Optics Puzzle - Relay Lens

[PaulFurman]

Tinkering around with a disassembled lens, 28-200, the rear group of elements produces a very curious & weird image when held to my eye but I can't get it to form an image on the camera.

What I have managed to do is project an approx. 4 inch circle on translucent paper when mounted in a box for shade about 10 inches away but I still can't get that into my SLR. I thought I should be able to focus on the aerial image with that 4 inch field of view with another lens, a 28mm f/2 in this case but all I see is the small blurry bright spot. Well, it looks like I can see a very small crop of the full image circle in focus inside that little bright spot, but that's worthless, I want the full image circle.

So I guess the only way is to shoot the projected image on the translucent paper or ground glass but that doesn't seem like a very good image really. There are adapters for using 35mm lenses on small format video cameras which project onto ground glass but that glass has to spin to avoid seeing the texture of the glass if I understand correctly. That's only my vague understanding of how they work.

The only other solution I can think of is to use a mirror to project onto white opaque paper, then focus on that and cover the whole rig in black cloth. Again, you're going to see the texture of the paper but at least it's a full brightness image not a half-diluted translucent image.

Held up to my eye, the image is not inverted but the projected image is inverted so maybe the mirror is the way to go. I happen to have a suitable front surface mirror.

This is all kind of like magic to me as I'm not an engineer and don't really understand the optics. It's just for fun, the resulting image is a fisheye view with *EXTREME* field curvature so that my finger tip is in focus filling 1/2 of the view and the edges are in focus at 5 or 10 feet away. It might produce interesting insect-in-their-surroundings scenes or for illustrating a book about dreams or something <g>. It is possible to arrange my hand so that the fingers recede in alignment with the field curvature and remain in focus while blocking most of the out of focus mid-ground and leaving a ring of distant background in focus. It would work for a closeup of a basketball or large mushroom so the whole thing was perfectly in focus and the center was very close.

[rjlittlefield]

I thought I should be able to focus on the aerial image with that 4 inch field of view with another lens, a 28mm f/2 in this case but all I see is the small blurry bright spot. Well, it looks like I can see a very small crop of the full image circle in focus inside that little bright spot, but that's worthless, I want the full image circle.
...
This is all kind of like magic to me as I'm not an engineer and don't really understand the optics.

Perhaps a brief review of the optics will help explain what you're seeing.

In your projection setup, what forms the image are rays of light that come from the world, go through the lens, and get focused onto your piece of paper. All of the rays get bent a lot in that process, so they're hard to think about. However, as a useful approximation, you can imagine that rays passing through the center of the lens do not get bent, while rays passing through other parts of the lens do get bent, so as to meet the central ray at the proper focus point.

Now, think about setting up your camera with its 28 mm f/2.8, so as to focus on the aerial image. The only rays that can possibly form an image inside the camera have to go through that 28 mm f/2.8 lens -- a 10 mm diameter hole positioned quite a ways in back of the aerial image. Which rays are those? Only the ones that form the very center of the aerial image! All the other rays that form the aerial image run at oblique angles that miss the 28 mm f/2.8 altogether.

So, what could you possibly do to capture some of those rays? There are (at least) three possibilities.

1. You can stick a transparent diffuser in the focal plane of the aerial image, thereby turning it into a non-aerial image that sprays its light across a wide range of angles.

2. You can stick an opaque screen in the focal plane of the aerial image, thereby turning it into a non-aerial image that sprays its light across a wide range of angles.

Notice that #1 and #2 are essentially equivalent. You're going to lose similar amounts of light either way. #2 does not give you a "full brightness image" in the sense you might imagine, because just like the diffuser, most of the light goes shooting off at angles that don't go through the camera lens.

3. You can stick a "field lens" in (or near) the focal plane of the aerial image. The field lens acts sort of like a condenser, redirecting light so that it does go through the camera lens, without significantly changing its focus. The small diameter of the camera lens then becomes the limiting aperture of the combination, so you have effectively stopped down the front lens, but at least you can see the whole field.

For just playing around, either of the projection schemes is much more practical than trying to find a suitable field lens. I think you'll find the transparent diffuser easier to set up with proper geometry, since everything is in a nice straight line. With the mirror approach, the focal planes won't be parallel because the camera will be having to look around the mirror. (I guess you could drill a hole in the mirror to look through. That's essentially what "mirror lenses" do.)

