Steven, I'll repeat Rik's comment: Great demo!
Your video deserves a wide, attentive, and thoughtful audience. Well done.
I also agree completely with your explanation, based on considerable experimentation I’ve done on the same phenomenon. (I have demo's of my own that I’ve been meaning to share for at least a year, but haven’t gotten around to editing for release—good on you for getting something nice out there on this subject. I've shared these demo's privately with a few forum members, and we've discussed them at length--so my agreement with your explanation is based not just on my own work, but also on private conversations of considerable depth.)
The surprising effects of utilized aperture are, I think, a vital subject for macro/micro photographers working with reflected light—especially those of us who sometimes use narrow-beam sources, and work with subjects that have mirror-like qualities. These effects seem—unfortunately—to be widely under-recognized. (Microscopists working with transmitted light, on the other hand, usually have equipment, training, and reference materials that deal with their version of utilized aperture phenomena, even if named differently.)
With narrow-source reflected light illumination, a number of things can happen:
- 1) Your subject can appear to move as you move the light. (As you’ve demonstrated here.)
2) You can get greater depth of field, but less resolution, due to increased diffraction. (A small utilized aperture acts like a small physical aperture.)
3) Some regions of your subject may be more or less mirror-like than others. The less mirror-like regions are not as affected by these phenomena (because they produce a more spherical wave front that utilizes—or "fills"—a greater portion of the lens’ numerical aperture) as the more mirror-like regions. So the subject itself contributes to resolution, diffraction, apparent subject movement, and depth of field. And all of these characteristics may vary from one detail of the subject to another.
All this can be confusing (I know from experience) until one understands what is going on.
One example: I usually use a single, very bright light source for focusing, and a dimmer, more complex set of light sources for shooting. These lights usually differ in size and placement (thus producing wildly variant utilized apertures). Imagine my initial confusion when, under the focusing light, I carefully positioned and focused a subject, then turned off the focusing light and took a shot with the shooting lights. What the devil??!!--the subject seemed to have moved from where I'd positioned it. Did I bump something? I turned the focusing light back on and took another look--and the subject was right where it was supposed to be. Then I turned off the focusing light and shot again. Huh? The subject had again (seemingly) moved! Of course, once I understood the perspective changes inherent in small utilized apertures, I also understood what was happening—but it drove me nuts at first.
Another example: In my early high-magnification work, I encountered a subject that seemed best imaged with reflected light from a small light source. In the resultant images, the lighting looked great, but the resolution was poor. At the time, I wondered what was wrong. But after understanding the effects of utilized aperture, it made sense: My point light source, with this subject, utilized far less than the full numerical aperture (NA) of the objective. In effect, I might as well have used an objective with a far smaller NA, which would have delivered far less resolution. Most of us understand that the maximum resolution of an objective lens depends on its NA, and that greater NA means greater resolution. But not all of us realize that we only get the resolution associated with the portion of the NA that we utilize ("fill with light"). If you shoot with an NA 0.70 objective, but your utilized aperture is 0.20, you get the resolution of an NA 0.20 objective. In this example, I decided to reshoot and use less dramatic lighting that permitted a greater utilized aperture. The result was a shot that had less "pretty" lighting, but better recorded the subject details I was looking to document.
--Chris
