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

480sparky

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glun

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In another thread, I read a post that said one of the key things to look for when taking good pictures of moving objects is a low f/x.x

I am just wondering what this stands for? And, I thought when you dish out the extra money and buy a better/stronger lens, it would have a lower f/x.x, but from checking some zoom lens, this seems not to be the case.

For instance, the Nikon D5000 I am looking at says 18-55 mm f/3.5-5.6 VG, and then another lens for it says 55-200 mm f/4-5.6 VG.

I remember reading the post about this and the poster saying that the key is to buy a camera with an f/x.x like in the 2 or 1 range.

Any thoughts on this?

Thanks.

It means smaller F-stop for larger aperture. The smaller F stop goes the more light your lens will get, hence you can shoot in darker condition.
 

Gavjenks

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Did anybody actually answer the question in the title of the thread....?

The "f" stands for focal length.
It's a formula for the aperture, in terms of the entrance pupil.

Aperture diameter = f[ocal length] / 2.8 for example. For a 200mm lens, the entrance pupil is 71.4mm.

But nobody really cares about the entrance pupil in absolute terms, because it doesn't mean much for the actual photograph. The ratio does, and so the nomenclature shows you the number that defines the ratio, which for some reason I guess doesn't really have a name of its own
 

Helen B

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Did anybody actually answer the question in the title of the thread....?

The "f" stands for focal length.
It's a formula for the aperture, in terms of the entrance pupil.

Aperture diameter = f[ocal length] / 2.8 for example. For a 200mm lens, the entrance pupil is 71.4mm.

But nobody really cares about the entrance pupil in absolute terms, because it doesn't mean much for the actual photograph. The ratio does, and so the nomenclature shows you the number that defines the ratio, which for some reason I guess doesn't really have a name of its own

The number itself (ie 2.8 in your example) is called the f-number. It is defined by the ISO (for general photographic usage) as the focal length divided by the diameter of the entrance pupil.
 

Gavjenks

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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*
 

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). In astronomy the aperture (diameter of the light collecting mirror) is much more important than in photography, so its important to know all parts of the equation to truly describe the scope. The focal ratio of course is mostly important for determining how worthy the scope is for photography, and faster (smaller "f-number" or smaller denominator in the ratio) is better because light is at a premium when it comes from billions of miles away and is the angular size on a pin prick.
 
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AlanKlein

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To clarify the second part of your question. Zoom lenses have a range of ratio f stops. So f/3.5-5.6 mean that the aperture will change on that range as you adjust the zoom from 18-55mm. Another point is not to compare the f stop range of a zoom lenses with another that has a completely different zoom. You need a much larger lenses (zoom or fixed focal length) to allow the same amount of light in a longer focal length than in a shorter one so the former will cost more.
In another thread, I read a post that said one of the key things to look for when taking good pictures of moving objects is a low f/x.x

I am just wondering what this stands for? And, I thought when you dish out the extra money and buy a better/stronger lens, it would have a lower f/x.x, but from checking some zoom lens, this seems not to be the case.

For instance, the Nikon D5000 I am looking at says 18-55 mm f/3.5-5.6 VG, and then another lens for it says 55-200 mm f/4-5.6 VG.

I remember reading the post about this and the poster saying that the key is to buy a camera with an f/x.x like in the 2 or 1 range.

Any thoughts on this?

Thanks.
 

Helen B

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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.

Don't you mean 'thus has a focal ratio of 10' rather that 1:10?
 

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.

Don't you mean 'thus has a focal ratio of 10' rather that 1:10?

The ratio is 1:10 spoken "1 to 10." Its a ratio of apeture to focal length (200:2000 simplified to 1:10 in my example). In other words, a dreaded fraction (1:10 = 1/10 = 0.1). People often sidestep the complications of speaking in terms of ratios and fractions and simply quote the denominator in the ratio. A focal ratio of 1:10 is often spoken as "has f-number of 10" or "has f-10". Saying "f-ratio of 10" is often spoken in the community and is technically incorrect, but no harm in my opinion. A ratio of 10 (implied 10/1 in math terms) would be silly and technically imply an apeture diameter 10 times larger than the focal length. But people know what you mean which is what's important.

Also the reason its often written as f/10, where the 1 in the numerator is implied.
 
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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.
 

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, because I've had the ratio backwards for quite a while and have done just fine with my telescope! It become interesting with telescopes because you can add focal reducers which effectively reduce your focal ratio by a factor (reducing the focal ratio, increasing field of view, decreasing magnification, etc). And you need to know the dimensions of your scope and eyepiece to determine the field of view and magnification so you can effectively compare the stars you're seeing to a star map and find stuff. For example, are the 3 stars in your view those on Orion's belt (a very wide field of view) or 3 other random stars that look like Orion's belt (a smaller field of view). Scale is important to identify what you're seeing.

http://www.google.com/url?sa=t&rct=...fJfP6dsCsUsHeT8Khar6a2A&bvm=bv.63934634,d.cWc

The focal ratio is the ratio of the focal length of the telescope related to its aperture. It&#8217;s calculated by dividing the focal length by the aperture (both in the same units). For example, a telescope with a 2032 mm focal length and an aperture of 8 in (203.2 mm) has a focal ratio of 10 (2032/203.2 = 10) or f/10.

It&#8217;s variously abbreviated as f-stop, f/stop f-ratio, f/ratio, f-number, f/number, f/no., etc.

Smaller f-numbers will give brighter photographic images and the option to use shorter exposures. An f/4 system requires only ¼ the exposure time of an f/8 system. Thus, small focal ratio lenses or scopes are called &#8220;fast&#8221; and larger f/numbers are called &#8220;slow.&#8221; Fast focal ratios of telescopes are f/3.5 to f/6, medium are f/7 to f/11, and slow are f/12 and longer.

Whether a telescope is used visually or photographically, the brightness of stars (point sources) is a function only of telescope aperture--the larger the aperture, the brighter the images. Extended objects will always appear brighter at lower magnifications. The main advantage of having a fast focal ratio with a visual telescope is that it will deliver a wider field of view than slower f-numbers.
 
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WayneF

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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".
 

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