--Rik

[PaulFurman]

Wow, thanks for the explanation. That made sense and I think I actually solved the puzzle. Hard to be certain hand held with everything upside down & bouncing around but I put a huge 4-1/4" medium format fisheye lens in the focal plane of the aerial image then held the camera with macro lens back & focused where the MF fisheye film would sit. This is a pretty crazy contraption!

BTW, on further thought, the mirror wouldn't help as I was describing, I still wouldn't be able to look at the projected image on an opaque screen but the mirror might help this setup work by flipping things right side up.

I'm not certain it really worked. I tried with a couple different telephoto lenses with about 3" diameter front elements and that definitely worked but not big enough to see the whole image circle. Hmm, I tried again & even the 4-1/4" fisheye is getting only a small central crop. And I tried with a 4-1/2" diameter 300mm f/2.8 and it produced something, almost, maybe... but all that squinting & holding my arms out has me exhausted for now.

OK one more check... throwing a black towel over my head looking into the box with a piece of translucent paper, it looks like the focused image circle might be more like 8 inches but it's really hard to tell because the focus plane is so strange. I have seen some huge lenses like that for aerial photography, from an airplane on very large film sheets, old military surplus online. That's a little too crazy though <g>.

Alright, one more test. I put the lens right up to a P&S digital and it comes close to working. I think it's getting the whole thing as a vignetted circle on the sensor which is not too tiny, about half the fame size of the 3MP camera. But the batteries died so... Hmm, it works pretty good on my cell phone <g>.

Off for the holiday feast in a bit...

[DQE]

My undergraduate degree was in physics, and in grad school I was in a hybrid program emphasizing medical imaging and more math and physics. Yet lens design and optics were deemed a special application of Maxwell's equations that did not merit a place in a physicist's curriculum. My grad school coursework and research required learning digital image evaluation and digital image analysis, but not too much that is directly related to macrophotography.

Practical, well-documented forum issues like avoiding excessive diffraction-induced image blur are not a concern - this information is covered in this forum and elsewhere.

Are there some macro photography-relevant optics and/or lens design books, web pages, etc, that that are (ideally) aimed at someone with a reasonable amount of mathematics and physics background but only a limited understanding of optics/lens design? Since photomicroscopy and photomacrography with affordable commercial equipment is a relatively narrow application, I'm hoping there are publications that are aimed at this specific area. I currently have an old college-level textbook, "Fundamentals of Optics" by Jenkins and White, and I also hope to find my short-course notes on lens design. I am trying hard to avoid becoming actively involved in photomicroscopy but would enjoy learning more about this area too.

Thanks for considering my request for some references.

[rjlittlefield]

Are there some macro photography-relevant optics and/or lens design books, web pages, etc

I am not aware of any. All the optics references that I have either 1) restrict themselves to the paraxial assumption, or 2) treat the general case and leave it up to the user to figure out what happens for the specific case in hand. I will be interested to hear what other people come up with.

One tool that has been very helpful to me is WinLens, a free ray-tracing program that can be used for setting up virtual experiments and showing you what happens to the light. That's what I used for exploring and illustrating the topic on Stopping down a lens combo (refs found there).

--Rik

[DQE]

Are there some macro photography-relevant optics and/or lens design books, web pages, etc

I am not aware of any. All the optics references that I have either 1) restrict themselves to the paraxial assumption, or 2) treat the general case and leave it up to the user to figure out what happens for the specific case in hand. I will be interested to hear what other people come up with.

One tool that has been very helpful to me is WinLens, a free ray-tracing program that can be used for setting up virtual experiments and showing you what happens to the light. That's what I used for exploring and illustrating the topic on Stopping down a lens combo (refs found there).

--Rik

Thanks for your reply. I had found things similar to the summary you provided and was hoping that my perceptions were mistaken. I guess it's a bit too much to expect textbooks to exist on macro lens design and their use with various accessories. While I'm at it I may as well wish for an appendix on lighting design for macro work too!

The info on the WinLens software sounds interesting and should help rebuild my fragmented lens design and optics knowledge.

