Pressing ESC will stop the rendering process.
However, if you are saving your render as a video file format your video may not be playable. In this case, I would recommend changing the render output file format to an image sequence like "JPG", "PNG" or "EXR". This will save every single frame as an image. You can then use ...
I think it's important to mention, focal length doesn't affect how the sphere is perceived, other than it's apparent size. That is, changing focal length alone, doesn't affect the "fish-eye" effect. Increasing the focal length moves the sensor away from the pinhole, making it see less. What really alters the way an object is perceived, is the ...
Nothing is broken, here, this is an intended design.
Camera view allows you to zoom in or out without permanently moving the camera object so you can close in on details or inspect the scene without exiting camera view.
This allows you to keep your carefully crafted camera position, possibly even animated, while zooming in on a specific detail of your shot ...
Yes it is possible but I am not entirely sure what exactly are you looking for... you can use a spherical gradient like this and this could be your starting base... use some plane as a reference object while using it on the more complex objects like this cube:
The viewport has to recalculate and redraw multiple times to give you a real-time display of your changes. When rendering, this should only happen once per frame during the initial scene build, so the difference should be negligible.
You mentioned doing it with multiple renders. This can be optimized beyond what you imagined (but still requires 3 images, in 2 separate renders, for every frame).
The first thing we'll do is set up an AOV. For our video monitor, we'll output UV in this AOV. For everything else, we'll output RGB 0,0,1.0. This will let us distinguish between the screen ...
A simple Emission shader will draw the exact RGB selected on screen. Use this for both materials.
Then, make sure to turn your samples all the way down as that's what's really going to make a difference. Consider reducing the detail of your circles, especially if they are spheres.
One way to generate a geometry like this is using arrays.
Details for Blender 2.93.4
Add a torus to a scene and edit it to have a lip like shown below.
Add an empty.
Select the torus and add an array modifier.
Modify the properties of the empty as shown below. This will cause the array to get smaller, rotate, and shift. For more details see Changing Size ...