What exactly is the "flange distance" on an SLR camera lens?

I am retrofitting instruments which used film camera backs to digital
SLRs, and one important property is what is called the "flange
distance". There are tables on the Web that users have collected for
various SLR mounts:

http://www.a1.nl/phomepag/markerink/mounts.htm
http://en.wikipedia.org/wiki/List_of_lens_mounts
etc.

But none of what I can find actually defines this "flange distance".

One would assume it is the axial distance of the focal plane of the lens
(for an infinity target) from "something" on the lens mount. The
question is whether that "something" is the shoulder that mates to the
opposing surface on the camera, or the tip of the bayonet ears which
project out from the shoulder, or what?

Notably some Wikipedia authors are trying:

http://en.wikipedia.org/wiki/Flange_focal_distance

Calling it, "the distance from the mounting flange--the metal ring on
the camera and the rear of the len--to the film plane". However, there
is no diagram and no defintion of what "metal ring" is meant. And to
add to the confusion, mechanical features on various mounts vary enough
(some are "male" bayonets, others female receptacles, etc) that no
universal definition is possible.

I would guess it's the shoulder, at least on mount types having a
shoulder, but then my measurements of focal planes on some lenses, and
measuring the film position on some old SLR bodies, do not seem to
consistently match the distances the tables give.

So, anyone have any authoritative definitions to offer?

In article , Richard J
Kinch wrote:

So, anyone have any authoritative definitions to offer?

I'm not sure if it's "authoritative", but this is what I've been using
for the last 40 years: Ignore the lens itself. It's the distance from
the front surface of the bare lens mount on the front of the camera, to
the film plane. The former should be obvious; the latter is universally
shown with a symbol - a horizontal line through a circle - inscribed on
the top of the camera. Even my DSLR has a "film plane" mark.

"Scott Schuckert" wrote in message
...
In article , Richard J
Kinch wrote:

So, anyone have any authoritative definitions to offer?

I'm not sure if it's "authoritative", but this is what I've been using
for the last 40 years: Ignore the lens itself. It's the distance from
the front surface of the bare lens mount on the front of the camera, to
the film plane. The former should be obvious; the latter is universally

"universally" ?

shown with a symbol - a horizontal line through a circle - inscribed on
the top of the camera. Even my DSLR has a "film plane" mark.

Deep Reset wrote:
"Scott Schuckert" wrote in message
...
In article , Richard J
Kinch wrote:

So, anyone have any authoritative definitions to offer?
I'm not sure if it's "authoritative", but this is what I've been using
for the last 40 years: Ignore the lens itself. It's the distance from
the front surface of the bare lens mount on the front of the camera, to
the film plane. The former should be obvious; the latter is universally

"universally" ?

shown with a symbol - a horizontal line through a circle - inscribed on
the top of the camera. Even my DSLR has a "film plane" mark.

"Universally" is undoubtedly an overstatement; but I'm quite surprised
at the range of cameras I've found that marking on -- cheap film P&S,
for example. And it's certainly on my D200.

"David Dyer-Bennet" wrote in message
. net...
Deep Reset wrote:
"Scott Schuckert" wrote in message
...
In article , Richard J
Kinch wrote:

So, anyone have any authoritative definitions to offer?
I'm not sure if it's "authoritative", but this is what I've been using
for the last 40 years: Ignore the lens itself. It's the distance from
the front surface of the bare lens mount on the front of the camera, to
the film plane. The former should be obvious; the latter is universally

"universally" ?

shown with a symbol - a horizontal line through a circle - inscribed on
the top of the camera. Even my DSLR has a "film plane" mark.

"Universally" is undoubtedly an overstatement; but I'm quite surprised at
the range of cameras I've found that marking on -- cheap film P&S, for
example. And it's certainly on my D200.

It's on all of our Canon DSLRs, too.

--
Skip Middleton
www.shadowcatcherimagery.com
www.pbase.com/skipm

On Wed, 14 Feb 2007 16:43:05 -0800, "Skip"
wrote:

"David Dyer-Bennet" wrote in message
.net...
Deep Reset wrote:
"Scott Schuckert" wrote in message
...
In article , Richard J
Kinch wrote:

So, anyone have any authoritative definitions to offer?
I'm not sure if it's "authoritative", but this is what I've been using
for the last 40 years: Ignore the lens itself. It's the distance from
the front surface of the bare lens mount on the front of the camera, to
the film plane. The former should be obvious; the latter is universally

"universally" ?

shown with a symbol - a horizontal line through a circle - inscribed on
the top of the camera. Even my DSLR has a "film plane" mark.

"Universally" is undoubtedly an overstatement; but I'm quite surprised at
the range of cameras I've found that marking on -- cheap film P&S, for
example. And it's certainly on my D200.

