Physics and Photography

Hi!
Just thought maybe an out of the ordinary post might be something to make you think...
I had not thiought about it much, in this sense that in 12th grade (1996-1997) I took a Physics class and was it extrememly hard, but I feel I got the most out of it moreso than the French 5 class I almost slept thru and gave like no effort and still walked out with an A.

Ok, we have these bits called index (indices) of refraction, or how light as we know it bends/changes speed (relative to speed of light at 186,000 miles per second) thru various media like water, a glass you drink out of, a plastic soft drink container, other translucent/transparen​t objects, and of course camera lenses. This is in contrast to diffraction which is what you see like the prism bit. I was really bored at work (which is nothing new) and was wondering what causes Canon L glass to be so insanely expensive, vs the regular Canon lenses. Enter indices of refraction...the cheaper glass diffracts light differently and make for a much broader spectrum of light. Let's use the prism, as to how white light spreads out under diffraction. Then enter Canon's budget line of lenses and will probably diffract light not too different from the prism, all things being equal. Note how wide it is and how the colours can appear almost blending or murky. This is a high incidence of refraction comapratively speaking. The L glass, on the other hand using UD or Flourite elements cut down on chromatic abberations which I understand to be much more visible in the results of budget lenses. By cutting the incidence of refraction way down, again comparatively speaking, you also cut down the chromatic abberations. It is also my understanding that it is almost impossible to eliminate them entirely, but SLD/UD/Flourite elements sure help a lot there. The result is the much better pics we know when we use L glass. There is also a sharpness that I recon comes from the much lower incidence of refraction, as the size of the incident spectrum is much tighter than the budget lens as well. I find the science end of photography fascinating. This is what I found out when going from budget lenses to L glass! This is the most simple and to the point explanation I can come up. Hope it explains it on a more understandable level.

Cheers from England,
Canuck

Bob G. wants me to ask: 'What's a Phusics class?'

Now, I'll read your post for real.

Tom

D'oh!!! Typo! Ok, I have corrected it. Please read it for real!

Cheers,
Canuck

I believe that's the Queen's english?

Cheers, mate! However, I don't think so! Such is life when posting at 0300 in the morning. Mega D'oh!!! I have a feeling I won't live this down any time soon.

Cheers, Canuck

Feedback anyone? 85 view and no real replies other than spelling corrections?

Hmmm...

It's a good read

But I was not a physics genius myself...

I read that similar thing somewhere when I wanted to know what makes L-glass so expensive. Good that are one of the few that applies knowledge gained in school to real-world situations

So to boil it down you're saying that L-Glass is really cool, is that correct?

I think you might be using the term "diffraction" when what you really mean is "dispersion." Dispersion is caused because the refractive index of a material depends on the wavelength of the light. For cameras, this is a problem because we need to focus (or bend) light of many wavelengths to a common point. The term "achromatic" describes compound lenses that are designed to limit the effects of chromatic aberrations. These compound lenses are made with several different-shaped simple lenses each with different indicies of refraction.

I don't see anything inherrently wrong with a high index of refraction...if every substance had an index of refraction of exactly 1.00 (as in a vacuum) then we would not be able to bend (and thus focus) light at all!

Certainly Canon L lenses are well engineered. However, the physics of the details is certainly beyond me!

No, I think what he's saying is that L-glass makes a crappy prism for making rainbows!

Hi!
Ilya:
L glass is very expensive but very cool. It has stepped up my ability to take pics another few levels! It amazes me the pics I'm taking these days. It is quite literally the difference between an ok pic and one that you could make money off of straight away type deal. You would be amazed by how many people notice the red ring around the lens and/or camera and enquire. Now when you mount it on the 10D w/ Big Ed you're talking serious pro looking kit. Not that it is or isn't. I really dislike the term prosumer. It pain sounds cheezy, much like nonalphanumeric digital cameras like CyberShot, Dimage, Elph, and Coolpix to name a few. Another point, yes the L glass would make for a really crappy prism as it isn't designed to spread out the light like a regular prism would. It would still happen but it would be only a fraction of the size. I also recon that it would be a much tighter colour rainbow that it produces.

Cheers,
Canuck

Furthermore, a prism makes a spectrum precisely because of dispersion.

Diffraction is something else altogether. It is the effect that rays of light will be bent slightly around an edge. A pinhole camera, for example, works because of diffraction, not refraction.

