f stop vs Front Element Diameter

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.

That would only be true for a simple lens, or prime lens, with no internal
aperture arrangement.

If you want to check out another anomaly, try measuring the actual focal length
and comparing that to what's written on the lens!

Mike Russell wrote:
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.

But that is not the purpose of the experiment.

Kennedy McEwen 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.

So.... what you want is the _projection_, onto the front element, of the
wide-open diaphragm?

It makes a lot of sense that there'd be a bit of extra "unused" space
around the edges of the front element, to give a margin of error, and
avoid lower image-quality at the extreme edges (something which seems
common). [Indeed for the non-zoom lenses, the differences he measured
seems to be just around right to account for such things, e.g. "f/3.5 vs
f/3.3"]

-Miles

--
In New York, most people don't have cars, so if you want to kill a person, you
have to take the subway to their house. And sometimes on the way, the train
is delayed and you get impatient, so you have to kill someone on the subway.
[George Carlin]

"J. Clarke" wrote in message
...

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.

That is a false asumption. It is the maximum diameter of the aperture. The
diaphragm is seldom in front of the front element, it's normally mid-lens.
Optical design normally doesn't use right to the edge of the element. so
there may be a 50mm diameter front element, but it *may* have a edge baffle
reducing the diameter a bit. Pentax did this with their 50mm f:2, which was
the same lens as their 50mm f:1.7, but just a small baffle added.

In article , Darrell A. Larose
wrote:

That is a false asumption. It is the maximum diameter of the aperture. The
diaphragm is seldom in front of the front element, it's normally mid-lens.
Optical design normally doesn't use right to the edge of the element. so
there may be a 50mm diameter front element, but it *may* have a edge baffle
reducing the diameter a bit. Pentax did this with their 50mm f:2, which was
the same lens as their 50mm f:1.7, but just a small baffle added.

the pentax 50mm f/2 was *not* the same lens as the f/1.7 with an added
baffle. it had a slightly different optical formula with one more lens
element (5-5 v. 6-5).

"J. Clarke" wrote in message
...
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.

A waste of time, since that isn't true.

Lens apertures are expressed as a fraction of the focal length, f. For
example, an f/4 lens is one in which the size of the effective aperture is
1/4 of the focal length.

This means obviously that the diameter of the front element must be AT LEAST
that large, since if it were any smaller the effective aperture would also
have to be smaller. But the diameter of the front element is often a great
deal larger than the effective aperture.

wrote:
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.

Thanks for this experiment! I've done that with some old fast primes and
the numbers were fairly close. Supposedly the stated focal length &
aperture are subject to (sometimes significant) rounding 'errors'.


That would only be true for a simple lens, or prime lens, with no internal
aperture arrangement.

If you want to check out another anomaly, try measuring the actual focal length
and comparing that to what's written on the lens!

Yep. The word 'telephoto' actually means the lens is designed to not be
as long as the math implies. I have an old non-telephoto 500mm f/4.5 and
it's a full 500mm long with a 111mm front element.

--
Paul Furman
www.edgehill.net
www.baynatives.com

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