f stop vs Front Element Diameter

I've seen numerous wrangles in which it was asserted that this, that, or the
other thing MUST be true because the f/stop is _always_ based on the
_diameter_ of the front element. So I decided to get out the ruler and see
if the facts supported this argument.

Methodology was to measure with an engineer's scale, convert to millimeters
(1 inch=25.4mm), round down to the nearest millimeter, calculate f/stop,
round up to 1 decimal place.

What I found was: (note-list was formatted in Courier New)

Minolta
28-56mm f/4-5.6 25mm diameter f/1.2-2.3 based on
diameter
56-170mm f/4.5-5.6 35mm diameter f/1.6-4.8 based on
diameter
50mm f/3.5 15mm diameter f/3.3 based on
diameter

Canon
10-22mm f/3.5-4.5 50mm diameter f/0.2-0.5 based on
diameter
17-85mm f/4-5.6 46mm diameter f/0.4-1.8 based on
diameter
24mm f/3.5 43mm diameter f/0.6 based on
diameter
50mm f/1.4 36mm diameter f/1.4 based on
diameter
100mm f/2.8 44mm diameter f/2.3 based on
diameter

Seems that while none of the lenses are _faster_ than what would be
indicated by the front element diameter, most of them are a good deal
_slower_.

Anybody want to add to the list?

"J. Clarke" wrote
...

I've seen numerous wrangles in which it was asserted that this, that, or
the
other thing MUST be true because the f/stop is _always_ based on the
_diameter_ of the front element. So I decided to get out the ruler and see
if the facts supported this argument.


I always thought it was based on the SMALLEST diameter the light would go
through any given complex lens.

On Sun, 20 Sep 2009 05:13:11 -0400, J. Clarke wrote:

because the f/stop is _always_ based on the
_diameter_ of the front element.

That's simply not true, at least not for modern lens.
Here's what Canon's own Chuck Westfall said about
that when we asked for some data about constant
aperture zoom lenses:

For a simple lens, the definition of f/stop is focal length
divided by the diameter of the front element. But SLR
zoom lenses are far from simple, and there are many
different types, such as wide-to-wide, wide-to-telephoto,
and telephoto-to-telephoto. In all of these lenses, it's the
apparent size of the aperture, i.e., the “virtual aperture”
that counts, not the size of the physical aperture. In the
case of a traditional telephoto zoom lens like the 70-200
f/2.8L, you can see the virtual aperture change if you
look through the front of the lens while you’re zooming
it. It increases in diameter as you zoom towards 200mm,
and decreases as you zoom towards 70mm. But the size
of the virtual aperture is directly proportional to the focal
length setting, resulting in an effective aperture that’s
constant at all focal lengths. So an 70-200/2.8L should
really be thought of as a 70mm f/2.8 lens with a sort of
“zoom teleconverter” in front of its iris diaphragm.
--
(o)(o)
"As for Photoshop", the LORD continued, "thou shalt not allow heavy
pixelation, for pixelation is detestable. And thee who might be having
trouble cropping, shouldst thou push aspect ratios greater than 20%
or so, thou shalt be stoned to death, for that is an abomination."

On Sun, 20 Sep 2009 05:13:11 -0400, "J. Clarke"
wrote:
: I've seen numerous wrangles in which it was asserted that this, that, or
: the other thing MUST be true because the f/stop is _always_ based on the
: _diameter_ of the front element. So I decided to get out the ruler and see
: if the facts supported this argument.
:
: Methodology was to measure with an engineer's scale, convert to millimeters
: (1 inch=25.4mm), round down to the nearest millimeter, calculate f/stop,
: round up to 1 decimal place.
:
: What I found was: (note-list was formatted in Courier New)
:
: [Meaningless measurements omitted]
:
: Seems that while none of the lenses are _faster_ than what would be
: indicated by the front element diameter, most of them are a good deal
: _slower_.

That's because the quoted (maximum) f-stop is based on the maximum diameter of
the aperture that lets in the light. That can't possibly be greater than the
actual diameter of the lens; but it may be much less, depending on the lens
design (i.e., of the glass elements and the aperture setting mechanism).

Bob

Guybrush Threepwood wrote:
On Sun, 20 Sep 2009 05:13:11 -0400, J. Clarke wrote:

because the f/stop is _always_ based on the
_diameter_ of the front element.

