What are F-Stops?

In article , says...

In message ,
Bob wrote:

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...

No, there have been f0.95 lenses.

This is correct. I have an Angineaux 25mm f/0.95 in C-mount for cinema work. I
also believe that some security cameras have lenses that can open beyond F/1.
0, though with the newer sensors that can do a good job of night-for-day, they
are probably fading.

Hunt

In article , says...

wrote:

In message ,
Bob wrote:

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

No, there have been f0.95 lenses.

nasa got a zeiss lens rated at f: 0.65 for a hasselblad.

Dang, those government types get all the good gear :-}

Hunt

On Fri, 27 Aug 2004 13:13:30 GMT, Buster wrote:

On Thu, 26 Aug 2004 23:55:11 -0400, Bob wrote:
snip

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...


Actually, any f-stop lower than 1 simply means that the opening is
larger than the focal length. Canon used to sell (a long time ago) a
50mm f0.9 lens. It was quite fat and a hefty chunk of glass. I think
Leica (or maybe Contax) also has (had) a faster than f/1 lens.
Buster

OOPS all you guys are right I was thinking of something else!!

BUSTED!


Damn now I want a 35mm F0.7 Nikkor!!

On Fri, 27 Aug 2004 13:13:30 GMT, Buster wrote:

On Thu, 26 Aug 2004 23:55:11 -0400, Bob wrote:
snip

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...


Actually, any f-stop lower than 1 simply means that the opening is
larger than the focal length. Canon used to sell (a long time ago) a
50mm f0.9 lens. It was quite fat and a hefty chunk of glass. I think
Leica (or maybe Contax) also has (had) a faster than f/1 lens.
Buster

OOPS all you guys are right I was thinking of something else!!

BUSTED!


Damn now I want a 35mm F0.7 Nikkor!!

"Peter Irwin" wrote in message
...
[SNIP]

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.


I'd always wondered about that 'other' series, which appears on a couple of
old folders I have - thanks for that. Amazing what the folks around here
know...


Peter

Huh? Foot candles (as well as lumens) are a measure of light intensity
and have nothing to do with f-stops.

A Concerned Contributer wrote:

Proportionate to film size the stops are based on the amount of
light falling on the film plane. In the old days it was first measured
using one standard sized candle at one foot distance from the front of a
lens. The measurement became known as the foot candle.

In article UjxXc.224279$J06.62156@pd7tw2no, "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...

Mark Hanson wrote:

In article ,
Bob wrote:
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...

This page says that Kubrick used a 50mm f/0.7 lens to film candle-lit scenes
in Barry Lyndon:

http://www.visual-memory.co.uk/sk/ac/len/page1.htm

Mark

I believe it was Cannon that advertised an f 0.95 lens long ago. The
previous poster is wrong as there is no reason a lens can have a
larger diameter than its focal length Probably lots of magnifiers
fall into this category.

Bob wrote:

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...

If I remember rightly, one of the big selling points of the Canon EOS
line when it first came out was that the lens mount was significantly
larger than anything previously available on a 35mm SLR - large enough
to accomodate the first f/1 50mm lens.

BTW, lower f-stops are certainly possible: a 50mm f/.8 lens would simply
be a lens with a 50mm focal length and a 62.5mm aperture (50/.8).

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)

That didn't make any sense at all...

Commonly "marked" f-stops generally start at f/1 and go up by full stops
(ie. 1/2 reductions in light). As you state, a full stop means a ratio
of 1.4 in the diameter of the aperture, so we get full stops at f/1,
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/44, etc
(every doubling or halving in aperture is two stops). The number listed
on a lens is the maximum opening that lens is capable of, and it doesn't
have to be on an exact stop (a 100mm, f/3.5 lens would have a max
aperture of about 28.57mm).

Bob wrote:

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...

If I remember rightly, one of the big selling points of the Canon EOS
line when it first came out was that the lens mount was significantly
larger than anything previously available on a 35mm SLR - large enough
to accomodate the first f/1 50mm lens.

BTW, lower f-stops are certainly possible: a 50mm f/.8 lens would simply
be a lens with a 50mm focal length and a 62.5mm aperture (50/.8).

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)

That didn't make any sense at all...

Commonly "marked" f-stops generally start at f/1 and go up by full stops
(ie. 1/2 reductions in light). As you state, a full stop means a ratio
of 1.4 in the diameter of the aperture, so we get full stops at f/1,
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/44, etc
(every doubling or halving in aperture is two stops). The number listed
on a lens is the maximum opening that lens is capable of, and it doesn't
have to be on an exact stop (a 100mm, f/3.5 lens would have a max
aperture of about 28.57mm).

Colin D wrote:

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,

That's the most coherent explanation I've seen so far on this thread
(mine included). Nicely done

but I hope it throws some light on the
subject for you.

Boooooo!