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Telecentric Lenses
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PostPosted: Wed Nov 14, 2012 2:00 pm    Post subject: Telecentric Lenses Reply with quote
I would like to see some images taken with Telecentric lens and a comparision to standard lens of equal focal length and stop.

As I understand a Telecentric lens has the iris at the front instead of buried among the elements and I wonder what advantages there are if any


PostPosted: Wed Nov 14, 2012 2:03 pm    Post subject: Reply with quote
Never seen a lens with the iris at the front, that would cause severe vignetting I think.


PostPosted: Wed Nov 14, 2012 2:16 pm    Post subject: Reply with quote
Check this video at 2:29

http://www.youtube.com/watch?v=MpZOFJg9SAA

iangreenhalgh1 wrote:
Never seen a lens with the iris at the front, that would cause severe vignetting I think.


PostPosted: Wed Nov 14, 2012 4:45 pm    Post subject: Reply with quote
There is explanations & example photos:

http://www.edmundoptics.com/learning-and-support/technical/learning-center/application-notes/imaging/telecentricity-and-telecentric-lenses-in-machine-vision/?&viewall


PostPosted: Wed Nov 14, 2012 6:02 pm    Post subject: Reply with quote
Yes I have seen that, they mainly use diagrams


I wanted to see ordinary images like flowers, pets, headshots maybe with background Bokeh


PostPosted: Wed Nov 14, 2012 6:34 pm    Post subject: Reply with quote
JohnBar wrote:
As I understand a Telecentric lens has the iris at the front instead of buried among the elements and I wonder what advantages there are if any
"telecentric" does not mean that iris is placed in front of the lenses

read this:

http://en.wikipedia.org/wiki/Telecentric_lens

iangreenhalgh1 wrote:
Never seen a lens with the iris at the front, that would cause severe vignetting I think.
not true;) http://japancamerahunter.com/2012/02/ms-optical-perar-28mm-f4-super-triplet/


PostPosted: Wed Nov 14, 2012 7:05 pm    Post subject: Reply with quote
@berraneck

it does if you read the edmund optics link above and I quote


"It occurs when the system stop is placed at the front focal plane of the lens, resulting in an entrance pupil location at infinity. A shift in the object plane does not affect image magnification."


PostPosted: Wed Nov 14, 2012 7:29 pm    Post subject: Reply with quote
Maybe you would enjoy finding out what a "front focal plane" of a lens is and where it is located, BEFORE you judge on others Wink


PostPosted: Wed Nov 14, 2012 8:16 pm    Post subject: Reply with quote
sorry, there was no intent to judge anyone. maybe it´s just my english.


PostPosted: Wed Nov 14, 2012 9:34 pm    Post subject: Reply with quote
JohnBar wrote:
@berraneck

it does if you read the edmund optics link above and I quote


"It occurs when the system stop is placed at the front focal plane of the lens, resulting in an entrance pupil location at infinity. A shift in the object plane does not affect image magnification."


This does not mean the iris is in front of the lens.

Check out this link and look at the picture at the top.

If you want to see pictures taken with lenses that are telecentric on the image side, look at any picture taken with an Olympus 4/3 system lens. One of the design principles behind that system was to make the lenses telecentric (or nearly so) to remove the problems with off axis rays on the sensor micro-lenses. Of course, apart from the high quality of such images, they are just normal photographs.

So perhaps what you really want is an example of a picture taken with a lens that is telecentric on the object side. Before getting excited about the prospect of the lack of perspective, you should note the following statement from the above link.

"A telecentric lens "sees" a cylindrical tube of space of diameter equal to that of the front lens element. It is limited to photographing objects whose lateral dimensions do not exceed the diameter of the lens."

So maybe you'll decide such lenses are not very useful in general photography. Smile

Mark


PostPosted: Wed Nov 14, 2012 9:56 pm    Post subject: Reply with quote
Mark

Schneider seems to think object space telecentrics ARE very useful for photography. Watch the video above

So please stop posting misinformation





SXR_Mark wrote:


This does not mean the iris is in front of the lens.

