Extra! Extra! Read all about it!!!!

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

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g4lab
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Extra! Extra! Read all about it!!!!

Post by g4lab »

Here you can read a webpage about an advance in lens design. Done by one of our list members. Who does happen to be a member of the Royal Society. You remember that little group whose members include sir Isaac Newton.

He posted this on another list but I am very positive that certain people associated with this list will want to read and look at these pages.

I consider it a great privilege to have "met" this gentleman (internet met) on the net and to have invited him here.
Last edited by g4lab on Thu May 13, 2010 11:36 pm, edited 1 time in total.

rjlittlefield
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Post by rjlittlefield »

Thanks for the link -- very interesting indeed.

To quickly summarize: 4X NA 0.47

The lens is also telecentric on the image side, ideal for digital sensors.

I count something like 16 elements in 14 groups, including the reflectors. This is definitely an advanced design!

--Rik

g4lab
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Post by g4lab »

Dr. Amos is one of the perfectors of the confocal laser scanning microscope.
I thought you would like it Rik. :D :D

elf
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Post by elf »

How do you keep the light that's reflected toward the subject from inside the lens from interfering with the light that's reflected back from the subject?

rjlittlefield
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Post by rjlittlefield »

elf wrote:How do you keep the light that's reflected toward the subject from inside the lens from interfering with the light that's reflected back from the subject?
There's no problem with light traveling to the right (toward the subject) interfering with light traveling to the left (coming from the subject, toward the sensor). The big problem is to make sure that all light getting to the sensor actually does come from the subject, as opposed to coming from lens and mount surfaces via stray reflections. One big part of this is often putting a "black hole" in the lens housing in back of the main half-silvered reflector, to trap illumination that does not get reflected toward the subject. Sometimes polarizers are used as well, to kill specular reflections off perpendicular glass surfaces. I don't know any of the details of this design. Perhaps the designer will chime in and tell us!

--Rik

enricosavazzi
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Post by enricosavazzi »

Looking at the lens diagram, my attention was caught by the use of two oblique plates to cancel out each other's aberrations. Am I correct to believe that this is feasible only if the light beams in this part of the equipment should be parallel to each other? It does seem so from the ray trace, so perhaps one could regard this part of the lens as an infinity system?

The two angled plates should cancel out each other's transversal chromatic aberration, but, unless they are made with materials of different dispersion, the axial chromatic aberration should be cumulative, right? I am following this train of thought because two angled plates that completely cancel out each other's aberrations would be an elegant solution for axial illumination, as well as for splitting light between a phototube and observation eyepieces. Neither a single oblique plate of finite thickness nor a cube beam splitter can do this without additional optics. However, as far as I can see, two identical plates at complementary angles would not cancel out all aberrations.
--ES

rjlittlefield
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Post by rjlittlefield »

Enrico, I like your analysis but I am having trouble putting part of it together in my head.

Let's presume that the angled plates sit in an "infinity" part of the system, so in each bundle all the rays are parallel.

Then at each wavelength the effect of each plate is only to shift each bundle sideways. The amount of shift will vary depending on the wavelength, due to dispersion. But no matter what the shift is, the second plate will introduce an exactly compensating shift in the opposite direction, thus putting the bundles back together again.

I think the above is what you mean by "cancel out each other's transversal chromatic aberration".

But I get lost when you speak of "axial chromatic aberration". Each bundle consists of parallel rays, which stay parallel regardless of wavelength. I think of axial chromatic aberration as being a shift in focus depending on wavelength, but in this case there is no shift -- all wavelengths stay focused at infinity. So I am confused.

What am I missing?

--Rik

PaulFurman
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Post by PaulFurman »

rjlittlefield wrote:To quickly summarize: 4X NA 0.47
What would that translate into as f/stop? (effective aperture)
I suppose focal length doesn't matter but also maybe interesting...

rjlittlefield
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Post by rjlittlefield »

PaulFurman wrote:
rjlittlefield wrote:To quickly summarize: 4X NA 0.47
What would that translate into as f/stop? (effective aperture)
f/0.85, with best performance wide open.

--Rik

enricosavazzi
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Post by enricosavazzi »

rjlittlefield wrote:Enrico, I like your analysis but I am having trouble putting part of it together in my head.

Let's presume that the angled plates sit in an "infinity" part of the system, so in each bundle all the rays are parallel.

Then at each wavelength the effect of each plate is only to shift each bundle sideways. The amount of shift will vary depending on the wavelength, due to dispersion. But no matter what the shift is, the second plate will introduce an exactly compensating shift in the opposite direction, thus putting the bundles back together again.

I think the above is what you mean by "cancel out each other's transversal chromatic aberration".
Yes, exactly
But I get lost when you speak of "axial chromatic aberration". Each bundle consists of parallel rays, which stay parallel regardless of wavelength. I think of axial chromatic aberration as being a shift in focus depending on wavelength,
also, exactly
but in this case there is no shift -- all wavelengths stay focused at infinity. So I am confused.

What am I missing?

--Rik
I am not sure myself and cannot quantify it, but I am thinking of the situation of a telecentric (on the object side) lens, which passes only rays parallel to the lens axis, but still the image it projects is affected by DOF (which is essentially the same as the DOF of a non-telecentric lens, at the same effective aperture and magnification) and therefore, presumably, by axial chromatic aberration. The rays stay parallel, but the effective length of the optical path is a function of dispersion, and changes with wavelength.

