Focal Length and Angles

Hello, All!

Eventually, it may not be a problem as sensors approach sizes
that give a unitary lens factor but, is there much reason apart
from tradition to use equivalent focal lengths? I wonder how
long it would take to get people to think in angles? From the
94° of equivalent length 18mm to 3.5° at a length of 800mm, the
angles are quite distinctive and informative. Even with the
longest lenses that a non-professional is likely to afford, 400
and 800mm, the angles are 6° and 3.5°.

Before someone else suggests it, just for the record, 94, 6 and
3.5° are 1641, 107 and 61 milliradians for those who like large
numbers :-)

James Silverton
Potomac, Maryland

E-mail, with obvious alterations:
not.jim.silverton.at.verizon.not

On Thu, 05 Jul 2007 14:24:46 +0000, James Silverton wrote:

Hello, All!

Eventually, it may not be a problem as sensors approach sizes
that give a unitary lens factor but, is there much reason apart
from tradition to use equivalent focal lengths? I wonder how
long it would take to get people to think in angles? From the
94° of equivalent length 18mm to 3.5° at a length of 800mm, the
angles are quite distinctive and informative. Even with the
longest lenses that a non-professional is likely to afford, 400
and 800mm, the angles are 6° and 3.5°.

Before someone else suggests it, just for the record, 94, 6 and
3.5° are 1641, 107 and 61 milliradians for those who like large
numbers :-)

James Silverton
Potomac, Maryland

E-mail, with obvious alterations:
not.jim.silverton.at.verizon.not

I think it's primarily because folks over the last 40 or 50 years have
been thinking 35mm - so the easiest thing is to give them equivalents. At
least for telephoto, I tend to convert to magnification power - also quite
distintive.

"ray" wrote in message
news
On Thu, 05 Jul 2007 14:24:46 +0000, James Silverton wrote:

Hello, All!

Eventually, it may not be a problem as sensors approach sizes
that give a unitary lens factor but, is there much reason
apart
from tradition to use equivalent focal lengths?

I think it's primarily because folks over the last 40 or 50
years have
been thinking 35mm - so the easiest thing is to give them
equivalents. At
least for telephoto, I tend to convert to magnification
power - also quite
distintive.


That's a point! It *would* be useful to have magnification
factors for long lenses.



--
Jim Silverton
Potomac, Maryland

James Silverton wrote:
Eventually, it may not be a problem as sensors approach sizes
that give a unitary lens factor but, is there much reason apart
from tradition to use equivalent focal lengths?

No.

I wonder how
long it would take to get people to think in angles?

Several generations. Maybe.
Take weight: Germany introduced the metric system back in 1870. My mom is
still buying half a pound of cold cuts instead of 250g.
Take power: Horse Power has not been a legal unit since 1977. Still
virtually everyone is still using it instead of kW.

jue

James Silverton wrote:
That's a point! It *would* be useful to have magnification
factors for long lenses.

Kind of, except, ... well, you are just adding an additional subjective
variable: what focal length (on 35mm analog film) corresponds to
magnification factor 1:
- 45mm?
- 50mm?
- 55mm?
There are advocates for each of those and some people even argue for 40mm or
60mm.

Besides, dividing the let's say 400mm focal length by 50mm to get the 8x
magnification factor is really not that difficult.

jue

J|rgen wrote on Thu, 05 Jul 2007 17:30:52 GMT:

JE James Silverton wrote:
?? That's a point! It *would* be useful to have magnification
?? factors for long lenses.

JE Kind of, except, ... well, you are just adding an
JE additional subjective variable: what focal length (on 35mm
analog
JE film) corresponds to magnification factor 1:
JE - 45mm?
JE - 50mm?
JE - 55mm?
JE There are advocates for each of those and some people even
JE argue for 40mm or 60mm.

JE Besides, dividing the let's say 400mm focal length by 50mm
JE to get the 8x magnification factor is really not that
JE difficult.

Admittedly, you could argue for factors other than 50 but it's
an easy one to use mentally and absolute precision is not all
that important if you can accept "equivalent focal lengths".

James Silverton
Potomac, Maryland

E-mail, with obvious alterations:
not.jim.silverton.at.verizon.not

On Jul 5, 8:24 am, "James Silverton"
wrote:
Hello, All!

Eventually, it may not be a problem as sensors approach sizes
that give a unitary lens factor but, is there much reason apart
from tradition to use equivalent focal lengths? I wonder how
long it would take to get people to think in angles? From the
94° of equivalent length 18mm to 3.5° at a length of 800mm, the
angles are quite distinctive and informative. Even with the
longest lenses that a non-professional is likely to afford, 400
and 800mm, the angles are 6° and 3.5°.

Before someone else suggests it, just for the record, 94, 6 and
3.5° are 1641, 107 and 61 milliradians for those who like large
numbers :-)

James Silverton
Potomac, Maryland

E-mail, with obvious alterations:
not.jim.silverton.at.verizon.not

I agree with you: angle of view would be the best way to go, but I
also agree with others that the 35mm "standard" has become so
ingrained that it may take quite some time before it's forgotten.

-Karl Winkler

In article Oa7ji.3387$Np2.3255@trnddc07,
says...

Eventually, it may not be a problem as sensors approach sizes
that give a unitary lens factor but, is there much reason apart
from tradition to use equivalent focal lengths? I wonder how
long it would take to get people to think in angles? From the
94° of equivalent length 18mm to 3.5° at a length of 800mm, the
angles are quite distinctive and informative. Even with the
longest lenses that a non-professional is likely to afford, 400
and 800mm, the angles are 6° and 3.5°.