Check out this link and look at the picture at the top.

If you want to see pictures taken with lenses that are telecentric on the image side, look at any picture taken with an Olympus 4/3 system lens. One of the design principles behind that system was to make the lenses telecentric (or nearly so) to remove the problems with off axis rays on the sensor micro-lenses. Of course, apart from the high quality of such images, they are just normal photographs.

So perhaps what you really want is an example of a picture taken with a lens that is telecentric on the object side. Before getting excited about the prospect of the lack of perspective, you should note the following statement from the above link.

"A telecentric lens "sees" a cylindrical tube of space of diameter equal to that of the front lens element. It is limited to photographing objects whose lateral dimensions do not exceed the diameter of the lens."

So maybe you'll decide such lenses are not very useful in general photography. Smile

Mark


PostPosted: Wed Nov 14, 2012 10:30 pm    Post subject: Reply with quote
I thought the video was talking about image space telecentrics. Avery talks about even illumination of the sensor.

When Avery talks about the aperture being "in front" he means that the aperture ring is in front of the focus ring ? If you go back a few seconds from 2.29 you can see some glass in front of the iris. Google "Schneider Cine Xenar" and you'll find lots of pictures showing this more clearly.


PostPosted: Wed Nov 14, 2012 11:04 pm    Post subject: Reply with quote
sichko wrote:

When Avery talks about the aperture being "in front" he means that the aperture ring is in front of the focus ring ?


The video has at 3:17 a slide indicating the same magnification at all distances which he says is more natural (not for me - I see with perspective), he also talks about to achieve the telecentric design you have to have the iris in the front, both these point to an object space design.

The glass at the front may just be to protect the iris from dirt which for a 50mm lens is approximately positioned 50mm in front of the main cell.


The reason I asked the question here is because of the seemingly different concept being put over in the video which differs somewhat from the google search info on telecentrics, which may be the reason for at least me, confessing to not understanding why.


PostPosted: Wed Nov 14, 2012 11:44 pm    Post subject: Reply with quote
JohnBar wrote:
sichko wrote:

When Avery talks about the aperture being "in front" he means that the aperture ring is in front of the focus ring ?


The video has at 3:17 a slide indicating the same magnification at all distances ....


The slide includes a reference (bottom right) to Wikipedia ...

http://en.wikipedia.org/wiki/Telecentric_lens

which tells us that ...

An image-space (or image-side) telecentric lens produces images of the same size regardless of the distance between the lens and the film or image sensor.

Quote:
... he also talks about to achieve the telecentric design you have to have the iris in the front...


OK I could certainly be wrong in my intrepretation.

Quote:
...both these point to an object space design.

The glass at the front may just be to protect the iris from dirt which for a 50mm lens is approximately positioned 50mm in front of the main cell.

The reason I asked the question here is because of the seemingly different concept being put over in the video which differs somewhat from the google search info on telecentrics, which may be the reason for at least me, confessing to not understanding why.


OK about the glass but I'm confused as well now. Back when I've done some more reading.


PostPosted: Thu Nov 15, 2012 1:08 am    Post subject: Reply with quote
Telecentric lens I know in use were attached with short tube to front of standard 35mm camera lens (Canon FD 2.8/35); the 35mm lens was attached to FD-C adapter and to a C-mount camera. IOW, the telecentric attachment acts like a close-up lens, with special properties:

SXR_Mark wrote:
"A telecentric lens "sees" a cylindrical tube of space of diameter equal to that of the front lens element. It is limited to photographing objects whose lateral dimensions do not exceed the diameter of the lens."


The Ebmund Scientific page linked above shows two photos of a male connector with multiple pins -- in the photo made with telecentric lens, the male pins appear to all point in the same direction, while in the photo made with standard camera lens, there is distortion, the male pins appear to point in a 'spread' direction depending on position.

Telecentric is useful for measurement, because same-size objects within the "cylindrical tube of space" will appear to be the same size -- without telecentricity, the same size objects will appear to be different sizes according to distance from lens.

