Where on the infinity sign does infinity start on the distance mark?

[Uhooru]

And to add, the "limit of resolution for the human eye". This sounds like its come from a study of "human factors" and is the practical important point to me. But DoF is looking at the point that the average human eye will perceive something to be in focus. Read London and Upton's "Photography" pg 50 of the 6th edition. A pretty well used and regarded college photography textbook. They don't use the 8 x 10 print test, they just talk about the sharpest focus is the area with the smallest CoCs.

[Tony Parsons]

Can I just ask - except for the Moon and Stars, when is true infinity focus really necessary ? I'm short-sighted (I'd say Myopic if I could spell it), so wear strong distance glasses (stronger than the optician suggested, as I do a lot of birding, and want to be able to see birds in the distance). Even with these, items such as the wind farms off the Norfolk coast (I thing between three and five miles away) seem somewhat fuzzy when I look at them, and to me, to see them in sharp focus would seem unnatural. Anyone any thoughts ?

[Ed_Ingold]

Should I have said "uncertainty," which is what I think of in terms of mathematics. For the present conversation, "circle of confusion" is a valid term for OOF conditions, and more attuned to the vernacular of photography.

 

What is photographic infinity? Technically, it is when the rear node of the lens is a distance from the image plane equal to the focal length. It gets tricky when the lens focuses internally by altering the focal length. In practical usage, you can see when distant objects are in and out of focus as you adjust the lens in magnified live view. In terms of distance, it might be 1000 meters, less for short lenses, more for super-telephotos. The image can't be sharper than atmospheric conditions allow, and most lenses aren't sharp at 12x magnification. There is a point which when crossed, the image sharpness is at a maximum.

 

Objects nearer or further from the plane of sharpest focus will be projected on the image plane as sections of a cone. When the diameter of that section exceeds about 0.02 mm on a FF sensor, it is considered to be at the limits of acceptable focus.

Edited February 20, 2019 by Ed_Ingold