Of course, the shape of the aperture blades is important for the out of focus highlights, the bokeh. I have previously seen that the Lumix 45mm f/2.8 1:1 macro lens has somewhat more rounded aperture when stopped down than the Olympus 45mm f/1.8. When not stopped down, i.e., at the largest aperture, the diaphragm blades move out of the way, and the opening is usually perfectly round.
Adding a higher number of diaphragm blades can make the aperture opening more rounded when stopping down. However, lenses seldom go beyond nine blades, probably for the reasons of cost, complexity, and the risk of having one of them break down.
It turns out that the number of blades is also related to the rendering of flare, strong light sources inside the image frame. I'll look at that later in this article.
Samyang and Lumix fisheye lenses
There are currently two fisheye lenses available for the Micro Four Thirds system, the Samyang 7.5mm f/3.5 and the Lumix G 8mm f/3.5:
Of course, the major difference between these lenses is the price (the Samyang is the cheaper), the image quality (the Samyang is better, in my opinion), and the focus mechanism (only the Lumix lens has autofocus). But in addition: The Samyang has six aperture blades, and the Lumix has seven. These close up pictures show the rear exit pupil at f/5.6 for both lenses. The Lumix G 8mm lens has more rounded aperture blades, but you can still make out seven segments:
|Samyang @ f/5.6||Lumix @ f/5.6|
Here is a video showing how the aperture blades open up on the Samyang lens:
Affecting the flare
It turns out that the number of aperture blades affect the flare in a fundamental way. It is common to get a star shaped flare when using small apertures. And the number of spikes in the star is the number of diaphragm blades (when that number is even), or twice the number of diaphragm blades (when that number is odd). Hence, using the Samyang should give us stars with six spikes, and using the Lumix should give fourteen. Let's check, by taking pictures at f/22:
|Samyang @ f/22||Lumix @ f/22|
To see this more clearly, here are enlargements at f/22 and f/11:
It's not so easy to count the number of spikes on the Lumix image to the right, but I think it is still quite clear that it is fourteen.
To my experience, most lenses have an odd number of aperture diaphragm blades, which produces the most spikes on the star shaped flare around bright objects, e.g., the sun. I guess this is because a larger number makes the flare look more blurred, whereas a small number makes the flare more distracting.
A bit of trivia is that Canon has chosen an even number for most of their lenses, while Nikon is going for an odd number. Hence, it is often easy to guess what brand a photographer uses, based on an image where the sun is inside the frame. However, the aperture also needs to be fairly small to see this effect, e.g., f/16 or f/22. So you might not be able to make this out based on any picture.