Now....I have disassembled a telecentric for cleaning. There is one cemented(?) group of two, possibly three, elements. I think I was able to get an infinity focused image from it, but didn't have enough extension tubes to be sure, time ran out, so I put the project away for a rainy day...it is raining now Wink I think this is what you are asking about...

http://forum.mflenses.com/the-corner-cube-and-the-telecentric-lens-t33119.html


PostPosted: Thu Nov 15, 2012 4:58 am    Post subject: Reply with quote
SXR_Mark wrote:


If you want to see pictures taken with lenses that are telecentric on the image side, look at any picture taken with an Olympus 4/3 system lens. One of the design principles behind that system was to make the lenses telecentric (or nearly so) to remove the problems with off axis rays on the sensor micro-lenses.



Here is a comparison of telecentric and non telecentric lenses, both at 50mm, with off axis light:

Telecentric:



Non telecentric:







You can see how the telecentric lens throws an image that is much less off axis than the non telecentric.


PostPosted: Thu Nov 15, 2012 8:27 am    Post subject: Reply with quote
JohnBar wrote:
Mark

Schneider seems to think object space telecentrics ARE very useful for photography. Watch the video above

So please stop posting misinformation





SXR_Mark wrote:

"A telecentric lens "sees" a cylindrical tube of space of diameter equal to that of the front lens element. It is limited to photographing objects whose lateral dimensions do not exceed the diameter of the lens."

So maybe you'll decide such lenses are not very useful in general photography. Smile

Mark


I am confused as hell now. I don't see how the lens can provide the same magnification regardless of working distance without having a cylindrical FOV instead of a conic one. Then such a lens would indeed be very limited for general photography. What gives?


PostPosted: Fri Nov 16, 2012 1:29 am    Post subject: Reply with quote
EO Imaging Lab Module 2.2: Telecentricity (video)


PostPosted: Fri Nov 16, 2012 2:16 am    Post subject: Reply with quote
Yes. It is very limited for general photography! As it cant perceive objects larger then the entrance pupil, Unless you want to photograph things what, 3-5" wide that is.


PostPosted: Fri Nov 16, 2012 6:46 am    Post subject: Reply with quote
fermy wrote:

I am confused as hell now. I don't see how the lens can provide the same magnification regardless of working distance without having a cylindrical FOV instead of a conic one. Then such a lens would indeed be very limited for general photography. What gives?


This is more or less what I knew telecentric lenses were: lenses with a cylindrical FOV, used mostly for machine vision or other industrial applications. I thought it could be funny seeing what would happen if they're used for normal photography, but for sure the applications would be extremely limited.


PostPosted: Fri Nov 16, 2012 1:28 pm    Post subject: Reply with quote
So is Schneider guy on drugs then? Look at 3:18 in the movie. They specifically highlight that telecentric lenses provide the same magnification regardless of distance, which means that their lenses must have cylindrical FOV. No doubt they would film some great movies with lenses that have a 10 cm cylinder as a FOV. Laughing Laughing Laughing


PostPosted: Fri Nov 16, 2012 5:07 pm    Post subject: Reply with quote
fermy wrote:
So is Schneider guy on drugs then? Look at 3:18 in the movie. They specifically highlight that telecentric lenses provide the same magnification regardless of distance, which means that their lenses must have cylindrical FOV. No doubt they would film some great movies with lenses that have a 10 cm cylinder as a FOV. Laughing Laughing Laughing

Figure 1 (below) shows simple ray tracing for the formation of an image using a simple lens.

If the lens is now moved towards the object (Figure 2), in order to focus on a second object, focus on our first object is lost. The image becomes blurred and it becomes bigger. Imagine now that the image is not formed by rays arriving at the image plane from different directions, but by a bundle rays all parallel to the principal axis of the lens. Whether or not the image is in focus depends upon the object lens distance but the size of the image does not. Moving the lens will move the image in and out of focus but it will not change its size. The lens is telecentric on the image side, or in the image space.

Or am I mistaken ?


