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Perspective shift from lighting change

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Joined: 22 Sep 2011
Posts: 299
Location: Florida

PostPosted: Mon Aug 25, 2014 7:16 pm    Post subject: Perspective shift from lighting change Reply with quote

I noticed while viewing a small washer that the OOF region would shift dramatically when I changed the lighting. My first thought when viewing was that perhaps that the aperture of the lens is not being filled all the way, and when the light moves, it is effectively shifting the utilized aperture's position. I also thought that this could be a totally bogus explanation, so I should post here for further opinions Smile

The video shows a metal washer being illuminated by an LED flash light that is moved side to side and then circularly, above the camera/lens. The optics are a nikon 10x 0.25n a finite objective from edmund on 150mm extension.

Also, this reminded me a little bit of this thread, http://www.photomacrography.net/forum/viewtopic.php?t=16736 with the difference of the focus staying the same, but with the illumination shifting.
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PostPosted: Mon Aug 25, 2014 8:07 pm    Post subject: Reply with quote

I agree that's exactly what's going on. Using directional illumination with a near planar subject causes much more light to enter one part of the aperture than others. As you move the light the utilized aperture moves around, which causes the OOF blurs to move around too. This is in complete accordance with "simple" ray optics, despite the highly counter-intuitive effect.

There is much more discussion of these issues in THIS thread, where the issue of utilized aperture arose as an unexpected side effect regarding DOF with a shiny subject.

Great demo, BTW!

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Chris S.
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PostPosted: Mon Sep 01, 2014 10:09 pm    Post subject: Reply with quote

Steven, I'll repeat Rik's comment: Great demo! Very Happy 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.

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Joined: 22 Sep 2011
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Location: Florida

PostPosted: Tue Sep 02, 2014 3:32 pm    Post subject: Reply with quote

Thank you Rik and Chris S for your comments, and the further information and reading.

It is good to get several verifications of what I had suspected. It is too easy in this hobby to find something that seems explainable and have it turn out to be something totally different.

I had not considered point 3 that Chris made at all. In retrospect it makes sense why this effect was so exaggerated on the metallic washer. It was mirror like and had plenty of detail to see for the eyes to fixate on, leading to the rather dramatic effect. Since I use a small light source for focusing as well I will have to keep this in mind when shooting shiny subjects.
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