calculating infinity.

[scott russell]

I know that when a lens is at its focal length from the film plane, it is focused for infinity. IE and 90mm lens 90mms from the film plane. But how do you find out what the lens to object distance is when that lens is focused toward infinity? What got me thinking about this is the wide angle lenses on 35mm. Now a 16mm fisheye on 35mm will have everything in focus after 3 or 5 feet or something ridiculously short like that, so if the object is 5 feet from the lens, you just set the lens for that sideways 8 and your done focusing. how could i figure this out for a 90mm super angulon?

[sparq]

Are you asking about calculating hyperfocal distance?
Google is your best friend: http://www.google.com/search?client=...focal+distance

[Brian Ellis]

I know that when a lens is at its focal length from the film plane, it is focused for infinity. IE and 90mm lens 90mms from the film plane. But how do you find out what the lens to object distance is when that lens is focused toward infinity? What got me thinking about this is the wide angle lenses on 35mm. Now a 16mm fisheye on 35mm will have everything in focus after 3 or 5 feet or something ridiculously short like that, so if the object is 5 feet from the lens, you just set the lens for that sideways 8 and your done focusing. how could i figure this out for a 90mm super angulon?

"How do you find out what the lens to object distance is when the lens is focused toward infinity? There's probably a forumula that can tell you how to calculate distance to infinity but when I did some reading before setting the infinity stops on my Linhof Technika I read that focusing on something at least a mile away would put the lens position at infinity. That's probably not precise but it seemed good enough for my purposes so I went out to a highway, found a billboard, drove a mile, and focused on the billboard to set the infinity stops. I'm sure that's not as precise as could be done with the right optical equipment or by applying a mathematical formula but it seemed close enough for practical purposes.

"Now a 16mm fisheye on 35mm will have everything in focus after 3 or 5 feet or something ridiculously short like that, so if the object is 5 feet from the lens, you just set the lens for that sideways 8 and your done focusing. how could i figure this out for a 90mm super angulon?" I'm not sure what you're asking but just because you can focus on something five feet from the lens doesn't mean that you should set the lens to the infinity position for anything five or more feet away or that five feet is infinity. Are you asking how to determine the hyperfocal distance?

[aduncanson]

I think that the hyperfocal distance is the answer to the question posed by the original poster. Focus on an object at the hyperfocal distance, (given in http://www.largeformatphotography.in...IntroToDoF.pdf ) and everything from half that distance to infinity is in focus. (ignoring swing & tilt and who knows what subtleties.)

The question of how distant a subject you need for setting infinity stops is different. I was a little surprised by the 1 mile figure because I recall reading that for lens testing, infinity was taken as anything greater that 43 focal lengths. Evidently that is a very different consideration.

For setting infinity stops, the objective would seem to be to allow an error in the standard's position which is small compared to the allowable focus error per depth of field considerations (less than 5 to 10%.) That allowable error would depend, of course, on all of the usual suspects including: focal length, aperture, degree of print magnification and ones personal standard for sharpness. Making a variety of assumptions, I get numbers between 0.5 and 2.55 miles for a 135mm lens and 1.2 and 5.7 miles for a 210.

I generally just use the moon.

[Peter K]

For the Super-Angulon 1:5.6/90mm the distance at infinity between lensboard and film is 100.6mm, for the f/8 SA it's 99.4mm. So from infinity up to the distance the circle of confusion has a certain dimension is "in focus". This depends of the size the final image will be seen.

[Leonard Evens]

When you are focused at infinity the distance to the object is infinite. You should get that straight before you do anything else, because otherwise you are going to remain hopeless confused. A object would be at infinity if it were further away than any real object can be. There do not exist such objects in the world we see around us, os it is best treated as a mathematical abstraction.

As I said, no real object is at infinity, but if there were such an object it would come to focus in the focal plane. Any object at great distance will come to focus very close to the focal plane, and if it is far enough away, it will come to focus so close to the focal plane that you won't be able to tell the difference. In that case, you could say that the object was `at infinity'. But note that ANY point sufficiently far way can be considered to be at infinity in this sense, so there is still no single distance that can be considered to be at infinity.

Just how far away a point has to be before you consider it effectively at infinity depends on the focal length of the lens and how closely you need to look. Generally shorter focal length lenses are effectively focused at infinity for shorter distances than longer focal length lenses.

A related but different concept, mentioned by others above, is that of the hyperfocal distance. Whenever you focus, there is a certain range called the depth of field which you would consider adequately in focus. This, of course, depends on how closely you look. Someone looking at a 40 x 50 print from five feet will consider much more adequately in focus than someone putting his nose right up against the print. But once you set your standard for what is sharp enough, you can calculate, from formulas, or using a DOF calculator, the nearest and furthest distance adequately in focus. Thus there will be a certain amount adequately in focus in front of where you focus---called the front DOF---and a certain amount in back of where your focus---called the rear DOF. At a certain distance and beyond, the rear DOF will be infinite. That means that everything in back of where you focus will be adequately in focus. The shortest distance at which this occurs is called the hyperfocal distance. When you hare focused at the hyperfocal distance, everything from that distance to half that distance in front will be in focus and everything in back will be in focus.

Focusing at the hyperfocal distance is not the same as focusing at infinity. If you had a lot of fine detail at great distance from the lens, and you don't much care about what is in focus in the foreground, you should focus at infinity, not at the hyperfocal distance.

The hyperfocal distance depends on the focal length, the f-number, and something called the coc, which is a measure of how closely you will look at a print of the scene. There are lots of DOF calculators on the web which allow you to enter those parameters and tell you hat the hyperfocal distance is.

[Alan Davenport]

"Infinity" for photographic lenses is 1500 X the focal length.

[Leonard Evens]

"Infinity" for photographic lenses is 1500 X the focal length.

It's always nice to have a definite number, but how many of us are going to calculate 1500 x the focal length in units we are familiar with, and then measure out that distance in the scene. Also, as I tried to explain above, there is no one distance which is effectively `at infinity'. It all depends on close you have to be to the focal plane before you can no longer tell the difference. That in turn depends on the focal length and how closely you look as well a what you are trying to accomplish.

Let me give a simple method which anyone can use without making distance measurements. This would apply when you really do want to focus `at infinity' rather than just having infinity adequately in focus, in which case you would use the hyperfocal distance.

Focus on two or three very distant points. See if there is any focus spread on the rail among them. If within the accuracy that you can focus they are all at the same place, then you are focused for all practical purposes at infinity. You won't do better than this by measuring out 1500 X the focal length or 750 X the focal length or 3000 X the focal length or whatever.

[Glenn Thoreson]

Very complicated stuff. I can tell you this much, if it helps - the average hyperfocal distance for a 90mm lens at f/11 will be 19 feet. That's how I set the focus on my home made fixed focus box cameras. At f/11 it covers everything from roughly 9 1/2 feet to infinity and gets better as it's stop down. Is infinity a set distance, or just something far enough away that focus will reach no farther? I don't know if that's the right way to put the question. I don't understand the math related to this.

[Vaughn]

I'm with you Glenn. We need Buzz Lightyear...anyone who can go "to Infinity...and Beyond!" must be able to calculate infinity, too.

""Infinity" for photographic lenses is 1500 X the focal length." Whoa! That means infinity for a 150mm (6" or .5 foot) lens is 750 feet. A 50 mm (3") lens would be 250 feet -- but on lenses it is usually marked at about 60 feet. Seems a bit much.

Vaughn