Know your camera - stars and rainbows

In the previous post I disected the exposure system of my camera (the Panasocic hc-x929/x920); I showed the combination of iris blades and ND-filter used and showed how it affected the sharpness of the image. In order to get an optimal sharpness of my images I must select an f-stop near f 2.8 or f 5.6. There are however also some other image effects which get affected by the choice of f-stop. Although these will generally not be decisive in choosing a particular f-stop I still wanted to show them to you in this post.
The first, most obvious effect is how the bokeh is rendered.
I must add to this that we are talking here about a small sensor camera which by the nature of it's design has a very large depth-of-field (or DOF) in most situations. So out-of-focus elements will not be as common in it's footage as in the footage of e.g. a full frame dslr. Only at long focal lengths (i.e. zoomed in) or when focussing at very close distances and shooting at wide open apertures will this camera be capable of separating the subject from it's environment by bringing the environment out of focus while keeping the subject in focus. And as the DOF increases with increasing f number I'm pretty sure that beyond f11 the DOF will effectively run from the lens to infinity. It is the shape of the aperture between f1.5 and f2.8 which effectively determines it's rendering of bokeh.
Out-of-focus dots of light will be rendered as a projection of the aperture. Knowing what I know now about the aperture system of my camera I can conclude that the bokeh will be a regular octagon between f2.5 and f2.8 and also between f5.6 and f11. However, above f2.8 and below f5.6 I might expect an irregular bokeh which may give out-of-focus elements an unusual look. Beyond f11 the bokeh will have changed to a diamond shape.
Deducing is one thing, testing another. So I took a couple of pictures of the same out-of-focus subject at different apertures but with identical brightness (as far as this was possible) to see if I would notice anything unusual between f2.8 and f5.6. Obviously the image gets sharper as the f-number increases. Here are a few (click to enlarge):

f2.8

f4.0

f5.6

I can't see anything unusual about the bokeh, The f4.0 picture looks normal to me, as did the other pictures with other f numbers which I will not bother to show. So the "unusual" bokeh which I expected between f2.8 and f5.6 did not appear.
The second effect is how bright point sources of light are rendered. These are effectively rendered as stars where the points of the stars are oriented perpendicular to the orientation of the edges of the aperture blades. Let's show a few examples (click to enlarge):

f2.8 f3.1

f5.1 f5.6

The following can be obeserved:

• The star gets smaller with increasing f-number
• Between f3.1 and f5.2 an additional pair of vertical legs are superimposed onto the star which are slightly mis-aligned with the original short legs. (Not very easy to see).
• At f5.2 a subtle diffraction pattern can be seen in the vertical legs where the light is separated in a rainbow pattern.

This can be explained by the design of the combination of the iris and built-in ND filter. As soon as the ND filter is shifted in front of the aperture, additional horizontal edges are introduced which add an additional pair of vertical legs to the star. Apparently the edges of the ND filter are not perfectly aligned with the edges of the iris giving them a slightly different angle.
When the ND filter covers the aperture almost entirely, two small slits remain which due to their small diameter cause a diffraction pattern like you might have seen in a typical high-school physics experiment. As the location of the minima and maxima depend on the wavelength of the light, the light effectively gets separated into it's individual wavelengths, giving a series of overlapping rainbow patterns. Once the ND filter covers the aperture entirely,  the star is regular again and has diminished in size.
Now, as a little encore I would like to show you some patters which occur at very specific combinations of focal length and focus distance. The aperture was in all cases wide open.

The top image appears when focussing on a short distance with the light source completely out of focus. In the center image the light source is brought slightly more into focus and on the bottom image the light source is almost in focus. The upper two images show artifacts which I haven't encountered yet in normal footage I shot with the camera, the bottom shot shows a familiar artifact; colorful "globes" surrounding a bright light source.
I'm not entirely sure what is causing this, but I suspect it is related to the fact that this model is a three-ccd camera (or 3mos as Panasonic likes to call it). Maybe the colorful patches of red, green and blue are internal reflections in the prism block which bounce back from the lens element, closest to the prism block, back towards the sensor. The distance between this lens element and the prism block will then determine if the reflections are in focus by the time they reach the sensors or not.
Notice how the camera interprets a range of wavelengths as one solid color of either red, green or blue. Only where the patches overlap as in the horizontal line to the right, a mix color is shown, in this case yellow.
Another effect caused by the fact that this is a three-ccd camera are the colorful fringes of the bokeh which can be seen at it's widest apertures. When you look at the image of the wide open aperture at the start of this post, or in the previous post, you can see a blue edge on the upper right, a red edge on the upper left and a green edge at the bottom. Sometimes when filming street lights in the background, such subtle colored edges can be seen, as if the colors don't align there perfectly. This is the telltale sign of a three-ccd camera.