PostPosted: Fri Nov 16, 2012 7:02 pm    Post subject: Reply with quote
sichko wrote:

Imagine now that the image is not formed by rays arriving at the image plane from different directions, but by a bundle rays all parallel to the principal axis of the lens.


I think the flaw here is in the above line. If all the rays are parallel to the principal axis, then the light is collimated and the image is at infinity. Of course, the beam of light coming from the lens has the same cross-section everywhere, but there is no focus at the sensor. Note, collimation is not the same as telecentricity, which means the chief ray of each bundle of rays is parallel to the principal axis. But there is still a convergence onto the image plane and so a focus.

Mark


PostPosted: Fri Nov 16, 2012 7:24 pm    Post subject: Reply with quote
SXR_Mark wrote:
sichko wrote:

Imagine now that the image is not formed by rays arriving at the image plane from different directions, but by a bundle rays all parallel to the principal axis of the lens.


I think the flaw here is in the above line. If all the rays are parallel to the principal axis, then the light is collimated and the image is at infinity. Of course, the beam of light coming from the lens has the same cross-section everywhere, but there is no focus at the sensor. Note, collimation is not the same as telecentricity, which means the chief ray of each bundle of rays is parallel to the principal axis. But there is still a convergence onto the image plane and so a focus.

Mark


OK. Many thanks for the correction. So if I'd written ...

Imagine now that the image is not formed by cones of rays arriving at the image plane from different directions, but by cones of rays with all principal axes of the cones parallel to the principal axis of the lens.

... it would be more accurate ? Although a diagram would help !


PostPosted: Fri Nov 16, 2012 11:11 pm    Post subject: Reply with quote
@ Sichko

The point you are making is (I believe, apologies if I am wrong) that, for a lens that is telecentric on the image space, the size of the image is independent of the position of the image plane (from the lens). Yes, this is correct. (sorry if I didn't get the point you were making before).

Image space telecentricity = exit pupil at infinity = image size independent of image plane distance

If the lens is conventionally entocentric on the object side, the image size still depends on the object distance. The lens still sees a normal perspective and has a field of view that extends beyond the lens diameter. Olympus 4/3 lenses are designed to be telecentric in the image space (at least the expensive ones are) but they still behave like perfectly normal camera lenses. With a camera lens, one does not really know that the lens is telecentric in the image space.

@Everyone Smile

The issue causing confusion here is that the Schneider video suggests that the lenses are telecentric in object side. Now, the guy talks about the lens giving even illumination. This means they are telecentric on the image side, just like the Oly lenses. This is fine and is a good idea.

But then you get the following two slides in the video





This is pretty unambiguous to me. It says these lenses are telecentric in the object space.

Object space telecentricity = entrance pupil at infinity = image size independent of object plane distance = no perspective = FOV is a cylinder equal to lens diameter

Making a photography/cine lens like this is stupid. Firstly it says this is MORE NATURAL!! Really. I see in perspective and I think everyone else does too.

Next, the aperture on these lenses is right at the front (someone pointed out there is glass in front of the aperture but this is probably a plane filter to protect the aperture blades). But a telecentric lens has the entrance pupil (the image of the aperture as viewed from the front of the lens) at infinity. Since there are no optical elements in front of the aperture on these lenses, the entrance pupil is exactly where the aperture is and definitely not at infinity. It is clear the these lenses CANNOT be telecentric in the object space. This is of course a huge relief to anyone who is thinking of buying one as it means they will be able to make video of more than just ants.

So why does the video have those two slides? My bet is that it was but together by marketing people who:
    heard the lenses were telecentric
    looked up what that meant in the Schneider catalogue
    found information in the section on machine vision lenses
    thought the same benefits must apply to these lenses.
    didn't realise the lens was only image-side telecentric


OK, maybe I'm being too cynical. Maybe Schneider have discovered some great way of making a pseudo-telecentric lens that has a meaningful field of view. I wait to be educated.

Mark

PS I've just realised there is a reference to wikipedia at the bottom of those slides. Doh. That says it all. Marketing people!!
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