Talk:Afocal photography

projection

It should be noted, that while many people seem to call it "eyepiece projection" – it's not!
A projection in the geometric/optical sense of "display of an image" always generates a picture on a flat plane. In fact I would say that afocality is exactly the opposite of a focused picture on a projection plane.
I'm sure this misuse of the term got started with people who were familiar with (genuine) eyepiece projection. The act of taking astronomical photos with an eyepiece in the telescope reminded them of eyepiece projection, and they never stopped to think about what the words really mean.
-- BjKa (talk) 12:30, 6 August 2014 (UTC)

Per a quick search there is nothing "erroneous" about the term "afocal projection". Need sources to word it the other way. Fountains of Bryn Mawr (talk) 14:24, 6 August 2014 (UTC)

Just because everyone else says it that way still does not make it right. How the physical world works is not subject to democratic vote and especially "Proof by Google" is evil on several levels. I'd like to compare this to the ball bearing phenomenon: So many people say "ball bearing" when in reality they mean "steel ball". The word "bearing" denotes specific engineering elements, and a bunch of steel balls by themselves will never make a bearing. This is just plain obvious to anyone who cares about what the words he uses really mean, although an internet search will always come up with the opposite.

Let me state it again: When you do an optical projection you want to focus your picture on a plane. When using a projector you must dial in the correct focal length for the distance to your wall, or else your audience will complain. That is the meaning of the word "projection", without any doubt even in the general public mind. As I said, this is exactly the opposite of afocality. If you know your Latin and/or Greek you will see at once that "afocal" literally meens "unfocused", and that is obviously not wat you want in a projection. Specifically in optical engineering it means that the rays drawn from the edges of the field lens are parallel and do not cross in a focal point on a focal plane in a focal distance. At a telescope afocality is needed as an interface for stuff which is constructed to view the world directly, like a human eye or a camera with a non-removable objective lens group. There is no projection going on between the telescope and the eye/camera and this is why speaking of projection in this context is just plain wrong. The projection part happens inside the eye or the camera as a matter of the normal everyday use of it.

There is no specific magic involved in taking photos at a telescope. I've been conducting tours at an observatory for many years, and I always set up a telescope for normal sighted eyes. In that setting visitors often just hold their camera to the eyepiece and make a picture. Nothing to it.

--BjKa (talk) 09:28, 7 August 2014 (UTC)

Let me state it again: WP:V requires that a statement of fact be verified. Even if you're sure something is true, it must be verifiable before you can add it. The term "afocal projection" comes up in published sources including NASA and Applied photographic optics: lenses and optical systems for photography, film, video, and electronic imaging by Sidney F. Ray. Why a particular optical setup is called a "projection" goes way beyond projectors and telescopes re: Ray pages 363-364. He is talking about "afocal projection attachments" for camera lenses there, basically tel-extenders - small telescopes that hang on the front of a lens. Fountains of Bryn Mawr (talk) 15:59, 7 August 2014 (UTC)