Anybody Willing to Further my Understanding of Optics?

[alexniedra]

Hey everybody...



Over the last little while, I've been increasingly interested in the science behind lenses and light. But, I won't be officially studying light until grade 11 physics next year[].


So, is anybody (*Cough* Daniel Browning[]) willing to help me out with the following topics?

• Understanding f-stop: I understand DOF, etc. I also know that F= f/d, where F is the effective F number, f is the focal length, and d is the aperture diameter. Can anybody help me dig a little deeper?
• Focal Length. What is the true definition of focal length - In terms of the optics?
• Light in general: What happens to light when it encounters a solid, like glass or fluorite? It refracts. What does this really mean? What is the refractive index of a material?
• Focus: How can objects be brought into focus? Simple observations have found that the optical system is shifted further from the film/sensor to bring closer objects into focus, and vise versa.
• Any other technical knowledge to share?

I know I'm asking for a lot of information, but I'm simply interested. I've tried some other sites on the Internet covering these topics, and I have gained a very general understanding. I'm looking for some photographers who are willing to explain these topics in much more detail.


Thanks!


- Alex

[Benjamin]

If you want to learn about the physics behind photography, you'll absolutely love this site. Here it is, photography optics. It explains most of the basic optical properties involved in photography. I read it and I loved it! []


Have fun in your upcoming Physics 11, that was the first physics course I attended in Canada!

[Jon Ruyle]

I wish I knew more about optics. But I can help a just a little.

Focal Length. What is the true definition of focal length - In terms of the optics?

I'm not sure what you mean by "in terms of optics", but the focal length is the distance between the image plane and the optic when focused at infinity. Equivalently, it is the distance between the optic and the focal plane when the image is at infinity.

Light in general: What happens to light when it encounters a solid, like glass or fluorite? It refracts. What does this really mean? What is the refractive index of a material?

The refractive index of a medium is the ratio between the speed of light in a vacuum and the speed of light in the medium. Refraction refers to the bending of light (or other wave) when it changes speed. The extent to which it does this can be computed by snells law


Actually, refractive index is a function of wavelength (ie, for a given medium it is different for different wavelengths of light). Thus different wavelengths refract differently, which gives rise to chromatic abberation. Roughly speaking, exotic glasses like fluorite are good because the refractive index doesn't change much at different wavelengths, so you get less chromatic abberation.

Any other technical knowledge to share?

The thin lens formula is useful. It says 1/f = 1/a + 1/b where f is focal length, a is distance between the optic and the focal plane, and b is the distance between the optic and the image. From this simple formula you can conclude quite a lot. (For example, you can use it to compute the amount of extra magnification you get with extension tubes).

[Oren]

The guys here gave you some pretty good info (I'm gonna read Benjamin's site when I have time). I can't add much - at least there is not much that I can explain to you over here on the forum.


One thing I'd like to tell is, although correct, Jon Ruyle's explanation for what is focal length is just well... an explanation and not the definition of "focal length"... it's a simple fact about lenses that derive from the formula he was talking about - 1/f = 1/u + 1/v (I use u and v instead of a and b since that's how we used to call it in physics class, but it doesn't really matter).


So as for focal length definition, you might want to check out: Focal Length

[Oren]

Take a look at the picture on the right side in the wiki page I gave you - the first lens in that picture is a convex lens and thus, rays of light coming in parallel to the optical axis (= the black dotted line in that picture), *after* the lens will pass exactly through the focal point - "F" in that picture. "F" - the focal point is located exactly at distance "f" from the lens, where "f" is the focal length of the lens.

[alexniedra]

Thanks for the help, everybody!


I'm definetaly going to take a look at the pages you gave me.

[Daniel Browning]

Great answers everyone. References that I recommend:


Paul van Walree's website that Benjamin linked to.


Joseph James' essay on Equivalence


Technical Books on Photography by Harold M. Merklinger (free download)


My favorite DOF calculator


Dead tree books:

• Optics in Photography, by Rudolph K.
• Sowerby's dictionary of photography (1956).
• Optics, 4th ed., by Hecht.

Canon's Technology Hall (fun marketing stuff)


Panavision's Demystifying Digital Cameras (good for MTF, bad info on sensors)


Tao of Leica by Erwin Puts (e.g. optics-> lens testing)


Other links:


http://www.imatest.com/docs/sfr_chromatic.html


http://www.bobatkins.com/photography/technical/


http://doug.kerr.home.att.net/pumpkin/index.htm


http://www.normankoren.com/Tutorials/MTF.html


http://www.cs.duke.edu/~parr/photography/faq.html


http://www.wrotniak.net/photo/index.html


http://www.photoscene.com/sw/tour/inside.htm





That should be enough to get you started. I find optics fascinating and would love to become an expert some day.

[Jon Ruyle]

So as for focal length definition, you might want to check out: Focal Length

Not trying to be argumentitive, Oren- but I read the definition at the link you gave and it seemed to define focal length exactly as I did (for a thin lens, anyway). How would you define it?


(And I do agree that some special cases of the thin lens formula follow from the definition. I don't think that disqualifies it as a definition).

[Oren]

"The focal length of an optical system is a measure of how strongly it converges (focuses) or diverges (defocuses) light. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the pencil of rays more strongly, bringing them to a focus in a shorter distance."


This definition is true also when the object is NOT at infinity while yours only referred to objects at infinity.

[Jon Ruyle]

With respect Oren, I wouldn't call that a definition of focal length because it does not quantify it. That is, if you hand me a lens and hand me the above quote, I can't tell you what the focal length of the lens is. (I think you'll agree that from a definition of focal length, I should be able to figure out what the focal length of a lens is)


Later in the article they give something I would consider suitable as a definition of focal length of a thin convex lens: "For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens."


This is almost identical to the definition I gave.


Keep in mind, a definition of focal length does not have to describe how a lens behaves (so the fact that my definition says nothing about how a lens focuses on objects not at infinity is irrelevant).