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.
http://www.vision-systems.com/display_article/356315/19/none/none/Feat/Matching-Lenses-and-Sensors

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.

The Kluge

When I first started this macro project I would have been super proud of this fly image. But I have moved on to the Kluge. Or more accurately the Kluge and a more commercial version now that I've added another flash to my collection.

My 35-70 mm Vivitar mounted directly on the camera give me images with magnifications that varies from a 1/10 (40mm) to a 1/4 (70mm) life size. Add extensions tubes and I can push it up to lifesize or greater imagery. With all the problems.

I don't 'hate' tripods. I just prefer not to be bothered with carrying one around. So if I'm doing 'studio' work in the family room I'll set a tripod up, mount the camera and xy stage on it and shoot away.

Shooting bugs out in the fields and gardens is a different matter. My old manual lens aren't image stabilized. I'm certainly not either as I lean forwards and backwards to bring the bug in and out of focus. And the bug (always) and the flower (with any breeze) are moving.

The one time I tried a tripod in the field or more accurately my back yard I followed advice I'd read in a manual. Find a flower that a bug will love, set up tripod , focus on flower, and wait patiently for bug to strike a pose. Took awhile but once one did, I snapped off five images before it flew away.

Woopee, golly gee. Bound to be keepers since the bug was twice as big as the tiny yellow flower I picked.

I ran in the house to examine my macro masterpieces on the computer. OOPS. Big HEAVY bug on TINY little flower bends flower stem and move everything out of focus. So much for the How-To-Take-Buggy-Pics-In-Your-Backyard manual. Time to think up Plan B.

The problem reduces down to light and speed. With a dash of focus thrown in. With the fly image I had bright sunlight. And a stationery bug for once or I wouldn't have posted the image. Something I didn't have in the roughly 100 other shots I took before I captured this keeper. My success to failure ratio wasn't good.

The obvious solution is a flash to freeze all motion--flower, bug and camera shake. Even better I could have the light I needed. Could shoot with a full stack of tubes for magnification and a tiny aperture for depth of field.

The D60 has a built-in flash with both TTL and manual mode. Unfortunately the lens with extension tubes sticks out so far it blocks the light when I move in close for the shot.

I could have also mounted my best flash on the hot shoe to raise it higher--a dedicated Canon flash unfortunately but the garage sale price was so good I couldn't pass it up. It works fine in manual mode on the hot shoe or with my radio controls and I can dial down the power from full to 1/128 power. Be perfect if I could have tilted the flashead downwards.

So I built the Kluge. Three feet of 1/8 by 1 aluminum strip, a metal switch plate, some hardware and tubing gathering rust in the garage. Add some sawing and drilling and tapping and there you have it. Not exactly something I can fold up and put in a camera bag but the flash mounts beneath the camera so the flash tube and the lens axis are in the same plane.

With it I took the Bee image. And I didn't have to take a hundred plus shots to get it. With good detail too as you can see with the 100% crop of the bee's eye.

Then just as the bee-buzzing--blooms season was ending I found another flash at a garage sale. A Sunpac 322 with a head that can be tilted down. It can't be mounted on the camera; the discharge voltage would fry the electronic, but with my second radio receiver it works fine on top of an old flash mount I had hanging around.

And since the mount folds up to fit in a camera bag, I suspect it will compete with the Kluge come spring when the bee are buzzing blooms again.

DIY Closeup Lens

Since the 500 mm lens works well if I don't push it beyond its limitations I'm obviously not going to tear it apart to make an achromatic closeup lens. But I did have another candidate--a zoom lens mounted on a twenty year old video camera with an Vidicon vacuum tube and all the high voltage electronic needed to make the tube work. It came with a separate VCR recorder for the big tapes that fit in a waist harness and--I assume--an equally large and missing battery power pack. Totally non working but since the main attraction of the deal was its big camera bag the camera stuff ended up being stashed away in my basement for some future optics project.

Removing The three tiny screws at the front of the lens didn't loosen anything. (Brought to mind some advice I received as a cub designer/draftsman-"If you goof up put a screw in the goof-up holes to fool them all not in the know.") Since I couldn't locate any other screws and don't own a lens retaining ring spanner out came the hacksaw.

Unfortunately I made a bad guess about the length of the lens assembly. I ended up sawing into what turned out to be an air spaced achromatic doublet. Fortunately I noticed the hacksaw chips between the two lens before I sawed the inner holder in half. After I pulled apart the pieces--they were held together by a press fit using paper shims--I ended with a closeup lens that looked terrible but still worked. Since the hacksaw chips were at or near the principle plane of the doublet they cut the lens transmission by a percent or two but didn't show up in the image.

A powerful closeup lens as it turned out with a focal length of 2.5 inches or 16 diopters. A lens that pushes this method of macro photography to its limits. But once I bought a 52-52mm coupling ring out of Hong Kong it turned my 18-55 mm kit lens into a macro lens. Of sorts. At the 18 mm end I'm in pincushion distortion and vignette heaven.

At the more reasonable 55mm end--judge for yourself. While the closeups aren't as sharp as those from the vivitar macro lens and extension tubes, the closeup lens takes up very little room in my camera bag and is far easier to use. There is something to say for auto focusing, metering and vibration reduction in macro photography.

18-55mm kit lens at 55 mm with 16 diaopter close up lens















without closeup lens