Hey Tobias! All very legit questions which we will try to answer in depth, and we hope down the road others will chime in to add things to this interesting topic.
Generally whether you shoot in a all in one anamorphic lens or a anamorphic adapter, there’s always a spherical lens design and cylindrical part.
In most Anamorphic lenses which yield the most distinguished anamorphic look the anamorphic element is placed in front, and thus is the first pass when the light comes in, the only thing that sits in front of it (some designs work differently) is the variable diopter. In the case of the Aivascope 1.5X anamorphic adapter focus is done with a variable diopter. The scope is tuned for infinity focus and if you rig it in front of the taking lens you need to make sure the taking lens also is adjusted for infinity focus, which is most of the case not where the infinity marker hits its mark (most taking photography lenses are badly bad focussed), so take a very distant object (1km away) and open up the aperture, use a digital magnify on your monitor and make sure infinity focus is nailed, preferably put some electrical tape around the taking lens, so it stays where its at. (not too much tape because you might want to redo this after a while because things can go off calibration), now its the variable diopter which does all the focussing work. we call this a ‘ single focus solution’ .
It sounds like we are drifting away from answering your questions, but its important to have a good understanding of how light enters your optical formula. Because the anamorphic element squeeze’s the image by 2X cilindrical in one direction, the aperture will be oval as well.
NOw let’s get this quickly overboard and if you need the exact scientific reason for it make a other topic… 🙂 bokeh works like a squared formula, so even though you desqueeze the 2X image by a number of two, the bokeh is still oval because its affected by a way bigger number (squared) than the 2X forumula. if you would put the anamorphic element behind the conversion point (aperture) it would just be squeezed by 2X as a whole and after desqueezing the aperture will return to a normal round circle. SO anamorphic boke is not 2X but instead squared, and by desquezing it 2X you still have a somewhat oval, depending on the dof the bokeh can look more squared or almost round, this is effected by both the distance to foreground, the focus point, how far away the distant object is, the squeeze ratio of the anamorphic lens, the f stop and of course the focal length of the taking lens.
PLease note that the anamorphic lens only produces the squeezed ratio, spherical bokeh is squared in its behaviour, thus a bokeh bubble of a 80mm lens with the same f stop as a 40mm is squared in terms of size, but since the anamorphic lens comes in play its squared with a offset in regard to the squeeze ratio of the lens. Hope this is somewhat understandable.
However if you want to render perfect circular (oval) bokeh you need to make sure that your taking lens has a circular aperture, most lenses wide open do have a circular bokeh but once stopping down, the amount of aperture blades and their system (you have different aperture systems, like rounded blades, flower shapes triangles etc.), will determine how circular thus how perfect your ovals will look like. Generally in cinema lens design (look at Cooke) a high aperture count is not really in its tradition and it can effect mtf (sharpness), due to airy rings, refraction , hence most cinema lenses (except lomo standard speeds) will have blocky bokeh once stopped down, but lets leave these discussion in other topics.
Generally a lot of photo lenses do have a high blade count including a rounded blade system. The famous 17 blade Helios (originated from biotar 58mm) , are very popular among anamorphic shooters. Generally stopping down on a taking lens will yield the best results when shooting anamorphic, thats because both the diopter (which is designed to work the best for one focal length in the case of the aiva a 50mm) and the anamorphic lens will cut off some high frequencies, by stopping down, you diminish these effects. But once stopping down a lot of lenses are not circular in terms of bokeh anymore. Thats why there are some lenses better for anamorphic shooting than others..
I myself am much more obessed with 3d depth distortion of anamorphic lenses and waterfall bokeh, than perfect circular ovals.. On top of that most anamorhhic lenses have cat eye bokeh (cut of bokeh by the housing ) on the edges, and slanted bokeh (only the ovals are straight in the middle but rotated on the sides) , this is a combined effect of both the anamorphic element and the spherical lens.
Due to the amber coating on the Aivascope (both front and back element), the scope will render somewhat warm, especially when a light source hits directly the glass, but again this is a combined feature of the coating of the taking lens and the Aivascope. If a taking lens has green coating, and the aivascope amber, than there will be secundary flares in green. I prefer to use lenses that are either single coated or multicoated with a balanced coat that has magenta green and yellow, and complementaries, that works super sexy together with the aiva.
The aivascope doesnt add much chromatic abberations, its a nice design, but generally if your taking lens has a lot of ca, the aivascope might articulate this more. BUt Ca with tha aiva is mostly coming from the taking lens, if you take a taking lens with lack of ca you probably find not much of ca in your footage. \
SO for bokeh, the scope mostly adds a squeeze ratio to your bokeh with a squared law, the same as bokeh works with a spherical lens but with the added squeze ratio of a anamorphic lens in that direction. But due to distortions (barrel mostly) the ovals are kind of messy on most scopes, and the wider you go or the further you put the taking lens away from the scope the more this is pronounced.
About the cat eye bokeh which is mostly produced inside the taking lens due to lack of coverage, the fun part here is that when you infinity focus on a lens, the element is as close to the sensor as it can be so mechanicala and optical vignetting are at its worst, if you focus closer with the taking lens (without the scope) the front element is further away and mechanical vignetting is lower, but again, just look for a nice taking lens that has characterstics that you would like to see to leak through in your anamorphic formula, a perfect lens doesnt exist in that regard, and that makes most Anamorphic lens collectors and shooters so obsessed. always on the haunt for that perfect lens!
If you happen to have more questions . please shoot!