Entocentric Lenses

No, I am not talking about excentric or egocentric lenses - and not about not centered lenses.

I am talking about quite normal photographig lenses, like all the Canon, Nikon, Pentax, Sony stuff we photographers use on our cameras.
Entocentric lenses show objects of same size smaller when they are more away. Quite normal as we see things too.

Telecentric Lenses (object space)

This is different - most people donĀ“t know this kond of lenses. It is more common in measurement applications - industry visual applications.
There objects of same size are shown in the same size - not depending on theit distance!
The entrance pupil of the lens system is at infinity. To need the iris aperture (the iris blades itself) in the focal plane of the lens.
Now one may think: STOP - with that no inifinty is possible. Yes - but this no big problem - cause the maximum object diagonal is off the same size as the entrance pupil. So the maximum imaged object size, is as big or smaller as the front lens of a lens!


Why should one need a object side telecentric lens?

It is very good fore measuring. If you can not be 100% sure about the exact distance between object and camera, one can not make exact measurments with an entocentric lens. But with a telecentric lens this is possible (this is a bit simplified)!

How to build a homemade telecentric lens?

This is quite easy.
One needs the working iris at the normal focal plane of the lens. I made this as an adapter for my normal used lenses - they have Canon EF mount. I used a cheap extension tube set, and put there in ~44 mm distance (register distance of Canon EF) an additional iris. This iris needs to be closed that the normal iris of the lens is no loger the main iaperture that stops the rays from entering the camera.

This homemade telecentric lens is not as exact as the ones one could buy from Sill and such companys!
It is only a small tinker project to get the telecentric effect.

Here some more example images. Made with this adapter and I think the Canon FD 85mmf/1.2 lens.

It seem like the grooving of the focus ring are spread - but they are only parallel. But our eye is not used to paralell lines on structures
that are obviously going farther away.