The L lenses all have some form of low-dispersion glass in them to minimize refractive chromatic aberration, as you point out. The original glass they use was flourite glass. The original FD 300mm/f2.8 Flourite lens was revoluationary in its day. But there were other manual-focus FD-mount lenses that were designated as L lenses that didn't use flourite glass. When the L designation became public and was put on the label, it designated only the use of special, low-dispersion glass. Thus, many non-L lenses are top-grade and intended for professional use, but don't use low-dispersion glass because they don't need it. The 50/1.4 is an example.

More recently, the L designation has become a value designation, such that Canon is now producing two distinct grades of L lenses. The 70-200/4, for example, uses low-dispersion glass (hence, the L), but it does not explore the boundaries of what is possible at that focal length the way the 70-200/2.8 does. The white lens shell does sell more lenses, though.

Rick "who is happy to see cheaper L lenses in slower speeds" Denney

I sure get it now. Still I wonder, is L glass the only answer when it comes to top quality question for Canon cameras?

Both refraction and diffraction produce dispersion. Dispersion is the change in direction of propogation. Refraction disperses the light due to the fact the index of refraction is a function of wavelength. Diffraction disperses the light due to interferance effects that vary with wavelength. Prisms, lenses, diamond rings and grandma's crystal knick knacks produce dispersed colors due to refraction (also rainbwos). The dots in the metal layer of a cdrom produce colors by diffrection. Diffractrion also causes spreading of the light when traveling past an edge, slit or through an aperture

A pinhole cameras work by narrowing the cone of light that reach the film. Because the hole has to be so small there is considerable diffraction that softens the image.

The maximum resolution of a lens is typically at about f/8 because when stopped down more, diffraction comes into play and spreads out the point response function of the system. When stopped down less, there is less diffraction, but the lens is not as well corrected and other abberations become large and spread out the point response function.

In order to get a well corrected lens that is very sharp at large apertures, you need to correct for both chromatic aberration and the geometric aberrations. If you stick with spherical surfaces you need a lot of individual lenses and glasses with the right combinations of indicies of refraction as a function of wavelength. Some of these glasses require some exotic materials to get the properties needed. Also, the larger the piece of glass the harder it is to produce a chunk that is free of bubbles, striations and other defects. Then there is the cost of the coatings.

What makes them so expensive?

1. lots of lenses
2. exotic materials in some of the glasses used
3. large diameter pieces of glass
4. sexy coatings

You can also go to aspherical surfaces to reduce the number of lenses, but aspheres are more expensive to produce than spherical surfaces, though some lenses use them.

There will be a quiz next week.

gsrossano:

Yes, I am well aware that there is a lot more to it, but wanted to keep it simple. This is why I left out bits like (index of refraction is a function of wavelength) and (In order to get a well corrected lens that is very sharp at large apertures, you need to correct for both chromatic aberration and the geometric aberrations. If you stick with spherical surfaces you need a lot of individual lenses and glasses with the right combinations of indicies of refraction as a function of wavelength. Some of these glasses require some exotic materials to get the properties needed. Also, the larger the piece of glass the harder it is to produce a chunk that is free of bubbles, striations and other defects. Then there is the cost of the coatings.)

(in parenthesis are cut and pasted from your post)

My goal for this was trying to explain it in the easiest way I could and make science of it fun. It is fascinating I must admit. BTW, I have been playing with lightwaves/radiowaves since age 15, 10+ years ago.

The end of the day I appreciate the input, really! My biggest problem is loosing people when I go off on tangents (heck cosines, cotangents, and sines too) describing very technical bits in great detail. It isn't that difficult a concept to understand, I think. I get told all the time that I'm off the wall smart and sometimes it really sucks! I just don't feel comfortable around dare I call them normal people, like at work. If I have something in common, that makes it a lot easier for me.
A lot of the stuff 'm interested in is really technical, even photography can be; ham (or amateur) radio is very technical; astrophotography is up there; history of the landmass known as the UK is. Just oh gee whiz info: the landmass that is comprised of England, Scotalnd and Wales is roughly the same size as New York State!
The British are usually amazed when I tell them that and kinda gives them an idea how big the US is. Kinda puts it into perspective from both sides of the Atlantic Ocean.

Cheers from England,
Canuck