What are F-Stops?

Okay, okay, I'm not asking THAT question. I've shot for years and I know
about aperatures and everything.

What I'm asking is... is there a standard for f-stops? I had always assumed
that each f-stop was the movement from the the indicators on a lens (ie.
2.8, 4, 5.6, 8, etc.). But when I read things like "moving from an f/1.8
lens to an f/1.4 lens is 1/3rd of an f-stop". Huh?

Y'all can begin laughing at my ignorance...

Look at http://www.uscoles.com/fstop.htm

Look at http://www.uscoles.com/fstop.htm

On Fri, 27 Aug 2004 02:50:28 GMT, "greg" wrote:

Okay, okay, I'm not asking THAT question. I've shot for years and I know
about aperatures and everything.

What I'm asking is... is there a standard for f-stops? I had always assumed
that each f-stop was the movement from the the indicators on a lens (ie.
2.8, 4, 5.6, 8, etc.). But when I read things like "moving from an f/1.8
lens to an f/1.4 lens is 1/3rd of an f-stop". Huh?

Y'all can begin lau ghingatmyignorance...


So you know that an f stop is the ratio of the focal length to the open aperture
of the lens?

And you know that the f stops represent a ratio that is a series that doubles
the light each time?

The best lens we could have would be f 1. I hear NASA actually made one... any
less then 1 is impossible since it would be inside out...

Now to half the amount of light, we should stop it down to half the area, now
since it is area it is a square function, so we want the square root of 2, or
1.4.

Now to half this again, we go to 2. And then to 2.8

As you can see, we have a series of numbers, and we can think of them in pairs.

EG just think about 8 and 11. To go down from here, just double them, so the
next 2 are 16 and 22, the next 2 are 32 and 44 etc. Easy! To go the other way,
it's 4 and 5.5 (ok they use 5.6) and as we saw, 2 and 2.8.

Understand?

BTW the last number on the lens doesn't count... a lens of 3.5 just happened to
be that size, and that's the biggest it can be! It is not a full f stop away
from the next standard number. (f4)

On Fri, 27 Aug 2004 02:50:28 GMT, "greg" wrote:

Okay, okay, I'm not asking THAT question. I've shot for years and I know
about aperatures and everything.

What I'm asking is... is there a standard for f-stops? I had always assumed
that each f-stop was the movement from the the indicators on a lens (ie.
2.8, 4, 5.6, 8, etc.). But when I read things like "moving from an f/1.8
lens to an f/1.4 lens is 1/3rd of an f-stop". Huh?

Y'all can begin lau ghingatmyignorance...


So you know that an f stop is the ratio of the focal length to the open aperture
of the lens?

And you know that the f stops represent a ratio that is a series that doubles
the light each time?

The best lens we could have would be f 1. I hear NASA actually made one... any
less then 1 is impossible since it would be inside out...

Now to half the amount of light, we should stop it down to half the area, now
since it is area it is a square function, so we want the square root of 2, or
1.4.

Now to half this again, we go to 2. And then to 2.8

As you can see, we have a series of numbers, and we can think of them in pairs.

EG just think about 8 and 11. To go down from here, just double them, so the
next 2 are 16 and 22, the next 2 are 32 and 44 etc. Easy! To go the other way,
it's 4 and 5.5 (ok they use 5.6) and as we saw, 2 and 2.8.

Understand?

BTW the last number on the lens doesn't count... a lens of 3.5 just happened to
be that size, and that's the biggest it can be! It is not a full f stop away
from the next standard number. (f4)

greg wrote:

Okay, okay, I'm not asking THAT question. I've shot for years and I know
about aperatures and everything.

What I'm asking is... is there a standard for f-stops? I had always assumed
that each f-stop was the movement from the the indicators on a lens (ie.
2.8, 4, 5.6, 8, etc.). But when I read things like "moving from an f/1.8
lens to an f/1.4 lens is 1/3rd of an f-stop". Huh?