That's simply not true, at least not for modern lens.
Here's what Canon's own Chuck Westfall said about
that when we asked for some data about constant
aperture zoom lenses:

You know that and I know that, but the people who argue on that basis
apparently do not.

For a simple lens, the definition of f/stop is focal length
divided by the diameter of the front element. But SLR
zoom lenses are far from simple, and there are many
different types, such as wide-to-wide, wide-to-telephoto,
and telephoto-to-telephoto. In all of these lenses, it's the
apparent size of the aperture, i.e., the “virtual aperture”
that counts, not the size of the physical aperture. In the
case of a traditional telephoto zoom lens like the 70-200
f/2.8L, you can see the virtual aperture change if you
look through the front of the lens while you’re zooming
it. It increases in diameter as you zoom towards 200mm,
and decreases as you zoom towards 70mm. But the size
of the virtual aperture is directly proportional to the focal
length setting, resulting in an effective aperture that’s
constant at all focal lengths. So an 70-200/2.8L should
really be thought of as a 70mm f/2.8 lens with a sort of
“zoom teleconverter” in front of its iris diaphragm.

Yep. So why not get out your ruler and add your lenses to the body of
physical evidence?

"J. Clarke" wrote in message ...
Seems that while none of the lenses are _faster_ than what would be
indicated by the front element diameter, most of them are a good deal
_slower_.



Maybe if you didn't do all that rounding, the numbers would come out more accurate.


John

J. Clarke wrote:
I've seen numerous wrangles in which it was asserted that this, that, or the
other thing MUST be true because the f/stop is _always_ based on the
_diameter_ of the front element. So I decided to get out the ruler and see
if the facts supported this argument.

Methodology was to measure with an engineer's scale, convert to millimeters
(1 inch=25.4mm), round down to the nearest millimeter, calculate f/stop,
round up to 1 decimal place.

What I found was: (note-list was formatted in Courier New)

Minolta
28-56mm f/4-5.6 25mm diameter f/1.2-2.3 based on
diameter
56-170mm f/4.5-5.6 35mm diameter f/1.6-4.8 based on
diameter
50mm f/3.5 15mm diameter f/3.3 based on
diameter

Canon
10-22mm f/3.5-4.5 50mm diameter f/0.2-0.5 based on
diameter
17-85mm f/4-5.6 46mm diameter f/0.4-1.8 based on
diameter
24mm f/3.5 43mm diameter f/0.6 based on
diameter
50mm f/1.4 36mm diameter f/1.4 based on
diameter
100mm f/2.8 44mm diameter f/2.3 based on
diameter

Seems that while none of the lenses are _faster_ than what would be
indicated by the front element diameter, most of them are a good deal
_slower_.

Anybody want to add to the list?


Especially some long telephoto lenses...

--
Bertrand

On Sun, 20 Sep 2009 05:13:11 -0400, "J. Clarke"
wrote:

....


Methodology was to measure with an engineer's scale, convert to millimeters
(1 inch=25.4mm), round down to the nearest millimeter, calculate f/stop,
round up to 1 decimal place.



When I worked professionally many years ago, I often used the
8X10 camera. The lens was an f8 as I recall and was was about the
size of a dinner plate. Well maybe not quite.

Sorry but the formual is a little more scientific than that.

On Sun, 20 Sep 2009 05:13:11 -0400, J. Clarke wrote:

Methodology was to measure with an engineer's scale, convert to millimeters
(1 inch=25.4mm), round down to the nearest millimeter, calculate f/stop,
round up to 1 decimal place.

This will be inaccurate, particularly as you've seen, for zoom lenses. A
more accurate methodology is to measure the apparent diameter of the
diaphragm, looking into the lens.
--
Mike Russell - http://www.curvemeister.com

In article , J. Clarke
writes
I've seen numerous wrangles in which it was asserted that this, that, or the
other thing MUST be true because the f/stop is _always_ based on the
_diameter_ of the front element.

If that is what is being asserted then it is obviously wrong. The
f-stop is based on the diameter of the pupil and the focal length. The
diameter of the front element is almost always larger than the pupil
since, except in unusual cases, the stop sits behind the front element
which must be large enough to accept all FoVs through the stop.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)