Alright you asked for it so here we go... #Anamorphic lenses thread incoming!
We will start with some history:
Sadly like a lot of cool things anamorphic lenses have a military background and were originally developed by Henri Chrétien during World War I in order for tanks to have a wider field of view.
In a way anamorphic lenses are a kind of wide angle converter. What makes them special though is they only compress one axis. This way you can build an optical system that is vertically a 50mm and horizontally a 25mm lens. They do this by using cylindrical lens elements.
I attached a picture of cylindrical and spherical lenses to this toot.
Cylindrical lenses focus light into a line, while spherical lenses focus light into a point.
Spherical lenses are typically used for normal photography lenses.
Back to history:
At some point in time movie theaters had to face a problem because there was something new in town - the television. Because of it less people went to see movies in cinema. Back in the day tvs and movies were mostly shot and shown in a 4:3 aspect ratio.
So in order to get people back into the theater something new had to be developed: Widescreen movies!
Anyway back to shooting. How can we get a wide screen image on a standard 35mm motion picture film?
Well using an anamorphic lens we can squeeze the horizontal axis giving us a compressed image which we can then by also using an anamorphic lens on the projector desqueeze to get a widescreen image.
Picture by https://en.wikipedia.org/wiki/User:Wapcaplet (CC BY-SA 3.0)
This is an anamorphic adapter for a 16mm film projector. You attach it in front of the normal spherical lens the projector has to desqueeze the anamorphic image back to normal.
As you can see - looking trough the front - the back looks like an oval shape even though it is round. When looking trough the back you also see an oval opening but it is wider instead of squeezed.
This can be quite annoying of course but thankfully you can also use a variable diopter for focusing!
What you do is basically set both spherical and anamorphic lens to infinity. And then add the diopter in front and use that for focusing.
They basically work like this: You have two diopters which cancel each other out when they touch (=infinity) and if you space them apart their strength increases focusing the whole system closer. I attached an image of a popular diopter for this.
This is how such a setup can look like assembled. You have a camera with a spherical lens attached and in front an anamorphic adapter lens. Add a variable diopter to the front and viola you have a usable setup where you only need to focus the front diopter instead of both lenses at the same time.
Focusing both lenses can be fine if you only shoot stills but for motion pictures it is not a usable solution.
This is the unedited picture I took. As you can see it is squeezed and looks pretty digital (because @lastfuture asked about the analoge look).
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