What Does The "f" in f/x.x stand For?

epeddy1

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In astronomy we use the same ratio to describe a telescope and call it a "focal ratio." My telescope is f/10 and thus has a focal ratio of 1:10. F-stop and f-number and all this other nonsense are just other shorthand versions. This is not the same as focal length (my f/10 has a 2000mm focal length and 200mm aperture).


Which is WHY it is f/10. 2000mm focal length / 200 mm aperture diameter = 2000/200 = f/10. Attached to a camera, it would act as 2000mm f/10 lens.

"Focal ratio" and f/stop are the same term. f/stop is the photographic use, where it is a variable on most lenses. Telescopes are fixed, typically do not have "stops".

Yeah, I realize that. Telescopes don't typically have the focal ratio quoted and must be calculated (or you'd have to dig pretty deep in the specs). The most important dimension in a telescope is the aperture. Only if you plan to use the scope for photography does the focal length and thus focal ratio really matter (although as I mentioned you need to know it to calculate FOV and magnification). The focal ratio doesn't really mean anything unless you want to understand the exposure settings you'd need on a camera. A typical conversation would be "what kind of scope do you have?" "I have a 6 inch Celestron" referring to the aperture. Whereas in photography you'll usually quote your lens as being "100mm f/1.8" referring to the focal length and focal ratio and ignoring the aperture (although it's indirectly reference with the f-number). You could just as easily get by quoting the focal length and aperture and just calculating the ratio for exposure (which is what we did in my high school photography class for whatever reason).
 

epeddy1

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It was confusing for me at first going back and forth between astronomy and photography because in photography the convention is quoting focal length and focal ratio (aperture is implied and is often ignored). So a lens is "50mm f/1.8." With telescopes the convention is quoting the aperture and focal length (focal ratio is implied and is often ignored). So a telescope is "200mm aperture, 2000mm focal length." In either case there are only 2 actual measurements - focal length and aperture.

For me to quote my telescope as being "a 6 inch aperture f/10" would be unconventional and be like you quoting a lens as "a 50mm focal length with 180mm aperture." Both are correct and provide all the info you need. Again, it's really just semantics and it would be nit-picky to split hairs IMO.
 

Helen B

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Could you give a reliable reference for it being aperture diameter divided by focal length rather than the other way round? I'm curious.

Your confusion is warranted, as I had the ratio backwards from the standard convention. As I said, luckily everyone knows what you mean when you say "f <whatever> of 10" when talking about photography, etc. It being the relationship of the aperture to the focal length.

The math all works out the same...

... but not if you try using the focal ratio or f-number in a formula. It's defined for a good reason. (I wasn't confused, I was just trying to remove your confusion by suggesting that you check elsewhere because you had no reason to believe me.)
 

epeddy1

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Could you give a reliable reference for it being aperture diameter divided by focal length rather than the other way round? I'm curious.

Your confusion is warranted, as I had the ratio backwards from the standard convention. As I said, luckily everyone knows what you mean when you say "f <whatever> of 10" when talking about photography, etc. It being the relationship of the aperture to the focal length.

The math all works out the same...

... but not if you try using the focal ratio or f-number in a formula. It's defined for a good reason. (I wasn't confused, I was just trying to remove your confusion by suggesting that you check elsewhere because you had no reason to believe me.)

Meh. Like I said, knowing aperture and focal length is enough. I don't know what formulas you'd use the f-ratio without decomposing the numerator and denominator. I haven't run across any; do you have an example? And any formulas comparing the ratios with each other would be fine as long as I'm consistent. Source: I'm a mathematician by trade, so tend to focus on understanding the actual numbers rather than conventions. I tend to find solutions "my way". Also why I could never be a math teacher, because "my way" usually doesn't translate well, as evident by this thread!
 

Helen B

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I know a lot of formulae that use the f-number without decomposing it into focal length and entrance pupil diameter. F-number has been found to be a useful combination, in something of the same way that Reynold's number, kinematic viscosity, the Prandtl number, the Grashof number and many others both dimensioned and dimensionless are used without decomposing but with understanding of what they represent. F-number is often used as-is in formulae for image brightness (with transmission if not replaced by the T-stop, and for extended images, not point sources), diffraction and focus calculations. It's very common. Have a look in any decent optics book like Born and Wolf, Hecht, Kingslake or Ray - especially if you are a mathematician and are comfortable with reading formulae.
 

epeddy1

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I know a lot of formulae that use the f-number without decomposing it into focal length and entrance pupil diameter. F-number has been found to be a useful combination, in something of the same way that Reynold's number, kinematic viscosity, the Prandtl number, the Grashof number and many others both dimensioned and dimensionless are used without decomposing but with understanding of what they represent. F-number is often used as-is in formulae for image brightness (with transmission if not replaced by the T-stop, and for extended images, not point sources), diffraction and focus calculations. It's very common. Have a look in any decent optics book like Born and Wolf, Hecht, Kingslake or Ray - especially if you are a mathematician and are comfortable with reading formulae.

Yeah, me saying 1:10 represents aperture:length is mathematically the same as saying 10:1 represents length:aperture when you start plugging them into formulae. Again, semantic convention. Like speed could be quoted as m/s or s/m and still represent velocity, but convention says use m/s in conversation.
 

pgriz

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This is a secret text.

What you see is not always what's there.

And maybe what is not visible is what you really want to know.

Not. Heh. If if was really so simple.

I blame Sparky for this. Because he made me check his "blank" post for hidden meaning.


:mrgreen:
 

snerd

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I didn't read the whole thread, but.................................

Why are there replies to a 4 year old thread?

:mrgreen:
 

pgriz

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Nostalgia. Or it took us THAT long to think of an appropriate response.
 

StoneNYC

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Well yes I know, but I mean it's a purely derivative word, which seems sort of odd for something so essential to photography.

Like if "garage" didn't have its own word, but were instead just called the "Attached-Car-Building"

Maybe it's not that odd, I don't know. I guess "driveway" and "outhouse" barely have their own words either *shrug*

Isn't that how Germans speak?
 

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