MDJAK wrote in post #2449845
So if I understand all this gobblydigook (j/k) no matter how large you made the front element of the lens, the field of view would not increase if the sensor size remains static.
In other words, if the front element of, say, a 600mm lens was twice as "round," what would that do to it?
mark
Assuming you increased the size of the aperture to match, it would make the lens faster (give it a smaller f/number). If you didn't increase the size of the aperture, it would do nothing except make the lens heavier.
Keeping it very simple, the aperture is the ratio between the focal length and the iris opening. Stick in the word maximum, and you have the speed of the lens.
For a simple (1-element) len having a focal length of 600mm and a speed of f/5.6, the diameter of the aperture would be:
f/5.6 = 600mm/5.6 = approx 107mm
You may notice that the "f" in "f/5.6" is the focal length.
If you increase the speed to f/4, the aperture becomes:
f/4.0 = 600mm/4.0 = 150mm
That's the aperture diameter. The whole lens must be bigger for physical reasons. The EF 600mm f/4L IS USM has a diameter of 168mm.
And, if you increase the speed of the lens to f/2.8, the aperture would become:
f/2.8 = 600mm/2.8 = approx 214mm
And that's why you don't find very many 600mm f/2.8 lenses.
And yes, if you increased the speed to f/1, the aperture would be 600mm! That would be a lens having a diameter of about two feet! Methinks it would be a bit awkward for birding.
With the design of modern multi-element lenses, some tricks can be played to make the physical length shorter than the focal length (the EF 600mm f/4L IS USM is 456mm long), but the required aperture size still applies.
Although it may not be intuitive, changing the speed of the lens does NOT affect its angle of view. A lens's angle of view is a property of its focal length and the size of the image circle ONLY. The diameter of the front element and the size of the aperture have nothing to do with it.
You can prove this to yourself with any lens by focusing on a bright scene wide open and taking a shot (ignore exposure). Then do the same thing stoping down one stop at a time to the minimum aperture of the lens. The angle of view in each shot remains constant, only the brightness and contrast changes.
This is because the light falling on each point in the image comes from every point in the scene. Hard to "feel," but true. The old light-as-rays drawings we are all used to are false (not lies, but an oversimplification). The light on one point in the image does not come from only one point in the scene. It comes proportionally from all points in the scene. Reducing the size of the aperture only reduces the amount of light entering the lens. It does not reduce the amount of scene entering the lens.
This is most easily visualized by presuming a point at infinity (a star, for example). Light from the star falls on every part of the object lens (the front)element) and, with the star at infinity, each "ray" of light from the star to the lens is exactly parallel. All these parallel rays of light converge to a focal point, where they become a single point of light (like the sun through a magnifying glass). They then expand to produce the image at the film/sensor plane, where they fall into focus. When they fall into focus, the position of the star relative to the rest of the image is revealed. The light from all points in the scene fall proportionally upon each point in the image. The point where the star exists gets the lion's share of the light in the image, just as it was the lion's share of light in the scene.
All this happens because light is both "rays" and "waves." Waves are really hard to draw, and their action is really hard to understand, without physics. And that is why the light-as-rays drawings are used.
I know this is a poor explanation, but it is the best I can do without heavy math.
