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While going through some pictures I noticed strange artifacts in the out-of-focus sunlight reflected on the surface of a lake. Most of the reflections appear as a full circle, but I marked the irregular ones.

I haven't seen this before, and I wonder if this is just a weird coincidence because of the movement of the water (so the reflections would come and go very quickly), or something in the lens optics or the camera processing somehow. The same can be seen in other pictures from the same session.

Note that the shape I'm getting doesn't exactly resemble the regular cat's eye effect, which is why in principle I rule that out as an explanation, but of course I could be missing something in the way the lens itself works and handles reflections.

Irregular bokeh

Technical details: taken with a Canon R5C and an EF 70-200mm f/2.8 IS III USM lens (with the EF to RF adapter from Canon) with the lens hood, at 102mm, f/2.8, 1/5000s, ISO 100. This was at 7:24 in the morning, a little over 2 hours after sunrise. It's a CRAW file edited in an up-to-date Lightroom Classic.

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This is just my speculation, based on my "mediocre" knowledge of physics and optics. It looks as if the light from those particular specular highlights are only hitting a portion of your lens.

As you know, waves on water act as little mirrors. Some of those "mirrors" are convex, and spread the light that hits them into a wide beam, just like the hood of a car. You can see the reflection from the hood of the car from almost any angle. However, some of those "mirrors" are concave, like the mirror in a reflective telescope. These concave mirrors focus light into a small spot or point. Or, if they are not perfect, onto some weird shaped spot, like when light reflects off of some windows of a building onto the building across the street. Because most of those small concave "mirrors" created by waves will not be pointing in the right direction to send that spot in your direction, you do not see them. You only see the light reflected from concave mirrors if they happen to be "pointing" that reflected light in your direction.

So, even though half of the "mirrors" on the water will be convex, and half concave (because what curves up must curve down, when it comes to water waves), most of the light reflected by the concave "mirrors" will be directed away from you. That is why only a few of the bokeh balls have weird shapes.

Now, what about those few concave mirrors that are sending their light reflections your way? Most of them are still going to beam their focused light in a wide cone, so they will appear just the same as any other bokeh spot. However, some will only focus their light on part of your lens. So, the bokeh ball then takes on the shape of the portion of the reflected light spot that happens to hit your lens, just the way an aperture with only a few blades causes polygon shaped bokeh.

My guess is that you only noticed them this time because you happened to be just the right distance so that enough of those misshapened, concave "water-mirrors" were just the right size and curvature to create light spots of just the right size to hit some of your lens while missing some of it as well. And, because you may be suffering from "Bokeh Ball Madness," where the afflicted analyse every image for perfectly round bokeh balls in order to meet some imaginary standard. ;^)


Edit: Here is a further explanation / "thought experiment" that may help people understand what I am saying:

Most specular highlights that people see are caused by convex surfaces, like the hood of a car. Those do spread light in all directions, and so, as some have stated, that light strikes the entire face of the lens. In this circumstance, the only thing that can "shape" the bokeh spot is internal to the lens or camera. Light that came from a point source, then reflected from a convex surface may appear to be what is commonly called "columnated," when observed from a distance, because all the light rays from that surface that strike a lens at that distance are essentially parallel. But those light rays from that convex surface are not "focused." They literally do spread out all over (within the range of that reflection). If you move from side to side, you still see the reflection, just from a different part of the car hood.

However, a concave mirror is different. It acts like a convex lens (a regular lens) and focuses the light. In other words, it causes the light from the point source to only go in some directions and not others. Only within the cone of light to the focal point. Then it spreads out again into another cone. Just as with a magnifying glass in the sun, the light from a concave mirror is focused into a cone of light which meets at a point, and then spreads out again. But this spreading is not the same as that of of a convex car hood. It is "contained" within the cone. That cone has edges. If you project that cone of light onto a screen, perpendicularly, it produces a circle. If your screen is far enough from the focal point of the concave mirror, then that circle is large enough that a small segment of the edge will appear to be straight. Imagine drawing a circle 40 feet in diameter, then looking at a small, 1 inch, part of its edge. It would look like a straight line.

Now, cut a 1 inch hole in a piece of cardboard to make an "aperture," and hold it near the screen, within the very large cone of light. All you will see behind the cardboard (inside your camera) is a small circle of light, the size and shape of that hole. Even from a concave mirror, this is what you will see most of the time, simply because there is more room in the circle than at the edge. However, adjust the position of the cardboard (or the mirror) till that edge of the cone intersects with the hole in the cardboard. Well, now you have only part of the hole illuminated at all. The shape you see on the screen will be the shape of the hole MINUS the part that is not illuminated. The part that is outside that cone of light. This gives you a bokeh spot that is the size of your aperture but has one side cut off with what appears to be a straight line.

