Eyes and cameras - a topic that has been troubling me

[jt7747]

Hi all. This is my first post. I'm seriously thinking about applying to optometric college in the fall. I'm trying to improve my knowledge as much as possible of the area, and one question that keeps coming up (I'm a hobby photographer...) is the difference between the eye and a camera.

Most photographers appreciate that the camera/film eye/retina analogy is only so good, and soon becomes impossible to reconcile with the influence of the brain, not to mention peripheral and foveal vision.

But the problem I'm having is to refute those photographers who blindly state that the eye = a 50 mm lens. I think this statement is nonsense, but I'd appreciate your help to clarify my thinking and back up my arguments.

First, a 50mm lens on a 35mm film camera covers a horizontal field of view of 39.6 degrees. A single human eye, on the other hand, from what I've read gives a field of view of 60 degrees inward, and 100 degrees outward. Though much of that is blocked by my nose. Anyway, the point is that thought of as a lens, the eye's field of vision is much wider than that of a 50mm lens.

I also gather that the foveal central field subtends around 5 degrees (I'm not sure if this is horizontal, vertical, or a solid cone angle), although the central 1/2 degree is the clearest.

So we have two competing views: one is that the human eye has a wide field of view, and the other that we only see a very small part of that clearly.

But neither view helps me explain to friends why the 50mm lens isn't going to replicate human vision: I don't think any lens can replicate human vision. Sure, 50mm lenses normally force people to take pictures (especially portraits) from sufficient distance that they look natural, but that still doesn't explain this obsession with 50 degrees. Furthermore, a 50mm lens on a 35mm SLR, when viewed through the viewfinder, projects an image with the same magnification as we see with our eyes, but this says more about the viewfinder magnification that some inherent property of the 50mm lens.

So you can see why I'm confused, and I would just LOVE to learn a bit more. If anyone has any accurate figures for focal length and viewing angles, and if they can help me understand this whole foveal/peripheral business, that would help me a lot.

Thank you!

[drk]

Dude, you'd make one great optometry student!

That's physiological optics. Most of what you discuss is valid, but very technical and esoteric. You'd have to talk to someone at a GOOD college of Optometry to get that kind of discussion.

[drk]

http://www.luminous-landscape.com/co...e-camera.shtml

http://www.hitl.washington.edu/publi...dwell/ch2.html

Does this help?

[braheem24]

You're comparing an inverted sphere for a retina to a flat cmos/ccd chip. DOF, VF, focal point all vary based on the shapes and distances from the initial refraction point (cornea/50mm lens).

[jt7747]

Thanks folks. Yes, the curved retina is really a spanner in the works. It must increase the visual field, otherwise our retinas would have to cover a greater area if they were flat.

What I really don't understand is what would happen if our eyes were more wide angle or more telephoto than they currently are. Certainly, our field of view would increase or decrease, respectively, but I wonder if our brains would still assemble a similar picture of the world to what we have now.

Is the retina's surface truly spherical, or is it slightly flattened? Also, how does the optic nerve allow the eye to swivel freely? There must be some slack in the system, a bit like the wires connected to an inkjet printed head.

Sorry for the banal questions

[braheem24]

PS...

The viewfinder is used as a focal point to view the image and is setup with an adjustable vertex distances to compensate for the user's eyes. "Zoom" requires a minimum of 2 lenses, the viewfinder focuses only.

[jt7747]

PS...

The viewfinder is used as a focal point to view the image and is setup with an adjustable vertex distances to compensate for the user's eyes. "Zoom" requires a minimum of 2 lenses, the viewfinder focuses only.

Really? Then how come different cameras have different viewfinder magnifications, even for the same film type? I'd submit there's more than just focussing going on inside the prism of an SLR viewfinder. Some definitely offer more magnification than others. Old cameras used to have interchangeable viewfinders and these would definitely alter the magnification, and hence the relative size of objects in the viewfinder.

[braheem24]

Below is a typical SLR, the viwefinder is a 1 piece convex lens, no matter the power on the lens or the vertex of the lens from the operator it does not effect the image on the film/cmos/ccd. It acts much like the focus ring on a prime lens.

[jt7747]

I'm sorry, but look at any camera manufacturer's site and you will see specifications for viewfinder coverage and magnification. See also here http://www.luminous-landscape.com/co...03-03-16.shtml

Bigger better cameras have bigger heavier prisms, and these give you better magnification. Cheap DSLRs give you small magnification viewfinders, well described on this page http://www.dansdata.com/20d_conf.htm :

"Viewfinder magnification is more important. Magnification tells you how big the image is. High magnification means a big image, like a movie screen viewed from a good seat; low magnification means an image like a distant postage stamp surrounded by a sea of blackness."

Viewfinder magnification is NOT fixed by the film format, otherwise all 35mm cameras would all have identical viewfinders. This is simply not the case

[braheem24]

Viewfinders are varied by the last mirror/prism image size, distance from mirror/prism to the eyepiece and the manufacturer's eyepiece aperature.

The magnification factor is relative to the focal point from the last mirror/prism to the eyepiece, if a manufacturer chooses to use a shallower focal length for better central detail, doing so is not truly zooming the picture it is much like a digital zoom where it crops the outer edges instead.

Magnified viewfinder image does not interpret to a magnified ccd/cmos image.

[jt7747]

Exactly! Viewfinder has no impact on the CCD. I never said it did. But don't you see....the very fact that some viewfinders have more or less magnification than others totally invalidates the oft performed experiment where people look through the viewfinder with one eye, and at the real world with the other eye, and adjust their zoom until both images appear the same size. Some people use this as a reason for the eye being the same as a 50mm lens.

But if we accept that some prisms are designed for different magnification than others, then it's evident that any kind of experiment based on looking through a viewfinder is totally flawed. Because some magnify more than others. This is the whole point I'm trying to make.

The prism has no bearing on the lens to sensor path, I don't recall expressing that view, but apologies if it came across that way.

Dyou see what I mean, about the 'looking through the viewfinder' methodology being totally, 100% flawed?

[braheem24]

I'm in agreement with you there, keep in mind prisms do not magnify or minify they only deviate light passing through a lens much like a mirror reflecting light based on the angle of incidence.

[jt7747]

That's what I don't understand, where the magnification comes from. I know nothing about lenses so that doesn't help. So the prism just relfects, so its job is to turn the light round the corner and invert it the right way up. That makes sense.

Ignoring the fact that the bigger the viewfinder, the dimmer it will be...if you want to see a huge image when you look through the viewfinder you're going to need a huge prism. These are heavy and expensive, and I guess that's why modern SLR's have such pathetic viewfinders. The finder on my old Pentax MEsuper is a joy to look through, everything is so bright and vivid.

[Thomas]

A 50mm lens is said to be "normal" because of the fact it doesn't change the magnification any. That is the only way it is like our own eyes.
The field of view is much changed because of the end needs of the image produced in the mechanisms of the camera system.
You could apply a wide angle lens to achieve the same viewing angle (as our eyes) but however you would change the focal length and thus the apparent size of objects viewed.
View finders are just that, one can get any range of adapters and put on the view finder to gain the desired results.
These do not have any effect on the image at the film/CCD plane.
You could also opt for a system with a High Eye Relief style view finder, like the one on my old Nikon F3.
just my .02

[DC Optix]

I'm really enjoying this...keep up the great discussion!