Iris Diaphram as pinhole

[wildbillbugman]

Hi to all,
I am wondering if a high quality iris diaphragm could be used as a multi aperture pinhole. With iris diaphragms being used with quality microscopes, I am wondering if the material can be thin enough and the holes smooth enough. My guess is yes. I think that with one investment, virtually any pinhole aperture could be realized.
Bill

 

[Jim Jones]

The smallest iris diaphragm in any of my lenses has a minimum diameter of maybe 1mm and has only four blades. It would be appropriate for about 1 meter focal length. With careful construction, an adjustable three- or four-sided pinhole can be made with a much smaller minimum aperture. A quick check of three blades of about .0012" thick hard material (stainless steel?) suggests a minimum length of maybe .010" for each side of the triangle. Such an adjustable aperture with two fixed and one moveable blade should be fairly easy to make. A four blade adjustable aperture could have a much smaller minimum size, but would require higher precision in construction. These apertures would have more diffraction than a round aperture of equivalent f/number. Since pinholes cost nothing to make and can be made in any size, spending money or time on adjustable pinholes seems more like a fun project than an investment.

 

[Gerald C Koch]

Any pinhole that is not perfectly circular will produce less than optimal results. Such a pinhole would be impossible to achieve with an iris type aperture.

 

[wildbillbugman]

Thank you,
That is another idea that I can dump into the shredder.

 

[NedL]

I think there are "zero aperture" diaphragms for use in microscopes, that close all the way? Somewhere I read about making an adjustable pinhole out of three or four razor blades.

As an aside, I've been wondering about the effects of non-circular pinholes, and whether they might have any artistic value, even if "sub-optimal". An obvious question would be the effect on sun flares, like the number of blades in a normal lens aperture affects the "sun star" flare. But it seems to me you might get softness in one direction or some other effect that might be interesting to play with. Might be interesting to play around with.

 

[AgX]

Any pinhole that is not perfectly circular will produce less than optimal results. Such a pinhole would be impossible to achieve with an iris type aperture.

What would be the ill effect of such a polygon compared to a circle?

Leaving issues of thickness aside (!), one would overlay a pinhole of optimum diameter (mean diameter areawise) with pinholes of other sizes.
Keeping in mind that the pinhole diameter is calcaluted for a single wavelenght anyway, I would not expect any ill effect.
The same goes for the distortion imposed by the edgy shape of the polygon.

 

[wildbillbugman]

Most of the zero aperture Irises on sale are double irises. Probably resolution disasters. But some microscope iris diaphragms get down to very low diameters with only one iris. They can also be had for less than a new car.

 

[Gerald C Koch]

Resolution suffers when the pinhole is not circular. The same problem exists for lenses. The image for a 4 blade iris is not as good as one with more blades. For example one of my enlarging lenses has 20 blades in the iris. The effect is similar to having two images super imposed, one taken at f/2 and one at say f/8.

 

[wildbillbugman]

PLEngineerinf has a video on Facebook. A motorized iris diaphragm with zero aperture.

 

[NedL]

Resolution suffers when the pinhole is not circular. The same problem exists for lenses. The image for a 4 blade iris is not as good as one with more blades. For example one of my enlarging lenses has 20 blades in the iris. The effect is similar to having two images super imposed, one taken at f/2 and one at say f/8.

Fascinating. Ralph has shown us that there is one "optimum" for resolution and another for contrast. If we had an elliptical pinhole, optimum for resolution on one axis, and for contrast on another it might lead to some interesting and subtle effects. For example, a photo of a forest of vertical tree trunks might look different depending on whether the camera was upright or rotated 90 degrees due to the direction that contrast is enhanced. I'd be surprised if it was easy to see the difference, but it would be interesting to try it.

Along similar lines, a few days ago, I saw an image that had a sort of "glow" in the highlights, it caught my eye and I went to look thinking it was going to be a zone plate image, but was just a pinhole. That made me wonder about just adding one or a couple of the zone plate rings ( maybe smaller, "outer" rings ) to add just a bit of that effect to a pinhole image....

 

[wildbillbugman]

Looks like my amateurish question got some useful dialog going among people who know what they are doing. BTW, in my research through as much literature as I can find about how pinholes are made and the plus/minuses of each method, I have seen some elliptical pinholes, although I do not remember where. I shall venture to find the sight where I saw them.

 

[RalphLambrecht]

try it first. pinhole images are not meant to be perfect.their charme is in their fuzzyness.

 

[wildbillbugman]

Ralph,
I wonder if the early pinhole photographers all agreed with you. I would bet that some were frustrated with the fuzziness and strove to sharpen things up.

 

[NedL]

I read an article in an "amateur" magazine from around the turn of the century extolling the virtues of pinhole for pictorial photography, and it suggested that the softness was "more realistic" and therefore better than a "technical" photograph made with a lens! I wonder if I bookmarked that... I'll have a look. It made me laugh to think maybe it was "more realistic" because not as many people had eyeglasses back then. Some of my larger ones have a nice soft look in hand, but get "too sharp" when reduced for viewing on the web. I do agree that the softness is part of the charm.

