How does hyperfocal distance relate to a simple lens-and-screen experiment?

Asked 5/20/2020

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In a simple convex-lens experiment with an object, a lens, and a screen, can hyperfocal distance be demonstrated or measured this way? I know hyperfocal distance is often given by H = f^2/(N c) + f. If a lens is focused at the hyperfocal distance, does everything from about half of H to infinity appear acceptably sharp? How do the object distance, lens position, and screen/image size affect this?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

6y ago

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"Hyperfocal" refers to the condition where depth of field allows the lens to be "in focus" from some minimum distance to infinity.

This depends on a core assumption: the size of the acceptable "circle of confusion," which is determined by the actual aperture diameter and lens focal length, but also by the amount of enlargement the image will receive before final viewing. That is, a 250 mm lens at f/11 will have greater depth of field on an 8x10 negative than it will on an APS-C or Micro 4/3 digital sensor, because the 8x10 is likely to be viewed as a contact print, while the crope sensor image will be enlarged at least to screen viewing size (around 20:1, give or take).

Once this acceptable circle of confusion is determined, it's a fairly simple calculation to determine how far in front of or behind the plane of critical focus objects can be and still produce images with this size or smaller circles of confusion. This takes the form of a table or graph that is condensed into a depth of field marking on a lens or focusing rack.

Finally, one can then set a focus that, at the specified aperture setting, will produced "in focus" depth of field that just extends to infinity, and generally to half the set focus distance (many old box cameras were actually focused at about 10 feet, but had aperture that made them hyperfocal, so they'd say "five feet to infinity" or "place your subject at least five feet from the camera").

It's important to remember that "depth of field" isn't a physical condition or quality -- it's a measure of how much defocus the user is willing to accept. If you scan an 8x10 negative that looks razor sharp all over to the eye at high enough resolution and examine it at 1:1 on a good monitor, you'll find that even at f/32 there's still a plane of critical focus, and everything not in that plane is at least a little bit fuzzy -- but if the fuzziness isn't visible in normal viewing, we consider that "within depth of field," and if that depth of field extends just to infinity, the setup was hyperfocal.

Originally by user89902. Source · Licensed CC BY-SA 4.0

user89902

6y ago

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Hyperfocal distance is not mainly about one exact object-lens-screen spacing in a simple school optics setup. It is a depth-of-field concept: when a lens is focused at the hyperfocal distance, the acceptable sharpness extends from roughly half that distance to infinity.

The formula H = f^2/(N c) + f is the standard approximation, where f is focal length, N is f-number, and c is the acceptable circle of confusion.

The key issue is that “acceptable sharpness” depends on the circle of confusion, which in turn depends on how large the image will be viewed or enlarged. In a small lens-and-screen experiment, the projected image is often too small for the eye to reliably distinguish slight blur from perfect focus, so it is not a very good way to demonstrate hyperfocal distance in a meaningful visual way.

So yes, focusing at H gives acceptable sharpness from about H/2 to infinity, but only relative to the chosen circle of confusion and viewing conditions. The simple screen setup can illustrate image formation, but it does not explain hyperfocal distance very well unless image size and viewing criteria are carefully defined.

UniqueBot

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6y ago

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