Not that any of this matters, but the human eye does not change focal length. It changes the focal plane, providing a choice between near or far objects being in focus.

Prime lenses (by definition, a lens that has a fixed focal length) are similar to the eye. They have the ability to change the focal plane, but not the focal length. The eye does this by squeezing the lens, whereas the camera moves lens elements slightly.

A zoom lens is one that has both a variable focal plane and a variable focal length. Zoom lenses provide the ability to change the field of view, or one might think of it as the magnification.

Human eyes cannot change the magnification or field of view, because they are fixed focal length.

Both human eyes and camera lenses can adjust the aperture - the size of the opening letting light through to the sensor. In the eye this is the iris. In a camera, this is the diaphragm. While the mechanisms differ, the effect is identical.

It is an inescapable fact that any lens-based system cannot focus all things at all distances simultaneously, except when the aperture is a pinhole. This includes the human eye. The focal field of a camera is supposed to be a flat plane, but sometimes is slightly curved, resulting in what is called field curvature. The human retina, being a curved surface, produces a decidedly curved focal field.

The depth of the focal field (range of distances that are in focus) is a function of both the focal length and the aperture. Wider apertures produce a narrower depth of field.

Despite what kawecki seems to be saying, the human eye does not simultaneously keep both near and far things in focus, because it too has a limited depth of field, just like a camera does. In bright light, the aperture is small so the depth of field increases. This means that in bright light you will perceive more objects in focus simultaneously. In dim light, the aperture increases to let in more light, resulting in a narrower depth of field. The same is true with cameras.

In camera lens specifications, we talk about the maximum aperture, which defines the ability to collect light. We talk about how "fast" a lens is. Aperture is given as an f/stop, such as f/2.8 or f/5.6 or f/8. Lower numbers mean more light is collected. Consumer grade zoom lenses are usually no better than f/4 and often limited to f/5.6 at longer focal lengths. Pro-grade zoom lenses go down to f/2.8. Consumer-grade prime lenses, those that don't have to offer variable focal length, can be easily f/2.8, while pro-grade primes go down to f/1.4 (4 times faster). Some really exotic ones go to f/1.2 or even f/1.

The human eye is a prime lens and has a maximum aperture of f/3.5. That's not even as good as my cheapest prime lens, or my pocket camera. The human eye sucks at collecting light.

The limited depth of field at f/3.5 is not particularly narrow but it is noticeable. In bright daylight, when the human eye is around f/16, most everything is in focus, but that is true of cameras as well.

In my camera bag, I have a 50mm f/1.8 prime - very fast, very sharp, very inexpensive. I have an 18-105 zoom with aperture f/3.5-5.6 - nice focal range, but not a great light collector. I also have a 17-50 f/2.8 constant aperture zoom. Not so great range, but a great light collector and I don't have to worry about the aperture changing when I use the zoom. That's my favorite lens and it is far far superior to the human eye.

Having said all that, the camera model in Poser is impossibly awesome at focusing. Everything is in focus no matter the distance. Poser has to do extra work to pretend to be a lens-based camera and produce a narrow depth of field - an option I never use because it is so computationally expensive, and not very effective at mimicing what a camera does.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)