For context, let me quote the most relevant snippets from the last several postings in that other thread.
rjlittlefield wrote:One caution about stopping down farther: diffraction blur.
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Stopping down farther would not have exposed much more detail in areas that are now OOF, and it would have softened even farther areas that are now fairly sharp.
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...inserting a 2X teleconverter also doubles the effective f-number, very much like extending the lens to get the same magnification. There's no free lunch -- the tradeoff between DOF and diffraction blur is the same no matter how you get the magnification.
Harold Gough wrote:On the matter of diffraction in lenses in general, I have always understood that the concern is about that arising from the edges of the diaphragm. As you close down to a smaller and smaller aperture, that part of the image with the worst of the diffraction effects contributes an incrementally increasing proportion of the image. I don't see how that can be increased by using a teleconverter, which uses the part of the image furthest from the diaphragm.
rjlittlefield wrote:The edges of the diaphragm really have no significant effect.
What matters is that the aperture restricts the maximum angle between light rays, which in turn restricts their ability to interfere with each other to produce differences in intensity that the sensor can detect.
Graham Stabler and I had a long discussion about this some months ago. The discussion is hard to follow, and then takes off on another point, but take a look at the illustration at http://www.photomacrography.net/forum/v ... 0&start=47 .
The point is that light rays striking the sensor at relatively steep angles with respect to each other can interact to form fine patterns of intensity, while light rays striking the sensor at narrower angles can only form coarser patterns.
DOF depends on the same angles, hence my comment that there is no free lunch -- the tradeoff between DOF and sharpness is the same no matter how you get the magnification. The nominal setting of the lens to get a particular DOF/sharpness will vary depending on whether you use extension or a teleconverter, but the combination of DOF and sharpness will not.
What we're wrestling with here is how to explain and understand what happens. The facts of what happens are quite straightforward and pretty easy to confirm by experiment: at fixed magnification, DOF and diffraction blur both depend on the effective aperture, and it makes no difference how that magnification and effective aperture are achieved.Harold Gough wrote:I'm not sure that all this information about interference has much to do with diffraction, the latter being about change of direction, in which scattering would be included.
The challenge is how to make sense of the facts. Here we are not helped by decades of photography literature that have used the word "diffraction" in several different ways, and have seldom used the word "interference" at all.
I wish I knew how to write an explanation that is simultaneously short, clear, and easily understood by people of many backgrounds. If I did, I could make a bundle of money selling copies to put in textbooks. But I don't. Nonetheless, I'm going to try again anyway, right now. Bear with me...
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The effects we're talking about are all due to the "wave nature" of light.
Like other waves, light waves do several interesting things --- they scatter off obstacles, they bend around corners, and they interfere with each other.
All of these effects are due to the same underlying physics. The different words reflect our human need to have simple models that adequately describe situations we care about.
Most explanations of image formation start at the subject and work their way toward the sensor.
I would like to try it in the other direction -- start at the sensor and work our way back to the subject.
At the sensor, what is important is "interference". All of our current sensors respond only to the intensity of light, not its phase. To form an image that the sensor can detect, light waves must come simultaneously from several different directions and interfere with each other to form a "standing wave" pattern in which regions of high and low intensity stay in the same place while the waves continue to oscillate. The high intensity regions are then "bright", the low intensity regions are "dark", and so on. In the words of most textbooks, "a real image has been formed".
To repeat, the process of forming a real image has everything to do with "interference", and nothing to do with "scattering" or "bending around an obstacle".
OK, now back up one step. Where do the light waves come from, that interfere with each other to form an image?
Of course they come from the lens, having gone through the aperture.
The primary action of the aperture is to restrict the range of directions from which light rays can reach the sensor. This has three principal effects:
1. It reduces the average intensity at the sensor.
2. It increases depth of field (by reducing the size of the blur circle for out of focus points).
3. It reduces the achievable resolution of the image formed at the sensor.
That last effect is what photographers normally call "diffraction blur", even though it's really an interference effect.
Perhaps this illustration will help:
Notice the caption on the image.
The famous "Airy disk" is simply the real image formed by a point source imaged through a circular aperture, producing a restricted range of angles. A narrower range of angles produces a larger Airy disk, just as in the illustration above a narrower range of angles produces a wider band spacing.Light coming from multiple directions forms a "real image" through interference that produces a standing wave pattern.
The size of detail that can appear in the image depends on the range of angles that the light comes from ---
a wider range of angles (larger aperture) can form a more detailed image.
To add some more context...
It's true that if you look very closely at the edge of the aperture, you can see some effects that would probably be called "scattering" and "bending":
But it's important to note that these "scattering" and "bending" effects are what you see when you focus your attention on the edge of the aperture. When you focus your attention on the image being formed at the sensor, it's the interference effect that matters.
OK, working our way back toward the subject, we run into the lens also.
For this discussion, let's treat the lens as just a lump of stuff that "changes the direction of light rays so they come to a focus" back at the sensor. That lump of stuff has some width (restricted by the aperture), and it's that width that provides the range of angles that we need to form a real image.
But there's a subtle detail here. Consider a single point on the sensor, representing a single focused point on the subject. Light exiting the back of the lens, at various angles and positions so as to focus on that point, had to enter the front of the lens at various angles and positions as well. In addition, all of that light has to be coherent enough to form a real image by interference.
So, how does it happen that a single point on the subject manages to send out a set of light waves that can travel different distances, be redirected by the lens, pass through the aperture, and end up sufficiently coherent at the sensor to form that real image?
The answer to that question really is "scattering", but this time it happens at the subject, and it's critical to image formation.
To cement this picture, let's run through the process again, but in the usual order -- subject to sensor.
Incoming illumination strikes the subject. Through scattering, each point on the subject produces waves of light that propagate as expanding spheres centered on that point. Some of these waves strike the front of the lens, where they get bent by refraction at the lens surface. Continuing on, the waves strike the aperture. At the aperture, a small fraction of the waves very near the edge of the aperture do get scattered/bent by diffraction, but most of the waves either get blocked entirely or make it through the aperture with no significant change. The waves that do get through the aperture with no significant change get refracted some more by other lens surfaces, then continue on to the sensor, where they form a real image by interference.
I hope this explanation clarifies the physics.
The photography community uses the term "diffraction" in a couple of different and incompatible ways. Sometimes it means "scattering and bending around obstacles", and sometimes it means "anything related to the wave nature of light". In the phrase "diffraction blur", the meaning is "wave nature of light". The blur is due to limiting the range of angles available for interference, not due to scattering or bending around edges of the aperture.
--Rik
PS. Thanks are due to Graham Stabler for pointing out this way of thinking about the process. It took me a while to switch gears, too, but now I find this model to be more clear than anything I had seen before. Graham will probably twitch a little at a couple of my phrasings in this post, but I've tried to strike a balance between precision and clarity.
