm thinking of building a smaller version with things I have at hand. Since the FOV is limited by front lens diameter, I started thinking about whether it would be possible to make my Canon 300mm f4.0 (75mm front lens) telecentric by placing an aperture behind it. 60-70mm FOV would definitely be good enough to begin with.

This led me to this old thread on here:
http://www.photomacrography.net/forum/v ... hp?p=58809
Where Enrico and Rik discuss the possibility of making a 300mm 2.8 objective telecentric in this way.

In that thread, however, the consensus seems to be that at low magnifications, it is better to place a large achromat in front of the lens instead. But from Enrico's calculations, I'm not quite sure why the telephoto+aperture approach wouldn't work. Is it because the extension needed to fill the sensor is too long? That wouldn't be a problem in my lab setup. Maybe I'm missing something obvious. Unfortunately I don't have any achromats that big laying around.

The camera would be a Canon aps-c or full frame DSLR.

Any input appreciated!

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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by rjlittlefield »

What magnification do you need to work at?

--Rik

viktor j nilsson
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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by viktor j nilsson »

My priority is to get as large as possible telecentric field at the subject side. Magnification and resolution and of lesser concern.
So if I can achieve a 70mm field, I'd be looking at a 36/70=0.51x magnification for full frame, 0.41x for aps-c.

This is for performing a study on predator-prey interactions (damselfly larvae eating Daphnia) in a glass arena, which I want to be large enough to enable realistic encounter rates and behaviors.

This is a schematic of what it was thinking if I had a 200mm telecentric lens:
Image
(Please ignore the fact that the actual telecentric field is round, so this wouldn't really work, but you get the idea).

But for now the scale will be smaller.

J_Rogers
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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by J_Rogers »

I would still wait for Rik to chime in, as he knows way more about this stuff than me. But I think the inherent difficulty of adding an additional lens in front rather than another aperture in the back is simply the additional lens itself. Most long focal length lenses are fairly large, and the physical size of its aperture/ pupils will vary. Regardless you'll still need an even larger additional lens.

Here is a Canon 300mm f2.8 with entrance pupil P. The first is wide open & the second is stopped down. In order to retain sufficient field of view, you start getting into some expensive lenses. Here is an APO that theoretically would work.
screenshot_1_c.jpg
screenshot_2_c.jpg

viktor j nilsson
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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by viktor j nilsson »

From my (admittedly rudimentary) understanding, I think that the idea was that, to make a low magnification telecentric lens, the easiest approach would be to place a large, weak achromat (say, 100mm+ diameter, 200mm+ focal length) in front of a regular objective. Since it is so weak, it doesn't have to be more highly corrected than a simple achromat. I don't think I've seen anyone place an additional lens in front of a telephoto lens, and can't see why that would help - the telephoto lens already has a big front lens.

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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by rjlittlefield »

viktor, thanks for the additional information.

I think you should give it a try, making the added aperture as small as you possibly can while retaining the minimum sharpness that you need. That will give you the largest possible telecentric field in front of the lens. Then adjust magnification to capture all or most of that field.

At 0.5X magnification, and 300 mm focal length, you'll be needing to add 150 mm of extension. With the added aperture positioned one flange focal distance from the front of the extension, that will leave 150 mm from the added aperture to the sensor plane. So, if you can tolerate stopping down to say effective f/50, the added aperture would need to be only 150/50 = 3 mm diameter.

Regarding the other approach of sticking another lens in front...
Since it is so weak, it doesn't have to be more highly corrected than a simple achromat.
Correct. Ideally the added lens will be not only weak but also close to the subject, so that each pencil of light is narrow as it goes through the added lens. This effectively stops down the added lens, as far as each pencil is concerned.

--Rik

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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by J_Rogers »

rjlittlefield wrote:
Thu Apr 07, 2022 3:56 pm

Correct. Ideally the added lens will be not only weak but also close to the subject, so that each pencil of light is narrow as it goes through the added lens. This effectively stops down the added lens, as far as each pencil is concerned.

--Rik
Rik, if the added lens was not weak and positioned so that its focus is being imaged by the entrance pupil of the main lens "at infinity" does that not produce the same effect? The chief rays should still cross the optical axis at the pupil and the bundles of light are narrower/angular field of view closer to 0. I'm just curious, and ultimately not trying to add confusion to the op's post.

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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by rjlittlefield »

J_Rogers wrote:
Thu Apr 07, 2022 4:24 pm
Rik, if the added lens was not weak and positioned so that its focus is being imaged by the entrance pupil of the main lens "at infinity" does that not produce the same effect? The chief rays should still cross the optical axis at the pupil and the bundles of light are narrower/angular field of view closer to 0. I'm just curious, and ultimately not trying to add confusion to the op's post.
I'm not having any success at visualizing what you're intending to describe. Can you make a sketch for me?

--Rik

Lou Jost
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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by Lou Jost »

I don't think I've seen anyone place an additional lens in front of a telephoto lens, and can't see why that would help - the telephoto lens already has a big front lens.
I am not sure that I understand this statement. It is quite common to place a diopter on a telephoto lens to get higher m than extension can give you. It works very well.

viktor j nilsson
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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by viktor j nilsson »

rjlittlefield wrote:
Thu Apr 07, 2022 3:56 pm
viktor, thanks for the additional information.

I think you should give it a try, making the added aperture as small as you possibly can while retaining the minimum sharpness that you need. That will give you the largest possible telecentric field in front of the lens. Then adjust magnification to capture all or most of that field.

At 0.5X magnification, and 300 mm focal length, you'll be needing to add 150 mm of extension. With the added aperture positioned one flange focal distance from the front of the extension, that will leave 150 mm from the added aperture to the sensor plane. So, if you can tolerate stopping down to say effective f/50, the added aperture would need to be only 150/50 = 3 mm diameter.
Thanks a ton, Rik. It does sound like it might work for what I want to do. I'll report back how it goes.

viktor j nilsson
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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by viktor j nilsson »

Lou Jost wrote:
Thu Apr 07, 2022 6:53 pm
I don't think I've seen anyone place an additional lens in front of a telephoto lens, and can't see why that would help - the telephoto lens already has a big front lens.
I am not sure that I understand this statement. It is quite common to place a diopter on a telephoto lens to get higher m than extension can give you. It works very well.
Oh, I meant that I haven't seen people adding an additional lens in front of a telephoto lens to make it telecentric.


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Re: Making a large-ish FOV telecentric lens from a 300mm f4.0?

Post by patta »

a bit late, confirm, it is feasible;
The method with extra doublet in front:
The objective of a telescope (100mm diameter, 1000 mm focal length, ~1000SEK from the chineses) should work for your 300mm, mounted at right distance in front of the filter thread. Note that telecentricity will change with focus because the telephoto configuration & pupil position change.
~80mm field diameter. Larger diameter, you need a larger telescope objective.
It works like a large, good quality diopter lens.
You could also take out the beautiful front lens group of the 300mm and use it as add-on lens in front of another telephoto.

The other method from 1st post, to move the aperture stop:
either you put an extra stop in telecentric position ( maybe can install it from the back, without disassembling the lens); but here the observaþion of Savazzi in the older thread. Or you screw out the front group of the 300mm, mount it forward with a bit more distance so the native stop iris appear telecentric.

Telecentric they get; give some aberrations and distortion; ain't be as good as the 300mm f/4 by itself.



For reference, that's one setup with the telephoto+doublet:
Lens_add_on_wide_near_vs_far_1000.jpg
That was about another discussion, that placing the add-on lens as near to the subject as possible reduces aberrations.

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