Y'all can begin laughing at my ignorance...

Yes, there is a 'standard', or at least an understood method of deriving
the series of aperture calibrations used on camera lenses.

Firstly, F-numbers are ratios, of the optical diameter of the aperture
to the focal length of the lens. A lens of 50 mm focal length (focused
at infinity) with an aperture diameter of 12.5 mm would be described as
f/4 lens (or more properly, f1:4). The same lens with an aperture of 25
mm would be an f/2 lens.

The value of using a ratio rather than just using the actual aperture
diameter is that the illumination at the focal plane will be the same
for any lens at a given aperture. A 50 mm lens at, say, f/8 will have
the same image brightness on the film as will a 500 mm lens at f/8, or
in fact any focal length lens at f/8. This enables the use of
shutter/aperture combinations without regard the lens focal length.

There has to be a starting point for any series of aperture numbers, and
in fact there have been several, some countries having different scales
than the standard one now in universal use. The obvious starting point
is f/1, i.e. a lens with an optical aperture diameter equal to its focal
length - not that there are many f:1 lenses around. Then, the series is
generated by successively halving the aperture diameter, giving F/1,
f/2, f/4, f/8, f/16, f/32 and so on.

But, because the area of a circle is proportional to the *square* of its
diameter (area = pi * r^2), such a series would in fact quarter the
exposure from one stop to the next. So, a second series of f-numbers is
interspersed with the first, this second series starting with an
aperture having half the area of an f/1 aperture. This will be f/1.4 -
1.4 being near enough to the square root of 2. This series runs f/1.4,
f/2.8, f/5.6, f/11, f/22, f/44 etc.

So, by combining the two series, we get f/1, f/1.4, f/2, f/2.8, f/4,
f/5.6 ... the standard series as we know it. Each successive stop
halves or doubles the amount of light transmitted through the lens.

Frequently, the maximum aperture of a lens is an odd number not fitting
the series exactly - for a number of reasons to do with the design and
manufacture of the lens. A lens described as, say f/1.8, is part way
between f/1.4 and f/2, so is some fraction of a stop faster than f/2,
i.e. about a third of a stop faster.

Apologies for the longish post, but I hope it throws some light on the
subject for you.

Colin D.

greg wrote:

Okay, okay, I'm not asking THAT question. I've shot for years and I know
about aperatures and everything.

What I'm asking is... is there a standard for f-stops? I had always assumed
that each f-stop was the movement from the the indicators on a lens (ie.
2.8, 4, 5.6, 8, etc.). But when I read things like "moving from an f/1.8
lens to an f/1.4 lens is 1/3rd of an f-stop". Huh?

Y'all can begin laughing at my ignorance...

Yes, there is a 'standard', or at least an understood method of deriving
the series of aperture calibrations used on camera lenses.

Firstly, F-numbers are ratios, of the optical diameter of the aperture
to the focal length of the lens. A lens of 50 mm focal length (focused
at infinity) with an aperture diameter of 12.5 mm would be described as
f/4 lens (or more properly, f1:4). The same lens with an aperture of 25
mm would be an f/2 lens.

The value of using a ratio rather than just using the actual aperture
diameter is that the illumination at the focal plane will be the same
for any lens at a given aperture. A 50 mm lens at, say, f/8 will have
the same image brightness on the film as will a 500 mm lens at f/8, or
in fact any focal length lens at f/8. This enables the use of
shutter/aperture combinations without regard the lens focal length.

There has to be a starting point for any series of aperture numbers, and
in fact there have been several, some countries having different scales
than the standard one now in universal use. The obvious starting point
is f/1, i.e. a lens with an optical aperture diameter equal to its focal
length - not that there are many f:1 lenses around. Then, the series is
generated by successively halving the aperture diameter, giving F/1,
f/2, f/4, f/8, f/16, f/32 and so on.