By the way, it occurred to me that multiple reflections within one of the angled plates cannot be cancelled out by the second plate, so this is another type of aberration that cannot be corrected by the two-plate arrangement. It can only be reduced (greatly) by anti-reflection coatings of the plates.
--ES

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Post by rjlittlefield »

enricosavazzi wrote:but I am thinking of the situation of a telecentric (on the object side) lens, which passes only rays parallel to the lens axis, but still the image it projects is affected by DOF (which is essentially the same as the DOF of a non-telecentric lens, at the same effective aperture and magnification) and therefore, presumably, by axial chromatic aberration. The rays stay parallel, but the effective length of the optical path is a function of dispersion, and changes with wavelength.
I see now. You are being led astray by a misunderstanding about telecentric lenses. It is not true that telecentric lenses pass only rays that are parallel to the lens axis. A telecentric lens passes rays at a wide range of angles, in proportion to its aperture, same as an ordinary lens. The difference with a telecentric lens is only that the central rays of all the cones are parallel. In other words, all the ray cones are oriented the same way, perpendicular to the plane of focus.
it occurred to me that multiple reflections within one of the angled plates cannot be cancelled out by the second plate, so this is another type of aberration that cannot be corrected by the two-plate arrangement. It can only be reduced (greatly) by anti-reflection coatings of the plates.
This is true, but the effect of multiple reflections is much less important than you may imagine. The offset secondary ray resulting from two reflections will be parallel to the primary ray that caused it. Assuming this occurs in an "infinity" section of the optics, the secondary ray will also be parallel to all other rays in the same bundle, corresponding to the same point on the subject. All these parallel rays will end up getting focused onto the same point on the sensor, and hence will not degrade the image. In essence, the secondary rays caused by double reflection just amount to a slight doubling of the edge of the aperture. Like all other changes to the aperture shape, this has no effect on in-focus image points.

This effect can be demonstrated with a piece of ordinary window glass. Hold the sheet of glass in front of your eye and look at a far distant object while rotating the sheet away from its perpendicular position. You will see that the far distant object is not doubled, no matter how much you rotate the glass. This is the infinity situation. Now, place your finger near the back side of the glass and repeat the rotation. In this case (somewhat dependent on lighting), you can see a clear ghost image of your finger, offset from the primary image by some amount that depends on the angle of the glass. This is the non-infinity situation. Diverging/converging rays give a ghost; parallel rays do not.

Another way to analyze this same experiment is to think about how offsets in ray position are perceived for the far distant and nearby subjects. Suppose the glass is angled so as to introduce a 1 mm shift between the primary and reflected rays. For the nearby finger, this 1 mm separation is easily visible; for the house across the street, it falls far below the resolution limits of your eye. Both analyses yield the same result, but one or the other may be more comfortable.

I hope this is helpful. Please let me know if I have been unclear or have scrambled the analyses.

--Rik

Blame
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efective aparture

Post by Blame »

rjlittlefield

Sorry and all that but you have fallen for a rather common error. don't be too upset. I have seen the same mistake in a lot of threads.

The F number is measured on the camera side. The NA on the target side.

You have converted the NA to F stop on the target side. A rough answer would be 1/2xNA. That would give about f1.06 (but only approximately. Your calculation may be more accurate).

However things are not so rosy on the camera side. You have to multiply by the total magnification. In this case maybe 4X giving f4.26 or there about.

Sadly 4X NA 0.47 isn't enough to determine the quality of the lens. Field of view is the all too often ignored factor.

As example. There are already plenty of cheap NA 0.65 40X objectives. Say I have such a lens with a typical field of view of 25mm. Say it is a typical infinity type designed for use with a 200mm tube lens. I can, with total honesty, announce to a slack jawed public that I have a 4X NA 0.65 objective for use with a 20mm tube lens. What I won't have mentioned is that drop in magnification doesn't come free. The Field of view has dropped to a tenth along with the magnification. It is now 2.5 mm. Just a pathetic little circle in the center of your resultant picture.

There is also one other factor. NA is the measure of definition for a perfect lens. The major manufacturers are said to get close with their plan apo designs. Less so with their economy lenses. There is nothing stopping them from marketing a lens up to about NA 1.4 with as low a magnification as they care. The problem is that beyond a certain point it is first economically and then technically impossible to deliver the definition that the NA promises.

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Re: efective aparture

Post by rjlittlefield »

Blame wrote:rjlittlefield

Sorry and all that but you have fallen for a rather common error. don't be too upset. I have seen the same mistake in a lot of threads.

The F number is measured on the camera side. The NA on the target side.

You have converted the NA to F stop on the target side. A rough answer would be 1/2xNA. That would give about f1.06 (but only approximately. Your calculation may be more accurate).

However things are not so rosy on the camera side. You have to multiply by the total magnification. In this case maybe 4X giving f4.26 or there about.
The number that I gave, f/0.85, is an accurate calculation for what an ordinary lens would have to be rated in order to perform at NA 0.47 when used at 4X. Your calculation is close, it just neglects a factor of m/(m+1).

I believe the number I gave answers the question that Paul intended to ask, although his use of the phrase "effective aperture" might suggest otherwise. This point is that this objective has a very wide aperture compared to conventional objectives or macro lenses intended to operate at the same magnification and field size.

The website that g4lab links to has a good description of this lens and its applications. I suggest reading it, if you have not already done so. The design is very sophisticated, and there is no hint of the unethical specifications that you suggest.

--Rik

Blame
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Post by Blame »

Well I could allways be wrong, and I admit that I didn't find that link. No doubt the Objective is as wonderous as advertised.

I was just pointing out that magnification and numerical aparture were not in themselves sufficient. I am not campaigning against this objective in particular, but against an oversimplified definition of quality that has all too often led to abuse.

As to the F number, I have given my explanation. I will let it stand.

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Post by Pau »

When we will be able to buy it for $10 like the JML lens? (lots of emoticons)
Pau

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