Now point several concave mirrors at the hole in the cardboard, from various directions, with the edges of their cones of light intersecting said hole in different ways. You would get several different images of the hole, but with varying amounts of that hole not illuminated. Just as in the image above. Is that improbable to happen in nature? Yes. That is why we only see it every once in a while. In just the right circumstances.

As to the brightness of the spots. Most specular highlights are well beyond the limits of the sensor. So the sensor cannot detect the difference in brightness. Most of the light from any specular highlight actually went somewhere else. Our cameras only catch the tiny part that happened to go through the aperture, whether they came from a convex or concave surface. And they all still overwhelm the sensor. So they all look the same brightness.

(I really wish I knew how to use my animation software. I could make a whole video on this topic.)

tl;dr - That's gonna happen sometimes when your specular highlights are coming from water waves, or other randomly moving concave surfaces.

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  • \$\begingroup\$ As to those who may say that you can "just stop down": True, this would reduce the size of the part of the front of your lens that this partial light spot would need to both hit and miss at the same time, thus reducing the frequency of these odd bokeh balls. However, there are many times when one cannot "just stop down." This image was likely one of them. If you stopped down too much, your squirrel would have been reduced to a silhouette. Besides, there is no reason that every single bokeh ball in your image has to be perfectly round. \$\endgroup\$ Jun 19, 2023 at 22:41
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    \$\begingroup\$ Very interesting. I'm thinking of a potential experiment to repeat this in similar conditions and see the frequency of the non-round bokeh balls. It might be possible to see it with a high-speed video using a similarly fast shutter speed too. Regarding bokeh madness or other perfectionism symptoms, no. I don't pixel-peep or complain about "noise in the shadows" either. It's just that in this case the odd shapes jumped to my eye immediately, especially as there are a few right next to the subject's head, making them distracting. \$\endgroup\$
    – Janoma
    Jun 19, 2023 at 22:53
  • \$\begingroup\$ @Janoma, Good, you have avoided the contagion. ;^) \$\endgroup\$ Jun 19, 2023 at 22:55
  • \$\begingroup\$ This seems to be a significant misunderstanding of how a lens works... there is a complete image of (light from) the entire scene at every point on the objective lens (as seen from that viewpoint). And increasing/decreasing aperture is simply adding/removing images from the "exposure stack." I.e. those partial spots exist at every point on the objective element. \$\endgroup\$ Jun 24, 2023 at 16:21
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    \$\begingroup\$ @GrantRobertson, We can fairly well assume that the specular highlights are distant enough to be considered generally parallel/collimated entering the lens. You are saying that some of those points may be farther concentrated/focused by a concave point on the surface of the water. Having that occur while remaining relatively the same size and brightness is problematic IMO. But since all of them are clipped, it is impossible to determine relative brightness. I withdraw my objection. \$\endgroup\$ Jun 24, 2023 at 18:05
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This is almost certainly due to the EF-RF adaptor and the cause is the same as what causes cat eye bokeh.

In a typical lens cat eye bokeh is due to the circular periphery of the lens itself when the aperture is at max (it is the lens barrel/vignetting). But the RF adaptor has straight edges at multiple angles which are the cause of the hard/straight/angular edges in this particular situation.

enter image description here

(I couldn't find a picture of the plain adaptor's baffling, but it is very similar)

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  • \$\begingroup\$ Cat eye bokeh occurs near the periphery of an image, and is more pronounced the closer it is to the edge. Also, each bokeh spot is similar in shape those nearest to it. (See sample images here:google.com/search?q=cateye+bokeh) Can you please explain how there are several completely differently shaped bokeh spots, in the center of the image, right next to each other, and right next to perfectly circular bokeh spots. \$\endgroup\$ Jun 24, 2023 at 17:19
  • \$\begingroup\$ The cause of the resulting bokeh shape (line/angle) is similar to the cause of cateye... a peripheral restriction/obstruction on one side. It's position away from the edge is because it is also due to an aperture restriction caused by the opening in the adaptor. It's being next to others which are unaffected would be due to it existing in only one portion of the objective element (as your answer described). \$\endgroup\$ Jun 24, 2023 at 18:14

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