aperature number vs f number

Len Philpot wrote:

guinness2 wrote:

Very interesting, thanks for the excursion into different world.

I suppose the eyepiece is represented by the rear lens glass on camera lens ?

Maybe someone more knowledgeable can elaborate since I'm no optician nor optical designer, but I suspect not in most cases. Here's why I think that...

A telescope's "objective" (i.e., its primary optic, whether a mirror or lens) delivers an image at the focal plane which is at its focal length. An eyepiece is simply a magnifier through which a larger view of that "prime-focus" image is seen. It is -- or should be -- a high quality magnifier for sure, but still just a magnifier.

Comparing a camera lens to a telescope isn't entirely apples-to-apples. I suspect a camera lens delivers a prime-focus, i.e., non-magnified, image. For example, when a 100mm lens' focal plane is coincident with the sensor (or film), the image is in focus at 100mm.

Do the last few rear elements in a lens effectively comprise an eyepiece? Dunno for sure, but I would be surprised if they do in any but the odd case. For example, maybe a lens designed to deliver a long effective focal length in a short physical package? Imagine a lens that's natively 80mm but the last elements magnify the image by 3x, so you have effectively a 240mm lens. I've not heard of that, but maybe it's possible. There are folded optic lenses, but that's a different design.

So there's my non-committal answer!

In a camera lens, the slew of elements behind the objective lens affect both the FL and other vital photographic concerns such as flatness of field and CA correction.

The difference from telescope lenses is the simplicity of the latter: a refractor has generally 2 objective lenses of same diameter for correction of CA, but the finest have three, with one of them being very pricey flourite glass, making the scope "Apochromatic", or CA -free. Simple reflectors have only a parabolic mirror and no lenses. And all these tele designs can project an image onto a sensor or film at the focal point, but it's generally not as well-corrected over the sensor width.

Since optical resolution is the primary concern of 'scopes, and this varies directly with objective diameter, the diameter of the objective lens, or mirror in reflectors, is the first number in the scope's specs. The scopes's magnification depends on viewing eyepiece FL: mag = objective FL/eyepiece FL So my 80mm f/6 refractor gives an 80x6=480mm FL. And with a 10mm EP, gives a 48X mag. view.

In photographic lenses, FL is the first concern, so as a camera lens, the above scope would be notated as 480mm f/6.