aperature number vs f number

guinness2 wrote:

A 10 inch f/5 telescope has an aperture of 10 inches (254mm) and a focal length of 50 inches (1270mm). A 10 inch f/10 scope would have twice the focal length and therefore twice the magnification with the same eyepiece, but it collects the same amount of light.

It is interesting numbering, is it different than in the world of a common photography?
10 inch f/10 usually means here the lens ~250 f/10 where the first is the focal length and the latter the lens speed= maximal aperture=maximal hole the lens is capable to open to.

So I feel your example is a bit misleading, despite it is physically correct.

I mean , it is worth to mention, that the lens with the speed f/5 collects more light than the one with speed f/10 of the same focal length, when both are full open.

I was hoping to provide an illustration of other slightly different but related applications of focal ratios. But you're correct, it could be potentially confusing if the basic principle isn't understood. My apologies for being potentially misleading.

That said -- To follow my tangent regarding telescope f-ratios and light gathering...

Remember we're talking about a ratio, which can be affected by changing any of the factors involved: aperture or focal length. Also FYI, telescope magnification is the ratio between the eyepiece focal length and the telescope focal length. E.g., a 10mm eyepiece in a 1000mm (focal length) telescope will be 100x, regardless of that telescope's focal ratio.

Broadly speaking, you can think of it this way: With telescopes the difference in focal ratio is due to differing focal lengths at the same aperture. With camera lenses, it's due to different apertures at the same focal length. So while you're correct that a camera lens at f/5 will collect twice as much light as one at f/10 (given the same focal length), that's not true for telescopes in my example. I was speaking the other way: constant aperture with varying focal length.

A 10" f/10 telescope has the same aperture (and therefore, the same light gathering ability) as a 10" f/5 telescope. It just brings the light to a focus at a distance twice as far away as the f/5 scope. So at the same magnification, the image will be equally bright in both scopes regardless of focal ratio. However, the same eyepiece will yield twice the magnification in the f/10 scope as in the f/5 scope so therefore with the image being twice as large, it will be one fourth as bright.

So you're right that a 'faster' telescope will yield brighter images than a slower scope of the same aperture with the same eyepiece (i.e., at different magnification), but at the same magnification they're equally bright.

Very interesting, thanks for the excursion into different world.

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

AnthonyL wrote:

Jethro B-UK wrote:

You are correct.

Oh dear, another mistake on my part.

I do know the difference between few and phew, believe me.

Phew, thank heavens for that. Now we just need to understand where this Aperture number comes from and we can sleep easy at night

Aye. 

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!  

Jethro B-UK wrote:

AnthonyL wrote:

Jethro B-UK wrote:

You are correct.

Oh dear, another mistake on my part.

I do know the difference between few and phew, believe me.

Phew, thank heavens for that. Now we just need to understand where this Aperture number comes from and we can sleep easy at night

Aye.

Lemmings351 answered how it  is derived. he did not leave its official name or when or who uses it.

Toronto Photography wrote:

Jethro B-UK wrote:

AnthonyL wrote:

Jethro B-UK wrote:

You are correct.

Oh dear, another mistake on my part.

I do know the difference between few and phew, believe me.

Phew, thank heavens for that. Now we just need to understand where this Aperture number comes from and we can sleep easy at night

Aye.

Lemmings351 answered how it is derived. he did not leave its official name or when or who uses it.

I think we understand what it is.   I think I'd be safe in saying that virtually no-one here uses it.

Can you clarify where it is that you saw it in the first place?

Pete Berry wrote:

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.

Yep, although flourite is falling out of favor due to expense and availability, being gradually replaced by FPL-53 and other more exotic glasses ...not to mention cheaper alternatives like FPL-51, FK-61, etc. From a visual standpoint my little 80mm f/7.5 achromat is decent enough, but with a camera not so hot. My 100mm f/9 ED doublet was much better. But of course it's been replaced by a 6" f/6 Newtonian.

I didn't go into SCTs, RASAs, Dall-Kirkhams, RCs, Petzvals, etc., etc., to keep from (further LOL) muddying the waters.

I would say optical resolution is one of two primary concerns, the other being light-grasp, at least from a visual standpoint. Imagers may say otherwise.