Telecentric lenses - s-3D

[istvanttt]

Sorry if this would be the wrong place, but as here is also discussed the "New German Lenses" I put it for now here. (Please move it if necessary)

Reading the describtion of those lenses I wanted to learn more about the telecentric concept and found this:

http://www.opto-engineering.com/telecentric.php

Reading it I started to ask myself what special impact leses like this could have for stereoscopic filming. My feeling would be that it could improve a lot the quality of a s-3D image. My scientific back-ground is too limited to explain it technically why, but somewhere in my stomach this suspect is growing.

Maybe NAB is not the time to ask things like that, but.. who knows maybe somebody else is thinking the same direction?
Nice weekend
Istvan

 

[Evin Grant]

I don't see why it should matter specifically to 3D. It mostly has to do with even field illumination.

 

[istvanttt]

I don't see why it should matter specifically to 3D. It mostly has to do with even field illumination.

If it gives less distorsions on the borders the parallax is cleaner. ie in the upper and lover borders barral distorsions can create vertical parallaxes which doesn't feel nice on the eyes. Again I read only that article and my questions are based on that.

 

[lensmaster]

Telecentric

Telecentric

Telecentric has field illumination advantages (not straight cos4 loss). However, there are mechanical size requirements for such a lens. In large format S35 and higher cine lenses, the limitation on rear element size is set by the ARRI PL mount.

If you look at a raytrace of a telecentric lens you will see that it's very easy to start vignetting the edge of the field when you impose the mechanical limitation of the PL mount. There is a hard limit to the lowest T stop you can achieve with a PL mount.

You can tell how "telecentric a lens is by looking at the size of the rear element (for the same given T-stop)

You will notice the Cookes have a small lens aperture. Many of the new lenses you will see (especially if they claim telecentricity) will be fairly larger on the back end.

Telecentricity eliminates breathing for the most part. There will be some angle change at the sensor with focus but not much.

My definition of "telecentric" is that the chief ray angles at the "image" plane are within a degree. If they are not within a degree, they are not technically telecentric. There is really no "sort of telecentric". It is or it is not.

For these types of camera (digital), there is a very delicate balance to strike to make the lenses perform well. From what I have read, the RPP's have been designed specifically for digital. If they were smart, there were image angle considerations taken (I can't wait to check the size of the rear lens) but it was not the only consideration.

There are many reasons I can think of where you don't want truly "telecentric lenses.

Lens design is a process and a balance of many factors. The more system factors you take into account (ie the sensor used), the better the result.

 

[istvanttt]

Lensmaster,
thank you for the detailed answer. It makes things more clear. I hoped that this German lenses could help to have less perspective distortions and therefor better stereoscopic lenses. I fell in love with the idea when I read that with telecentric lenses a tube seen from the front will always be the same even if the camera gets closer to it. In s-3D it is a fact that the objects closer to the left/right side of the frame have a different distance to the left/right lenses and therefor different pespective distortions. This is why sometimes even before those objects cut the frame and may create frameviolations it starts to be less pleasant in the perception.

I'm wondering why I never find discussions on the lens types for s-3D. Here in Las Vegas during the session "Digital Cinema Summit" all is going about s-3D, even a Professor was invited to talk about human perceptions, but never ever came the question of what kind of lenses have what kind of advantages/disadvanages in s-3D.

Anyway, thank you
IStvan

 

[Pawel Achtel]

Telecentric lenses, by definition, are completely useless for 3D (stereoscopic) work. In fact, they are also useless for almost all other 2D filming situations except maybe some specialised macro work and not even good at that. Telecentric lenses are mostly used for machine vision because they make parallel projection instead of perspective projection. :Yawn:

You are also limited with subject size being at most the size of the front of the lens. You would need a very large lens to film a person...or a crowd :rofl:

 

[lensmaster]

Tecentric

Tecentric

Pawel,

There is object side and image side telecentricity. Machine vision lenses are object side. You are correct that the subject has to be smaller than the front lens in that case. They are used for eliminating perspective errors in machine vision inspection.

You can also be image side (sensor side) telecentric.

You see these types of lenses first and foremost in electronic projection systems (DMD, LCOS). Those image devices lose contrast as a function on image angle on the sensor. So you need a telecentric lens to make sure you project with even contract.

For electronic imaging lenses, telecentricity will help reduce color fringing caused by the ray bundles cutting across he bayer pattern as a function of sensor size. Telecentric or "near telecentric" can help reduce this effect.

However, telecentricity has a few penalties including rear lens size and the fact that constraining the image angle space to zero costs the designer a degree of freedom in the design. This degree of freedom will make some other aberration (like distortion) harder to correct.

Telecentric lenses aften have more, bigger and thinker lenses near the rear group so the transmission (T/# ) can be harder to maximize.

Like everything, there is a balance. A 100% telecentric lens won't breathe (a nice bonus for cine) but a near telecentric lens will breath much less (if noticeable) than something like an old F35 still design.

(ha, I typo'ed telecentric in the title)

 

[Pawel Achtel]

Thanks lensmaster. Yes, but Istvan wanted to use an object telecentric lens as described on the website he referred to, not image-side telecentric. Besides, image-side telecentric lens does not give you any advantage specifically for 3D (stereoscopic) acqusition.:attention9ha:

 

[istvanttt]

Well, my hope was to find lesnes which reduce perspective distortions and the describtion of the parallel ray seemed appealing.

But like always in this forum you learn from whatever level you come and ask. Thanks to everybody
Istvan

 

[lensmaster]

Yep,

I got caught up in the telecentric term and did not read his intention carefully. I am a lens geek so I wanted to spew on about telecentricity.

 

[Pawel Achtel]

Yep,

I got caught up in the telecentric term and did not read his intention carefully. I am a lens geek so I wanted to spew on about telecentricity.

No worries, the info you gave was good to share. :thumbsup: