Extremely Dumb Questions about Lens Systems (Specially Cooke triplet)

[fred_radmehr]

Hi everybody and sorry for initiating such a ridiculously beginner level thread.

I am confused after reading some documentation regarding lens systems as I have very poor knowledge and background.

Here they are:

1) What is the meaning of the focal length of the system?
2) what are the field angles and why are there 3 separate value in some papers and helps (e.g. 0,10,20)?
3) Why is wavelength mentioned and there are 3 separate sets of rays focusing differently (mainly in red, green and blue colors)? does that mean the image will have different focus for different colors of the source?
4)what is paraxial F/#?

Thanks in advance for any helps and responces.

[erian]

These are good questions but as I am not knowledgeable enough to answer precisely then will wait till somebody else will address these.

I understand that you are investigating some optical schematics. Would it be possible for you to post them here (or the link to them)?

[Nodda Duma]

1) focal length is how far from the film the lens has to be held to produce an image that’s in focus. It is a design value, and indicates how wide a view the lens projects onto the film.
2) field angles : imagine looking straight ahead, 10 degrees off to the right or left is a 10 degree field angle.
3) yes, for shorthand the lens is analyzed at a few discrete wavelengths which are chosen to indicate performance across the full range of wavelengths it is designed for.
4) f/# calculated on-axis ie straight ahead. The calculation is easier and representative of the f/# over much of the scene.

[Jim Noel]

#1 answer is not quite correct.
Focal length is the distance from a point (usually the middle of the lens, or close to it) to the surface of the film or ground glass when the lens is sharply focused at infinity (or a very distant object.

[Nodda Duma]

It is correct: a super non-technical variant of the most generic “inverse of optical power” definition for the benefit of the OP while tying it in to his other q’s. “It is a design value” is key. Your definition is the more specific effective focal length. Either one is fine tho.

[fred_radmehr]

Thanks everybody for your reply.
I still did not get Jim Noel's point and the answer 4 from Nodda Duma. (Would have been great if clarifies more).

Sorry guys but I might be super noob to ask this. If the focusing from physics means that rays gather at one point then does not it mean that the image will be only one super hot point? I am seriously confused at this.

Another point that concerns me badly now is that I am trying to work on lens for projectors and the answers you give me are mainly for camera lenses. How much of a difference does it make about this and that?

[Dan Fromm]

Fred, a lens is a lens is a lens. Lenses for projectors, like taking lenses for cameras, map a point in the subject plane (in front of the lens) to a point in the image plane (behind the lens). It is point to point, not everything to one point.

You may be thinking of the days when you used a simple biconvex lens to map an image of the sun on to an innocent little ant.

[gnd2]

Sorry guys but I might be super noob to ask this. If the focusing from physics means that rays gather at one point then does not it mean that the image will be only one super hot point? I am seriously confused at this.

Your questions don't really have simple answers without a fairly in-depth understanding of physics. The thing about physics is that the details are important. "rays gather at one point" is ambiguous and leaves out important details.

It's not ALL rays that gather at one point. All rays *parallel to the lens axis* will pass through the *lens focal point*. This is what happens when you use a "magnifying glass" to focus the suns rays to a single point and start a fire.

If you take a single point of an object you're photographing, it's rays aren't parallel, they radiate out in all directions. The rays from that one point that reach the lens will then converge at a single point, and the rays from another point on the object will converge at a single point, but those two convergence points will be different points that aren't the lens focal point (unless one of those points happens to be on the lens axis and very far away, then that point will converge very close to the lens focal point).

https://www.physicsclassroom.com/cla...s-Ray-Diagrams

To expand on #3 a little more:
the path of a light wave bends when it transitions between two different media (between air and glass in this context). How much it bends depends on the wavelength of the light wave (among other things). So different colors of light will bend to different degrees, which means yes, the focal point for different colors of light will be different. This is what leads to chromatic aberration. More advanced lens designs include design features to correct this problem. The achromatic doublet was one of the first ways to correct for this (at least partially) https://en.wikipedia.org/wiki/Doublet_(lens)