But, because the area of a circle is proportional to the *square* of its
diameter (area = pi * r^2), such a series would in fact quarter the
exposure from one stop to the next. So, a second series of f-numbers is
interspersed with the first, this second series starting with an
aperture having half the area of an f/1 aperture. This will be f/1.4 -
1.4 being near enough to the square root of 2. This series runs f/1.4,
f/2.8, f/5.6, f/11, f/22, f/44 etc.

So, by combining the two series, we get f/1, f/1.4, f/2, f/2.8, f/4,
f/5.6 ... the standard series as we know it. Each successive stop
halves or doubles the amount of light transmitted through the lens.

Frequently, the maximum aperture of a lens is an odd number not fitting
the series exactly - for a number of reasons to do with the design and
manufacture of the lens. A lens described as, say f/1.8, is part way
between f/1.4 and f/2, so is some fraction of a stop faster than f/2,
i.e. about a third of a stop faster.

Apologies for the longish post, but I hope it throws some light on the
subject for you.

Colin D.

"greg" wrote in message
news:UjxXc.224279$J06.62156@pd7tw2no...
Okay, okay, I'm not asking THAT question. I've shot for years and I know
about aperatures and everything.

What I'm asking is... is there a standard for f-stops? I had always
assumed
that each f-stop was the movement from the the indicators on a lens (ie.
2.8, 4, 5.6, 8, etc.). But when I read things like "moving from an f/1.8
lens to an f/1.4 lens is 1/3rd of an f-stop". Huh?

Y'all can begin laughing at my ignorance...


Well, a full f-stop will either double or half the amount of light that gets
to your screen or film plane. This means the area of the hole will either be
half as great, or doubled. So if the change only increases the light by1/3
as much as a full stop, then it is a third of a stop.

"greg" wrote in message
news:UjxXc.224279$J06.62156@pd7tw2no...
Okay, okay, I'm not asking THAT question. I've shot for years and I know
about aperatures and everything.

What I'm asking is... is there a standard for f-stops? I had always
assumed
that each f-stop was the movement from the the indicators on a lens (ie.
2.8, 4, 5.6, 8, etc.). But when I read things like "moving from an f/1.8
lens to an f/1.4 lens is 1/3rd of an f-stop". Huh?

Y'all can begin laughing at my ignorance...


Well, a full f-stop will either double or half the amount of light that gets
to your screen or film plane. This means the area of the hole will either be
half as great, or doubled. So if the change only increases the light by1/3
as much as a full stop, then it is a third of a stop.

In rec.photo.equipment.35mm greg wrote:
Okay, okay, I'm not asking THAT question. I've shot for years and I know
about aperatures and everything.

What I'm asking is... is there a standard for f-stops? I had always assumed
that each f-stop was the movement from the the indicators on a lens (ie.
2.8, 4, 5.6, 8, etc.). But when I read things like "moving from an f/1.8
lens to an f/1.4 lens is 1/3rd of an f-stop". Huh?

The standard sequence is from the square roots of the powers of two.

So (2^0)^.5 = 1
(2^1)^.5 = 1.414...
(2^2)^.5 = 2
(2^3)^.5 = 2.828...
(2^4)^.5 = 4
(2^5)^.5 = 5.658..
(2^6)^.5 = 8
(2^7)^.5 = 11.31...

If you want to find out what is one third of a stop above f/5.6
you can use the formula (2^5.33333)^.5 which is roughly 6.35.

If you want to find the half-stop above f/8, you can use the
formula (2^6.5)^.5 which is about 9.51.

By custom the irrational numbers are truncated rather than
rounded up so you get f/5.6 instead of f/5.7 which would be
a bit more logical.

There used to be another common sequence which went:
1.1, 1.6, 2.2, 3.2, 4.5, 6.3, 9, 12.6, 18, 25, 36, ...

This was called the Continental sequence. It is very nearly 1/3
of a stop above the standard sequence. It is formed from the
square roots of the series 1.25, 2.5, 5, 10, 20, 40 ...
The continental sequence does round up instead of truncating.

Peter.
--