# Lenses explainedMarch 28, 2004 2:58 PM   Subscribe

Camera Lenses are something i've never really understood, but should. This was the best stab at explaining I've ever read.
posted by mrben (8 comments total)

[this is good]
An excellent way of explaining something in simple terms without treating the reader like a child.
posted by dg at 3:34 PM on March 28, 2004

Yeah, that's pretty good. Along the same lines, I recommend this text to anyone who wants to get super-comfortbale with these concepts, and all the controls on an SLR camera. It's the best comprehensive guide to light, cameras, lenses, and film for beginnners I could imagine, and gets better with every edition.
posted by scarabic at 3:46 PM on March 28, 2004

When I was younger, I once focused (pun possibly intended) my wandering thoughts on how lenses worked. It seemed magical that they could somehow know where a light ray was coming from to point it to the right spot (with all of the others from the same source) onto the film/retina/etc. I bugged my mother and whoever else was around at the time, but no one could explain it for me. It would have helped to have had some concept of depth-of-focus/depth-of-field. When the lens is just being magical for light coming from a certain distance away, instead of all light, it's not *quite* so amazing. Considering the amount of effort that goes into lens design, well, I guess it should be.

I just now found a pretty good explanation of the relationship between depth-of-field and aperture size. And another. (Most that I could find simply stated the relationship for photographers but didn't give any explanation.)
posted by whatnotever at 4:08 PM on March 28, 2004

What I'm curious about is how the lens is able to refract the light at all. I understand the principles of refractional index, but how is slowed down when it enters the lens?
More importantly, how does it speed up when it leaves the lens?

If it is 'bounced' between the atoms of the glass, how come it is not refracted in completely arbitrary directions, rendering the object translucent rather than transparent?
posted by spazzm at 7:02 PM on March 28, 2004

spazzm: enter the wave/particle duality of light. Waves travel at different velocities when passing through different mediums - however since the 'particle' momentum is present, the wave will speed up once it encounters a medium of lesser density (I may not be entirely correct here, it's been a few years since I took my physics classes). This is an instance where thinking of light as a stream of photons is not useful.

Light is weird. But purty!
posted by blindcarboncopy at 7:29 PM on March 28, 2004

Light is weird..

Indeed. Slightly OT one my favorite light storys is how Newton figured out colors, a very simple experiment anyone can do but takes a genius to see the significance. At the time it was thought colors were created by mixing various amount of pure white light with darkness, white light is the only pure color everything else is a shade of white. Makes perfect sense and was the accepted doctrine for thousands of years.

Using a prism (ancient device nothing new) Newton broke white light into its rainbow of colors, then he put another prism in front of that and returned it back to white, in a dark room with no other source of ambient light. The conclusion: Pure white can not be created from in-pure "shades" of white (yellow, green, etc..) .. kind of like you cant breed a pure-bred dog from mutts. In fact, colors are the pure light and "white" is the combination of all colors combined.
posted by stbalbach at 9:18 PM on March 28, 2004

There is so much wrong with the text and at least one of the illustrations on that site. You'd be better off picking up a basic undergraduate non-calculus physics text to get a better idea of how lenses work.

As just one example:

Point A is a point on a well lit, brightly colored object. (In this case, a bright red chili pepper.) It gives off rays of red light in billions of different directions, but only one of those billions of rays travels from Point A to Point B. Because (in our illustration) only one of the billions of rays reaching Point B is red, there is no chance for a human observer to perceive Point B as having any sort of reddish color to it.

They neglect that there are billions of other red rays coming from other parts of the chili pepper and thus, you would perceive red light at the point B.

Just my \$.02.
posted by Qubit at 9:55 PM on March 28, 2004

blindcarboncopy:
Yes, but even if we choose to look at it as a wave phenomena would it still not be diffracted in all directions, instead of just one?
And I can't see how the wave/particle duality explains light speeding up after leaving a medium.

Can someone explain it to me, real simple like?
posted by spazzm at 12:33 AM on March 29, 2004

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