 

[NedL]

Found it! I hope you will enjoy this as much as I did. f/60 men and fuzzwuzzyists! Here's the quote:

...I commenced systematic experiments that have convinced me that in the production of most of the qualities that distinguish pictorial from technical Photography the pinhole is not only superior to the lens but can never be equaled by the latter instrument. Apart from line and mass and values which are largely matters of artistic judgment and selection one of the greatest difficulties of pictorial Photography has been a satisfactory solution of the question of diffusion of focus. How to reproduce in a picture the various planes as the normal eye sees them nothing microscopically sharp nothing woolly. This the lens cannot do and every attempt is a compromise The f/60 men got everything sharp and lost aerial perspective; many of the so called fuzzwuzzyists, going to the other extreme produced results equally unpleasant. Sane workers stuck to the via media and attained a compromise that however satisfactory was never ideal. With the pinhole however no such difficulty exists. With a proper sized opening say a No 11 needlehole objects are just about as sharp as the average eye sees them and allowing for aerial perspective they are equally in focus whether distant six inches or six miles.

The article is on page 213 of this book: http://books.google.com/books?id=BWZUAAAAYAAJ

Cheers!

 

[Gerald C Koch]

Fascinating. Ralph has shown us that there is one "optimum" for resolution and another for contrast. If we had an elliptical pinhole, optimum for resolution on one axis, and for contrast on another it might lead to some interesting and subtle effects.

The problem with this idea is that ALL portions of a pinhole (and also a lens for that matter) are involved with formation of an image. Therefore a particular position in a pinhole does not form the image of a particular portion of the image. So a noncircular pinhole will still result in a degraded image.

 

[NedL]

Thanks Gerald! Not trying to argue with you, but "typing out loud". I'm going to have to think about this more, I'm not getting it. I think Ralph's optimum aperture for contrast was slightly smaller than Rayleigh's optimum for resolution, and maybe the contrast is increased by the dark part of the first diffraction ring? When we're this close to diffraction does the shape of the hole really matter that much? ( In microwaves, which I know more about, it does not matter very much and what matters is that the longest dimension can admit the wavelength and polarization. Elliptical or circular or rectangular slot antennas all function well, with the main difference being polarization. ) In most physical camera shapes, as soon as you are the slightest bit off axis, there is squint in the pinhole so effectively it is an ellipse at all points but one on a flat film plane anyway. And that ellipse has a different orientation if the displacement from center is horizontal or vertical. I wonder if the polarization of the light would matter, like it does for a microwave slot antenna? I never took an optics course in college, so I'm completely ignorant here. Some "light summer reading" is in order now...

This is making me want to build one to see for myself!

 

[Jim Jones]

When pinholes are used for extreme panoramic photography, the "squint" that NedL mentions above is exaggerated. In theory an elliptical pinhole with the major axis horizontal should reduce astigmatism and increase exposure at the ends of the image with a loss of sharpness in the center. I haven't wrestled with the problem of making elliptical pinholes small enough for compact panoramic cameras to test this. However, tilting the pinhole backwards when using the equivalent of a rising front in a wide angle "tall building" camera is effective in improving sharpness near the top of the building. It also makes exposure more even over the entire image. This is one practical application of a pinhole that is effectively elliptical.

 

[AgX]

Making a circular hole and then bending the foil will get you somewhere near to that elliptical hole. Well, more oval than elliptical...

 

[NedL]

I think this is a complicated topic. If we have a non-circular pinhole, the diffraction pattern will change. Instead of a uniform Airy disk, we get and "Airy square" or "Airy ellipse".

I've just spent the past couple hours looking at diffraction patterns for different aperture shapes... here are a couple neat links I stumbled across ( one for microscopy and another for acoustics ):

http://www.leica-microsystems.com/products/confocal-microscopes/leica-tcs-sp8-configurable-confocal/technology/square-pinhole/

http://courses.physics.illinois.edu...Rect_Aperture_Thy/Diffn_Rect_Aperture_Thy.pdf

But the image formed on our film or paper is a complicated overlay of the diffraction patterns of all the points on the image. For me at least it is not obvious what the overall effect will be from overlaying this "point' pattern. Now I'm more curious than ever about the effect on perceived sharpness or other more subtle effects ( like the characteristic "glow" of highlights in a zone plate photograph. )

After thinking more, I don't think my microwave analogy above works at all. Those are waveguide properties, when the aperture less than the wavelength. Pinholes are huge compared to that and it's diffraction around the edges that we're talking about here, which does happen in microwaves ( around the edges of buildings for example ) but is not important at the edges of a slot antenna.

Non-circular apertures are used in various applications like astronomy and microscopy. I think it might be fun to experiment with a more "qualitative" and artistic goal in mind.

Also an aside: I'm getting sucked in to making things too technical, which I have a strong tendency to do. The reasons I like pinhole photography are almost the opposite of "technical"... it's the magic of the light landing directly onto a piece of paper in a coffee can making a wonderful photograph! I need to be careful not to suck all the joy out of it and making it into "work" instead of play. I have no fear of differential equations but my next step needs to be to make something and play with it instead of trying to solve this with 2-d convolution or whatever.

AgX has a good idea and a simple one that would be easy to mount. Also it will not be hard to construct two rotating slit plates from razor blades, so that the aperture could be adjustable parallelograms... Time to stop typing and start doing!