Tuesday, October 20, 2009

When does diffraction---

When does diffraction rear its ugly head? A lot sooner than I had thought.

In an earlier post I'd spoke about empty magnification--bigger blurs circles with no extra detail in images with greater than 1:1 magnification. I also hinted about plans to find better lenses that would drag the hidden detail out of those pesky blur circles. I even bought two nikkor enlarger lens--50 and 75 mm--that were touted on a few websites as being almost as good as genuine optimized closeup lens. With winter coming on, now is the time to think of indoor photography projects..

Then I happened on this article published in a machine vision trade journal.

An eye opening article that rearranged my thinking.

I'd known about Airy disks for decades. They are the blur pattern a perfect no aberrations lens would produce. If such a lens existed. At some time during my optical education, I must have even calculated their diameters. But I had no gut feelings about them. Diffraction limits aren't important when you design infrared spectrometers.

A general observation before I confess all my sloppy thinking to my multitude of readers (humor). The Airy Disk Diameter (ADD) where 83 percent of the energy resides is approximately equal to ADD=2.44*f#*wavelength. For red light and f2.8 the ADD is 4.32 microns.

My carry-it-everywhere 8 MP Oly Sp350 has a max aperture of f2.8 and a pixel pitch of 2.2 microns. So with the lens wide open I'm covering 4 pixels with my smallest blur spot because of diffraction. So if you ever questioned whether jamming huge numbers of pixels into tiny sensor had more to do with marketing than with physics, this is your answer.

With macro photography the diffraction problem get worse. The f# in the equation is an approximation for the numerical aperture--a very good approximation with normal photography but one that breaks down with macro photography. So we have another approximation--the exact equation for numerical aperture has the sine of the angle in radians in it so we won't push beyond the approximation--where the real f# is the f# from the camera lens times the (magnification plus 1) So at 1:1 magnification the diffraction blur becomes twice as big as it would be with normal photography.

For good depth of field with 3D objects you need to use small apertures. Apertures with big diffraction pattern--you can see the problems piling on

Bottom line. There are definite limits to how much detail I can see in my macro photos but they were not caused by my lens. So on to other projects.

As for my nikkor enlarger lens, they weren't that expensive and a web site claim they are good ultraviolet lens. And since I have an ultraviolet slide mounted in a slide projector holder----

Until I was writing this I couldn't figure out why anyone would do this. In the visible the slide is black. Then it hit me--slide projector with bright bulb and a cooling fan, add the UV transmitting slide and you have a source for UV fluorescence photos.

Yup--a replacement winter project.

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