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PostPosted: Thu Feb 05, 2009 11:19 pm 
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Joined: Sun May 25, 2008 12:48 pm
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Location: UK
I know I think too much. But as I've been in a macro frame of mind recently, I was wondering if the characteristics of telecentric designs will be good for macro.

For those not familiar, the short version is light entering and/or exiting the lens only does parallel to the lens' axis. Light at other angles doesn't go through. Assuming a bi-telecentric design for now (applies to both ends), taking a simplified case with a magnification of 1, then basically what's in front of the lens is what's presented on the sensor. Magnification will be fixed at 1, no more, no less. That applies regardless of distance. DoF will be infinite. Yes! Imagine infinite DoF for macro. It would make things a lot easier. But the above is only for the ideal case.

The simplest design of such a lens would consist of two identical convex lens elements and one aperture. Each lens element is placed one focal length away either side of the aperture. The aperture will be "small". Light entering the front of the lens parallel to the optical axis will go through the aperture and is straightened out by the 2nd lens. Light entering at any other angle will be blocked by the aperture. The actual magnification would be varied depending on the ratio of the focal lengths of each end. Actually I just realised this is a basic 2 lens telescope design with the addition of an aperture.

Practical problem 1: The system will be most telecentric with the smallest aperture possible. Bigger apertures will make it less telecentric. But small apertures will cause diffraction softening/limiting.

Practical problem 2: Spherical and chromatic aberrations will vary the focal distance depending on where the light enters and its frequency. That would need correcting for.

In a quick google search, I came across this page selling such lenses for other applications. It seems in practice there are limits to working distance and DoF.

I think I need a lie down now, but I'll try making one on the weekend. Are there any other practical considerations I may have overlooked?

Edit: In hindsight, I'm not sure if this is the best section to post this in. Mods feel free to move without redirection as needed.

_________________
Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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