View Full Version : Where is Infinity?

20-Oct-2005, 14:20
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
20-Oct-2005, 14:26
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...


tim atherton
20-Oct-2005, 14:28
sounds a bit like those photographers who are still stuck in perspectivism... take us to the vanishing point Scotty

20-Oct-2005, 14:33
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
20-Oct-2005, 14:34
50x the focal length.

Struan Gray
20-Oct-2005, 14:42
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
20-Oct-2005, 14:44
I have heard that 200x the focal length is basically infinity.

John Kasaian
20-Oct-2005, 15:01
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
20-Oct-2005, 15:01
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: ∞ ≠ ∞

20-Oct-2005, 15:10
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.)

David Luttmann
20-Oct-2005, 15:47
.....my wife's credit card balance ;-0

John Kasaian
20-Oct-2005, 17:02

Give Mr Photo a try. He'll know if the spools of film he has will fit in a K-24. I'd email him through his eBay store---hes the guy who sells frozen, dated Kodak aerial films like Aerecon and Panatomic in 5" and 9-1/2" rolls.

John Cook
20-Oct-2005, 18:28
I asked my pal, the Southern Baptist preacher, about the location of infinity.

He said it’s right by the place he likes to go fishing in Northwestern Maine, about “seventy miles beyond the Great Commission”.

dave t
20-Oct-2005, 18:46
I like Peter Collins' answer. But just to be pedantic, there are, of course, a whole lot of infinities -- an uncountably infinite set of infinities in fact. So there's nothing special about an infinity not equalling another infinity. (See what happens when you live in the circle of confusion?)

20-Oct-2005, 19:28
For my Leicas, it's the lattice work gate across the lake behind my house. For my Technika it's the Moon.

20-Oct-2005, 20:12
Funny guys! Okay, then what's an Infinity STOP?

It's like a safety device to keep you from looking beyond Infinity and scareing the heebie-jeebies out of you? Eh?

Pete Caluori
20-Oct-2005, 20:15

50 and 200 x the focal length have been suggested, but I believe that is way too conservative. Years ago I remeber reading a paper on lenses and optics and the number I remember being touted for calculations requiring infinity was 1000 x the focal lenght of the lens.

Regards, Pete

20-Oct-2005, 20:39
"but I believe that is way too conservative"

do you mean not conservative enough?

i was just wondering about this the other day ... no one ever told me where infinity was. it just seemed a bit closer with wide lenses.

Steve Clark
20-Oct-2005, 20:44
I saw a personalized license plate that read NBYOND. Closer inspection revealed that the vehicle was an Infinity...hehe...

Donald Qualls
20-Oct-2005, 21:12
The rule I've seen for selecting an object to set the infinity focus of a lens is that it should be at least 50x focal length -- but with focal length in mm, and distance to the object in feet. So, for a 200 mm lens, you'd want to use an object at least 2 miles away (about 10,000 feet).

Dan Jolicoeur
20-Oct-2005, 21:46
Wouldn't infinity be as far as the eye can see on that particular day, and then beyond that? If it is a clear day it may be unlimited or about 50-70 miles. Or on a hazy day it could be 500'.

Struan Gray
21-Oct-2005, 00:24
How do I get Aleph on this keyboard?

The really long definitions of infinity are derived from the optical engineering meaning of "diffraction limited". Simply, all other forms of blur should be one tenth of the blur from diffraction. This increases my second formula by a factor of 10.

Note that simply multiplying the focal length by a number, and ignoring the aperture setting, is doomed to be wrong. Good enough for goverment work, but wrong enough to bite you in the backside when you really need it.

My favourite definition came from Pete Andrews in a photo.net discussion:

Infinity is quite a lot further away than most people think. It's even further than the horizon, which is also quite far away. Try pacing out the distance to the horizon, and then multiply that distance by several.

Doremus Scudder
21-Oct-2005, 02:37
I never focus on infinity. It seems I am always focusing on something a bit closer, like the horizon or a distant tree... Following this logic, it would seem that, even for the most distant objects we photograph, the point of focus should be slightly less than infinity (in an ideal world with no optical aberrations, of course).

Seriously though, unless we are "guess focusing" or using stops that are set for a certain distance, why do we need to worry about infinity? I just make sure that what I want to be sharp is as sharp as I can get it.


Louie Powell
21-Oct-2005, 04:38
New Jersey

Pete Caluori
21-Oct-2005, 05:51
Thanks for catching that Paulr, I did indeed mean not conservative enough.

Regards, Pete

Walt Calahan
21-Oct-2005, 06:26
The best I can remember from intro to photography way back in 1975, the answer to the exam question was "the moon" is considered "optical" infinity.

mark anderson
23-Oct-2005, 07:57
after a long absence from computer forums it is good to see that intelegent conversation still is still out there, how far out there is another question ;-).

if the moon is to be considered infinity, why then must i refocuse when i turn my telescope to mars.

i think the true answer to the question was found many years ago on a back road in maine

"ya cant get thare from hair"

23-Oct-2005, 09:13
Seriously though, unless we are "guess focusing" or using stops that are set for a certain distance, why do we need to worry about infinity?

I'm messing with some aerial cameras that have fixed focus.

Walt Calahan
23-Oct-2005, 10:55
"optical infinity" for camera lenses and camera rangefinders, not telescopes.

Since I didn't think I was writing for an astronomy forum, I left off that qualitying clause.


Graeme Hird
23-Oct-2005, 18:51
Infinity is the point where two parallel threads meet.

I bet if you visit Photo.net's LF forum you would find a parallel thread there - when they converge we will have reached infinity ...


24-Oct-2005, 12:12
"if the moon is to be considered infinity, why then must i refocuse when i turn my telescope to mars."

do you really?

what's the focal length of the telescope?

mark anderson
24-Oct-2005, 14:33
mead 1000mm f-11