Sorry to go on about mathematical abstractions when you would all rather consider Art, but I find the corollaries to the above analysis very interesting.

For instance, want to know how big the disks of distant OOF lights will be in your image? Imagine your lens opening sitting at the plane of focus. If you are shooting a 50mm at f/1.0, then the opening is 5 cm, or 2". So imagine a 2" diameter disk however far out you are focused, and that's what the distant lights will look like. Focus closer, disks get bigger. Here's an example:

That's

**probably** a 50 f/1.4 wide open, so those disks should be 1.4" = 50mm/1.4 across at the plane of focus, which is the window grill. That middle section is 10" across, so that looks right. In another shot, where I'm closer to the window, the disks look bigger:

but they still look about 1.4" across

**at the plane of focus**, which is now closer.

For specular lights not so far away, it's a bit more complicated. I'll spare you.

We now return you to your photography forum...

--Matt