Where is Infinity?

[John_4185]

Really, where is Infinity? Is there a multiplier against focal length I can use to presume infinity? I am guessing wildly that it's 200 * focal-length.

[Denis Pleic]

By definition, infinity is not finite. So, it's always further than you want it to be

Seriously, for practical purposes (like adjusting a rangefinder on a camera), it is usually said that you should use (i.e. focus on) an object at least a mile away...

Denis

[tim atherton]

sounds a bit like those photographers who are still stuck in perspectivism... take us to the vanishing point Scotty

[John_4185]

Aw, Denis! Come on. For all practical purposes, it's got to be finite, a real multiplier.

Tim - from the Star Trek we never saw, "Very funny, Scotty. Now beam down my clothes."

[Scott Rosenberg]

50x the focal length.

[Struan Gray]

Infinity is when the light rays coming from the object you are photographing are parallel. Practical infinity is when they are so close to parallel, you can't tell that they're not. Like most things, you have to decide how closely you want to look.

The simplest thing for photographers is to assume that the defocus of an object at infinity is acceptable if the defocus disc is equal in size to the circle of confusion. Infinity is then just the hyperfocal distance:

infinity = (focal length)^2/(f-number * c.o.c.)

That's pretty blurred. A more stringent criterion is to make the defocus equal to the diameter of the Airy disc - essentially you are saying that diffraction limits your ability to detect parallelism. If you measure the focal length in millimeters infinity in meters is given by:

infinity = 0.75 * (focal length/f-number)^2

For a 150 mm lens at f22, the first formula gives about ten meters, the second about thirty five.

[Sidney Cammeresi]

I have heard that 200x the focal length is basically infinity.

[John Kasaian]

I saw one in the parking lot ;-)

On my aerial cameras, infinity or whats in focus when I'm focused at infinity, starts somewhere kind of close up which runs contrary to the archtypical definition of infinity----having no beginning or end. From my position behind the camera infinity has a beginning but appears to be endless.

My head hurts.

[Peter Collins]

Can't tell you because most days, my life is entirely within the circle of confusion. But perhaps this is a case of special relativity in which: ∞ ≠ ∞

[John_4185]

Okay, I'm good with Struan's formula. Cool.

John K - I have a K-24 that I want to play with. Ya know you can get 5" spools for it now?

Next: How to eyeball tilts. (or misspell tilts and end up in an adult search.)