View Full Version : Finding infinity on a specific lens

22-Dec-2008, 13:31
Is there a simple formula for calculating where infinity in on any particular lens?
I know...the practical answer in look at the ground glass and focus...but is there a way, say to know if a 14" lens of a specific design would need x amount of feet of studio to hit infinity or hyperfocal distance?

22-Dec-2008, 15:08
Some wag is sure to inform you that infinity is, well infinite, so it might as well be me.

To be a little more helpful, from:

hyperfocal distance = focal length ^2 / (f-number * Circle of Confusion)

As a further aid, forum participant, Emmanuel Bigler helpfully suggested that the circle of confusion can be taken as equal to negative diagonal/1720 yielding:

hyperfocal distance = (1720 * focal length ^2) /(f-number * negative diagonal)

Be sure to keep your units consistent for focal length and negative diagonal and recognize that if you use millimeters, you should expect get a very large number under most conditions.

Peter K
22-Dec-2008, 15:33
As a rule of thumb infinity of a specific photographic lens is

focal lenght x 300

But for measuring the focal-lenght the use of a collimator telescope is the best way.

Peter K

22-Dec-2008, 15:40
Thanks, maybe it's the wrong way to ask the question...It looks like I get the award to the dumb question of the day!

The hyperfocal distance formula is very helpful, thanks.

It would seem that hyperfocal distance is what I should have asked for since what I need to know is the minimum distance in focus (given an fstop) when a lens is focused to infinity.

Sorry for the bass-ackwards way of asking!

22-Dec-2008, 15:45
Thanks Peter!
I was hoping I could find a quick general rule to use to get a basic idea. I'll still calc the h-focal to get accuracy where I need it.

22-Dec-2008, 18:19
You may want to try a free program that I wrote. You can use it with any film size camera to determine DOF, and you can generate hyperfocal charts. You may find it useful. It is WinXP based. There is also a PocketPC version.

Talbot.DOF (http://gogebic-pc.home.comcast.net/~gogebic-pc/dofcalc/win/)


23-Dec-2008, 06:16
I know...the practical answer in look at the ground glass and focus...
Not so easy. Can be done at night, aiming at a far-away street-lamp.

Alan Davenport
23-Dec-2008, 07:59
As a rule of thumb infinity of a specific photographic lens is

focal lenght x 300

I'm sure we could argue about this all day, so I'll only mention it once:

I learned (somewhere in the journey) to use 1500 X FL for infinity.

Probably the important thing is to realize that "infinity" is not as unimaginable as some might think. There is a point, in this case a distance from the lens, at which focus is indistiguishable from that obtained at any longer distance. Whether it's 300 or 1500 times the focal length, we can discuss over brews sometime.

Maybe it depends on the size of your thumb...

Emmanuel BIGLER
23-Dec-2008, 09:07
Hello good friends of the Hyperfocal Club !

If I may add something, I confess that I have always found irritating that the actual, practical infinity setting or the depth of field (DOF) scales are not intrinsic to a given lens.

In fact when I started photography, some smart engineers in Germany had computed for me the distance settings on my [OFF-TOPIC !!] beloved 35 mm Bessamatic.
So I was convinced that the DOF scales were proper to the 50mm focal length of my Skopar(TM) (how could THEY engrave something wrong ?)
... too bad ! when you change formats, everything seems to become fuzzy regarding DOF, and one of the most disappointing thing you eventually discover using large format cameras is: instead or relying on the science of austere German engineers, you HAVE to re-compute your DOF scales & hyperfocal distances yourselves all the time, according to your application !

I have a nice example about the fact that the hyperfocal distance depends on the final quality, sharpness criterion and conditions of display of your final, enlarged or contact print.

A French cinematographer & photographer, Raymond Depardon, is now working on contract with the different local/provincial governments of France to document the country as he likes it.
He has chosen to work with a 8x10" view camera to do the job and part of the work (still in progress) has been displayed in some parts of France where the job was completed, including the place where I live, Franche-Comté and the Jura.
The prints I've seen were displayed as huge outdoors panels, size about 2mx3m (about 7x10 feet)
When you look at the print at a distance equal to the diagonal of the image, namely about 3.6m (12 feet) all prints seem incredibly sharp from the foreground to the background.
Raymond Depardon did not use a wide angle lens, apparently he used a standard lens with not many visible "Scheimpflug aids"
When you come closer to the print, you clearly see that the depth of field is not unlimited but that, as usual, the focus has been made on the main subject; but the background that looked perfectly sharp from 3.6m away is not exactly sharp, as expected when you use any regular camera with no tilts and no Scheimpflug (or no digital combination of frames)

So the question : "What was Raymond Depardon's hyperfocal distance ?" is perfectly irrelevant since it depends whether you stay quietly at a viewing distance equal to the diagonal of the (huge) prints, hence the f/1720 rule is meaningful to compute your circle of confusion, or you choose to get closer and closer, and then the previous formula becomes grossly wrong...

One of the joys of high-resolution, large format photography is that not only you ARE allowed to get closer and closer, but that you HAVE to get closer in order to really enjoy all the fine incredible details that can be recorded on a 8x10" negative !!

Ole Tjugen
23-Dec-2008, 10:10
Personally I've just about stopped using hyperfocal settings. After all, hyperfocal means that the nearest point will be just as sharp as the distant horizon, and both will be just at the limit of "acceptably sharp" as you have defined it. That again means that both will be just on the borderline between "sharp enough" and "too unsharp".

I find that it is generally much better to focus on what is important, and then stop down enough that the rest is as sharp or unsharp as you want.

See H. M. Merklinger's "The Ins and Outs of Focus" for more on this - it's available online as a download.

23-Dec-2008, 11:51
Emmanuel - Thanks for your clarification about the importance print size and viewing distance in determining Circle of Confusion. I knew that I had slid right over those subtleties. As for those "Off-Topic" cameras and their DoF scales; I find them inadequate unless I apply a (typically two stop) correction, reducing their assumed CoC to one that serves my purposes.

Ole - Thank you for your answer to my query yesterday about the meaning of "Doppleanastigmat". You had great information as usual.