Hyperfocal distance v. far-field

[bigtrueba]

Hi, in class we were talking about lenses and how there is a certain focal length where all images 'far enough' away are in focus. I was doing some wikipedia searching and found the hyperfocal distance. Is that what this is?

I was also reading somewhere about near/far-field. from what i understand the far field is when light comes in perfectly parallel, but this usually takes a very long distance (light years). These two phenomena are not related are they? (hyperfocal distance and the far field) i mean, the idea behind the hyperfocal distance is not the angle at which the light meets the lens, correct?

 

[Andy Resnick]

The idea of 'depth of focus' (or depth of field) is intuitively obvious, but quantitatively, it's very difficult to express. One concept that is important to understand is that your eye has a limit on what it can resolve- anything smaller than this 'circle of confusion', 'blur circle', or what have you, is considered indistinguishable from a perfect point.

The hyperfocal distance is found by setting the depth of focus to a maximum- a lens that is focused on an object located at this distance H- say, 16 feet- will produce an image with everything in focus from H/2 to infinity. The hyperfocal distance is a function of the focal length and the f-number, and there's a lot of online depth of field calculators that can help you understand the concept better.

Near and far field is totally different- far-field simply means that certain mathematical approximations (Fraunhofer region) to diffraction hold and starts around 10*wavelength, while near-field is (generally) the region within 1 or 2 wavelengths of the source/object. The near field has both propagating and nonpropagating (evanescent) components to the electromagnetic field, while the far-field has only propagating modes.