Eli5 how does bokeh happen?


Like why does something being out of focus create weird circles

In: 1

Bokeh is a lens geometry side effect. A lens with variable light input is very important when you want a camera to work in sunlight (which is very bright) and twilight (which is not). Even the human eye (which is NOT A CAMERA) uses a variable aperture (the hole the light goes through) for this function.

If you put the aperture in a focal plane, making it smaller makes the image smaller, not what you want. You want to put it in a conjugate plane, a place where all the light is out of focus. Changing the aperture here, like the iris between the two lenses in the eye, changes the volume of light but not the size of the image.

This is all great, as long as everything is in focus and there are no reflections inside the lens. However, some lenses can focus across a broad range of distances, as close as a couple of feet and as far away as miles away. When these lenses are out of focus, you see artifacts rather than images. You’re supposed to use this information to focus the lens, but some people keep it for artistic reasons. The only real problem is UFO nuts that think it’s a focused image of an alien spacecraft.

If the aperture is round, the bokeh are round. You can test this with your eye. If you wear glasses, take them off in a dark room and look at a tiny light, like on the smoke detector. You’ll see a large, dim, fuzzy colored circle. (If you don’t wear glasses, borrow a pair from someone who does, you can reproduce the effect that way).

If the aperture has blades, like a camera lens, you see polygons with the same number of sides. Sorry, you’ll need a camera to experiment with this.

Bokeh, lens flares, caused by focus of the shape of a lens, creates all kinds of effects, like star sparkles and rings, by capturing a smear of light over a filter in that shape.

A fisheye lens will usually produce circles and long curving streaks.

When you squint, you induce bokeh streaks caused by the shape of your eyelashes and if a wer tear is on your eye a diffent bokeh form arises and if a tear is on your eyelash and you squint, a whole different shape arises.