Angles are not distinctive, and would not help the "equivalent" problem
you mention. You are quoting lens angles for 35 mm film frame cameras,
but the angle for one lens would be a different angle for the same lens
on a different sensor size... the same 1.5x factor or whatever
(inversely for angles). This differing angle is the reason why we need
the concept of equivalent focal length.

Most of us are already used to what 28mm eqivalent means (it wasnt that
long ago). We may not be able to state the exact degrees, nor do we
care about degrees, but we do know the situation that it will cover.
And focal length is marked on the lens (35 mm film reference). So we
really dont need some new system with the same problem.

But agreed, the time will come someday when no one has ever seen a 35 mm
film camera, and the reference will be hard to explain then. But the
angles of those same 35 mm film camera lens wont be any better help.

--
Wayne
http://www.scantips.com "A few scanning tips"

Wayne wrote on Fri, 06 Jul 2007 01:26:10 GMT:

?? Eventually, it may not be a problem as sensors approach
?? sizes that give a unitary lens factor but, is there much
?? reason apart from tradition to use equivalent focal
?? lengths? I wonder how long it would take to get people to
?? think in angles? From the 94° of equivalent length 18mm to
?? 3.5° at a length of 800mm, the angles are quite
?? distinctive and informative. Even with the longest lenses
?? that a non-professional is likely to afford, 400 and
?? 800mm, the angles are 6° and 3.5°.

W Angles are not distinctive, and would not help the
W "equivalent" problem you mention. You are quoting lens
W angles for 35 mm film frame cameras, but the angle for one
W lens would be a different angle for the same lens on a
W different sensor size... the same 1.5x factor or whatever
W (inversely for angles). This differing angle is the reason
W why we need the concept of equivalent focal length.

W Most of us are already used to what 28mm eqivalent means (it
W wasnt that long ago). We may not be able to state the exact
W degrees, nor do we care about degrees, but we do know the
W situation that it will cover. And focal length is marked on
W the lens (35 mm film reference). So we really dont need
W some new system with the same problem.

W But agreed, the time will come someday when no one has ever
W seen a 35 mm film camera, and the reference will be hard to
W explain then. But the angles of those same 35 mm film
W camera lens wont be any better help.

I can't really disagree with what you say but the "equivalency"
values are mostly used for 35mm type cameras and by those who
use them. An angle does give an exact measure of what the lens
will cover. I suspect those people using larger cameras would be
professionals with a deeper knowledge and I just wondered if the
time was coming when the traditional notation would be given up
but I'll admit that conservatism is a very powerful emotion in
many fields.

James Silverton
Potomac, Maryland

E-mail, with obvious alterations:
not.jim.silverton.at.verizon.not

"ray" wrote in message
news
On Thu, 05 Jul 2007 14:24:46 +0000, James Silverton wrote:

Hello, All!

Eventually, it may not be a problem as sensors approach sizes
that give a unitary lens factor but, is there much reason apart
from tradition to use equivalent focal lengths? I wonder how
long it would take to get people to think in angles? From the
94° of equivalent length 18mm to 3.5° at a length of 800mm, the
angles are quite distinctive and informative. Even with the
longest lenses that a non-professional is likely to afford, 400
and 800mm, the angles are 6° and 3.5°.

Before someone else suggests it, just for the record, 94, 6 and
3.5° are 1641, 107 and 61 milliradians for those who like large
numbers :-)

James Silverton
Potomac, Maryland

E-mail, with obvious alterations:
not.jim.silverton.at.verizon.not

I think it's primarily because folks over the last 40 or 50 years have
been thinking 35mm - so the easiest thing is to give them equivalents. At
least for telephoto, I tend to convert to magnification power - also quite
distintive.

Each way of referencing the lens view has its own plusses and minuses that
may be of different importance to different photogs.

For those who have become used to many decades of single focal length lenses
on a 35mm film camera they have trained their "eye" to compose an image with
relation to the focal length of a particular lens. For these people the 35mm
equiv number is of great use. When looking at a scene they can "see" that
they need a particular FL to catch the scene they want. Then by converting
to the digital version they can catch the scene to match what is in their
mind.

For those of us who were early adopters of zoom lenses and who were used to
composing in the eyepiece and zooming to match our intended view (with
almost no concideration of the precise FL used) the equiv numbers are of
little concern. We can still compose in the eyepiece.

For the idea of using "angle of view" this could be a good idea, but when
you figure that the same lens may be used on several cameras with different
sensor sizes and thus different angles, the number can become less than
useful. Also many of the formulas for things like DOF and such depend on a
standard FL across all cameras. Also we would have to standardize what
"angle" we are talking about. A simple statement of an angle of 45 deg of
view could mean 45 degrees along the horizontal plane, the vertical plane or
the diagonal.

So as I said each of these ideas has their own plusses and minuses. Without
any overwhelming reason to change, the "old method" will probably prevail.
Of course this means that when an EXIF shows the image was taken with a
particular camera and a 85mm lens, anyone who wants precision measurements
will have to learn the specifics of the sensor dimensions for that camera
and do the math to find the specific angle of view along the particular
plane of interrest for that particular image. Most of us won't worry about
it too much.

JMHO

Randy

==========
Randy Berbaum
Champaign, IL