I'm having difficulty find any info on determining where infinity focus begins. I'm assuming there's a formula involving focal length and... Can anyone help?
Answer
The key to this answer is to first settle on the tolerable size of the circle of confusion. Most depth-of-field tables set this value at 1/1000 of the focal length. Kodak, for critical work set this value a 1/1750 of the focal length. For our purposes, we will use 1/1000. Thus if a 50mm lens is mounted the permissible size of the C of C is 0.05mm. This size will tolerate a 10X enlargement viewed from standard reading distance.
Now most depth-of-field on line calculators can be played with to discover what subject distance for a given focal length set to a given aperture will just kiss infinity. If you play with them, imputing different values, you will discover that infinity is about 4000 times the working diameter of the lens for a C of C of 0.05mm. So a 50mm lens is mounted and set to f/2. The working lens dimeter is 50 ÷ 2 = 25mm. Using 4000 times the working diameter, infinity is 4000 X 25 = 100,000mm = 100 meters = 3,937 inches = 328 feet.
The same 50mm set to f/8 has a working diameter of 50 ÷ 8 = 6.25mm. Using the 4000 rule of thumb, infinity is 25,000mm distant = 25 meters = 984 inches = 82 feet,
Let me add that I think the value of 4000 X the working diameter is too stringent. I think that 3000 X or somewhere in-between 4000 and 3000 X the working diameter is the more practical value. However, those that know me,, know I am overloaded with gobbledygook.
P.S. Infinity for an optical system is defined as that distance from the camera whereby the light rays from the subject arrive as parallel rays.
Hyper focal distance formula using 1/1000 of the focal length as C of C diameter answer in feet.
Focal length ÷ f-number X 0.033 X 1000
Lens is 100mm focal length operating at f/11
100 ÷ 11 X 0.0033 X 1000 = 30 feet
The hyperfocal distance is simply the working diameter of the lens multiplied by the agreed upon diameter of the circle of confusion. Thus if a 200mm lens is used set to f/8, the working diameter is 200 ÷ 8 = 25mm. If the circle of confusion is 1/1000 of the focal length, then 25mm X 1000 = 25,000mm, this is hyperfocal distance. 25,000mm X 0.0033 = 82 ½ feet. It’s as simple as that!
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