When I started this blog entry several days ago I was certain I knew the answer.
I wrote: "---the 35ish year old Canon f1.4 50mm lens--even if it did trip me up a few times before I figured out its quirks. "
I had traced the most confusing of the quirks to a little catch hidden under the lens's locking ring. If I accidentally twist the locking ring and release the catch, it limits how far the iris can close. The iris adjusting ring turned freely and it might say f16--the limit on the lens-- but the opening stops at where the iris happens to be when I accidentally trip the catch
While I'm sure this enables some neat function when the lens is mounted right side up on a proper camera, in macro work the depth of field--the height where everything is in reasonable focus-- is proportional to the iris opening, So why did one set of images look sharp when the next set look so... You get the idea. A strip of tape fixed that problem but it took a while to figure out what was going on.
A second quirk was caused by a second catch. It is spring loaded and keeps the iris open on my old FTb Cannon camera until I press the shutter. In theory, easily fixed by a rubber band. In practice, not so easily fixed since there was no good place to hook the other end and still be able to adjust the stage up and down. Took a trip to the hardware store before I could kluge up an attachment rod that moved up and down with the stage.That is where I ended several days ago. I would have have written a few more lines and published the blog if there hadn't be one other mystery to work out. The Canon lens imaged 4.5 mm on the 24mm long sensor for a magnification of 5.3X while the Nikon lens imaged a little less than 6mm for a magnification of 4.1X. Another clear advantage to the Canon-- but why? Both were 50 mm lens. Using geometric optics formulas it shouldn't have made any difference--50mm is 50 mm period.
But real world didn't line up with theory. Which meant it had to have something to do with the principle planes.What are principle planes? you must be asking.
Today you would use a computer to trace all light rays needed to calculate a magnification through the series of glass (and sometimes other materials) lens elements that make up a camera lens. Before computers and with more effort you could do the same calculations with a hand calculator. Before calculators and with much much more effort you would use a slide role--the precision machined five foot long version that came complete with its own magnification optics so you could extrapolate between the lines for the necessary accuracy. Before that you would have used a book filled with tables of sines, cosines, tangents and logarithms, plus paper and goose quills.
Sometime near the end of the goose quill era someone came up with the idea of principle planes. With them you could use the simpler formulas of geometric optics that Issac Newton and Carl Friedrich Gauss devised--formulas showing where you would find an image of a candle flame and how big it would be.
With a single symmetrically convex spherical lens-- one with the same amounts of glass ground off on either side and one of the easiest lens to grind--the two principle planes are together in the middle of the lens. With a plano convex lens with one flat side and one strongly curved side, the planes separate. One is against the flat side; the other is at the tip of the curved side. Add another lens element to make a doublet and you can calculate another set of principle planes. Add a third element... and so on. Add certain combinations of lens element and the principle planes don't even end up near any of the glass elements. Like in a telephoto lens. One of its principle planes to floating in space somewhere in front of its lens barrel.At this point, "So what" and "Why should I care," are valid thoughts.
In normal photography you wouldn't care--the reason you won't find principle planes in the index of most how-to-photography books.
But with micro photography--bigger than lifesize photograph-- it's different. Take a lens. Measure the distance from its front principle plane to your bug's eye--z mm. For a magnification of 10X do the simple calculations. Distance from from the second principle plane to the camera ccd =10z. Oops. Your 300mm telephoto lens is not going to work. 10z puts your camera through the ceiling and into the girl's upstairs bedroom.But enough with the handwaving theory and back to real world.
I worked out the position of the principle planes of the Nikon 50 mm using the fact that for 1/1 magnification the distance between the object--a machinist rule-- and the sensor must be 4X the focal length (8 inches) plus any separation of the principle planes. Measured out to be 8 1/16 inches. That lens is clearly symmetrical with the principle planes at the center of the lens
With the Canon lens it was different. One principle plane was close to mounting flange, the other plane was about a inch farther up in the lens. Another clear advantage for the Canon lens--20% more magnification. But during these tests, I came to realize what I took for lower lens resolution was less than perfect focusing. Three dimensional flowers might be more photogenic than lens charts but they aren't ideal for resolution comparisons.
A super cheap resolution chart would be a fresh flower pedal with interesting detail held flat to a card with double sided sticky tape.
But first I had to work out a major problem with the Nikon lens--a major lens flare in the center of the image. How had I managed to miss that earlier?
Turns out that earlier I'd mounted the camera, extension tubes, bellows and lenses in that order. This time I had the bellows above the extension tubes. When I flipped them the glare spot disappeared. Bellows folds make excellent baffles for stray light.
Summery of what I've learned-- I better have my technique down pat before I blog about lens comparisons. And good macro/micro photograph is both science and art.
Where do I go from here? Despite how it might look, I dud take some non-macro pictures recently. Like about ten gigabytes worth this weekend. Butterflies and kids looking at butterflies at Olbrick Gardens on Friday. Circus World and a Wild West reenactment on Sunday. RAW images and burst modes can fill up memory cards quick.
So it might be a while before I'm blogging again about macros. But it will happen.