It's on all of our Canon DSLRs, too.

Not on my 50 year old Leica (at least not on a cursory examination)
but is on my 40 year old Yashica.

"J. Clarke" wrote in message
...
On Wed, 14 Feb 2007 16:43:05 -0800, "Skip"
wrote:

"David Dyer-Bennet" wrote in message
e.net...
Deep Reset wrote:
"Scott Schuckert" wrote in message
...
In article , Richard J
Kinch wrote:

So, anyone have any authoritative definitions to offer?
I'm not sure if it's "authoritative", but this is what I've been using
for the last 40 years: Ignore the lens itself. It's the distance from
the front surface of the bare lens mount on the front of the camera,
to
the film plane. The former should be obvious; the latter is
universally

"universally" ?

shown with a symbol - a horizontal line through a circle - inscribed
on
the top of the camera. Even my DSLR has a "film plane" mark.

"Universally" is undoubtedly an overstatement; but I'm quite surprised
at
the range of cameras I've found that marking on -- cheap film P&S, for
example. And it's certainly on my D200.

It's on all of our Canon DSLRs, too.

Not on my 50 year old Leica (at least not on a cursory examination)
but is on my 40 year old Yashica.

Hmm, are you sure? 'Cause it's on my 50+ year old Exactas, IIRC.

--
Skip Middleton
www.shadowcatcherimagery.com
www.pbase.com/skipm

On Wed, 14 Feb 2007 19:28:44 -0800, "Skip"
wrote:

"J. Clarke" wrote in message
.. .
On Wed, 14 Feb 2007 16:43:05 -0800, "Skip"
wrote:

"David Dyer-Bennet" wrote in message
se.net...
Deep Reset wrote:
"Scott Schuckert" wrote in message
...
In article , Richard J
Kinch wrote:

So, anyone have any authoritative definitions to offer?
I'm not sure if it's "authoritative", but this is what I've been using
for the last 40 years: Ignore the lens itself. It's the distance from
the front surface of the bare lens mount on the front of the camera,
to
the film plane. The former should be obvious; the latter is
universally

"universally" ?

shown with a symbol - a horizontal line through a circle - inscribed
on
the top of the camera. Even my DSLR has a "film plane" mark.

"Universally" is undoubtedly an overstatement; but I'm quite surprised
at
the range of cameras I've found that marking on -- cheap film P&S, for
example. And it's certainly on my D200.

It's on all of our Canon DSLRs, too.

Not on my 50 year old Leica (at least not on a cursory examination)
but is on my 40 year old Yashica.

Hmm, are you sure? 'Cause it's on my 50+ year old Exactas, IIRC.

If it is there it's well hidden. Bear in mind though that Leicas are
not SLRs.

"J. Clarke" wrote in message
...
On Wed, 14 Feb 2007 19:28:44 -0800, "Skip"
wrote:

"J. Clarke" wrote in message
. ..
On Wed, 14 Feb 2007 16:43:05 -0800, "Skip"
wrote:

"David Dyer-Bennet" wrote in message
use.net...
Deep Reset wrote:
"Scott Schuckert" wrote in message
...
In article , Richard J
Kinch wrote:

So, anyone have any authoritative definitions to offer?
I'm not sure if it's "authoritative", but this is what I've been
using
for the last 40 years: Ignore the lens itself. It's the distance
from
the front surface of the bare lens mount on the front of the camera,
to
the film plane. The former should be obvious; the latter is
universally

"universally" ?

shown with a symbol - a horizontal line through a circle - inscribed
on
the top of the camera. Even my DSLR has a "film plane" mark.

"Universally" is undoubtedly an overstatement; but I'm quite surprised
at
the range of cameras I've found that marking on -- cheap film P&S, for
example. And it's certainly on my D200.

It's on all of our Canon DSLRs, too.

Not on my 50 year old Leica (at least not on a cursory examination)
but is on my 40 year old Yashica.

Hmm, are you sure? 'Cause it's on my 50+ year old Exactas, IIRC.

If it is there it's well hidden. Bear in mind though that Leicas are
not SLRs.

No, but the film plane indicator was there so there could be critical
measurements from film plane to subject, for instance for macro. Parallax
could be a problem for RF cameras, at close range, at least I guess it would
be, and would make such a measurement necessary, I'd think.

--
Skip Middleton
www.shadowcatcherimagery.com
www.pbase.com/skipm

Scott Schuckert writes:

It's the distance from
the front surface of the bare lens mount on the front of the camera, to
the film plane.

OK, now define "front surface". Seems obvious on the Canon EF mount, but
on the old Canon FD mount that used an opposite "gender", the bayonet stuck
out still further from what I would call the "front surface", so does that
bayonet then count as the real "front surface"?