I find your essays and other publications on these and related topics to be very informative, enjoyable, and very helpful.

[rjlittlefield]

One other thought...

I guess I'm casually presuming that you're spent some time with books like "The Manual of Close-Up Photography" (Lester Lefkowitz) and "Photomacrography, 2nd edition" (Kodak Technical Publication N-12B).

Those books do spend a lot of time talking about lenses and lighting, but with an emphasis on the practical aspects: what can you put together to get a good result?

They definitely do not talk in detail about the paths of light as it winds its way through a system.

Thanks for the kudos about my articles. I learn a lot by trying to tie theory and practice together into a consistent story complete with demos. When I work that way, the world has a way of whacking me alongside the head with my own misconceptions.

--Rik

[Harold Gough]

Are there some macro photography-relevant optics and/or lens design books, web pages, etc, that that are (ideally) aimed at someone with a reasonable amount of mathematics and physics background but only a limited understanding of optics/lens design?

The Blaker books have lots of technical stuff in them but I cannot be sure what would suit you. They do not not cover current or recent lenses. At present one of them is somewhere in the huge jumble I call my office. I'm not going in there alone!

See my post in this thread:

http://www.photomacrography.net/forum/v ... 60&start=0

Harold

[PaulFurman]

DQE, you have found the text book right here :-) it does take some digging but there is a great wealth of info.

I think I've given up on the weird lens. It looks cool held up to my eye but I cannot get it to project onto my SLR sensor plus it actually vignettes badly on the P&S and pumps too much light into the cell phone camera so that blows out. Because it works with my eye, I figured the P&S would do better since their size is similar to my eye... I went off on a tangent for a while trying to set up a relay lens that peered into it with a tiny 7mm webcam lens but that was a lost cause in terms of practicality. I had to put my 35mm Canon bellows lens on a huge extension to get it to fill the frame with the back of the webcam lens and by then the image quality was horrible. Here's an example with the webcam, which of course is pretty awful quality and has so much depth of field anyways that it barely makes a difference. Looking through it with my naked eye, it looks much sharper.

[DQE]

DQE, you have found the text book right here :-) it does take some digging but there is a great wealth of info.

I think I've given up on the weird lens. It looks cool held up to my eye but I cannot get it to project onto my SLR sensor plus it actually vignettes badly on the P&S and pumps too much light into the cell phone camera so that blows out. Because it works with my eye, I figured the P&S would do better since their size is similar to my eye... I went off on a tangent for a while trying to set up a relay lens that peered into it with a tiny 7mm webcam lens but that was a lost cause in terms of practicality. I had to put my 35mm Canon bellows lens on a huge extension to get it to fill the frame with the back of the webcam lens and by then the image quality was horrible. Here's an example with the webcam, which of course is pretty awful quality and has so much depth of field anyways that it barely makes a difference. Looking through it with my naked eye, it looks much sharper.

I very much agree that the internet and in particular this set of web pages are the best available source of a full range of images and information about macro photography.

Yet I thought it was worth posting a query to see if I had somehow overlooked some printed references and books. The more I think about it, the more naive it seems to expect that books would be written on a subject as specialized and limited in commercial value as macro photography. However it means so much to those who enjoy the field/addiction. I assume there is a different situation with respect to photomicrography since it has so much commercial interest and there is much expensive equipment available for sale to research groups. Yet that probably leaves interested non-commercial enthusiasts without comprehensive support in terms of a wide range of textbooks and journal articles and affordable equipment. Thanks to whatever gods may be for e-bay!

In any event, there is a wealth of information in both the macro and micro areas within these web pages. I also enjoy the photos at the other popular macro forums such as the fredmiranda.com pages and others.

Your lens experimentation is very interesting as well as enjoyable to read and think about - thanks for posting the images and the discussion.

[Harold Gough]

Some of these images are getting a bit scary!

Harold

[PaulFurman]

> Some of these images are getting a bit scary!

Just be glad they aren't sharp!

PS I just found the perfect optics 101 primer, neatly packed into 17 typewritten pages: http://www.janrik.net/PanoPostings/NoPa ... xPoint.pdf It probably doesn't cover every conceivable detail of everything known to man but feels like it comes close.