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Focal length...in plain language?

 
 
Jackson Bryan
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      07-24-2007
Anyone able to explain focal length in plain language?

JB


 
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WalkingMan
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      07-24-2007
On Jul 24, 7:31?am, "Jackson Bryan" <(E-Mail Removed)> wrote:
> Anyone able to explain focal length in plain language?
>
> JB


Distance from the lens to the film, in millimeters (Simple enough?).
Due to the lens configuration, the physical distance is not always
same as the stated length.
Marshel

 
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Jürgen Exner
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      07-24-2007
Jackson Bryan wrote:
> Anyone able to explain focal length in plain language?


From http://en.wikipedia.org/wiki/Focal_length:
The focal length of an optical system is a measure of how strongly it
converges (focuses) or diverges light. A system with a shorter focal length
has greater optical power than one with a long focal length.

jue


 
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Roy G
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      07-24-2007

"Jackson Bryan" <(E-Mail Removed)> wrote in message
news:f84nuc$cdf$(E-Mail Removed)...
> Anyone able to explain focal length in plain language?
>
> JB
>


If the lens was a single piece of Convex Glass, then its Focal length would
be the distance between the centre of the glass and a sharply focussed image
of the Sun produced by it.

The precise definition requires parallel rays of light falling on the lens,
but for most practical purposes, the rays from the Sun can be considered as
parallel.

A Concave lens cannot produce a sharply focussed image of the sun behind it,
so it ends up with a negative Focal length, but I can not remember how the
figure is calculated.

Every real life Camera lens has a number of pieces of glass within it, so
the focal length is computed from a theoretical point, which may not even be
within the physical lens barrel.

Most Telephoto lenses are designed to be shorter than their effective Focal
length, and most Wides are designed to be longer.

Roy G


 
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Don Stauffer in Minnesota
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      07-24-2007
On Jul 24, 6:31 am, "Jackson Bryan" <(E-Mail Removed)> wrote:
> Anyone able to explain focal length in plain language?
>
> JB



Maybe the "optical triangle will help. Draw a triange from the center
of the lens to the edges/corners of the scene being photographed.
There will be a similar triangle (same angles) from the center of the
lens to the edges of the format (active area of the CCD). The object
distance is the distance from the center of the lens to the objects in
the scene. The image distance is the distance from the center of the
lens to the image plane. In optics, to simplify the drawing, the
"object" is usually considered to be an arrow perpendicular to the
line from the lens to the center of the arrow.

Now, if the object(s) is/are located at or near an infinite distance-
that is, very far from the camera, the image distance will be the
focal length. That is, the focal length of the lens is the distance
from the lens to the image when the lens is imaging a very, very
distant object.

In anything other than macro photography, the image distance is just a
little further from the lens than the focal length.

 
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Allen
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      07-24-2007
Jackson Bryan wrote:
> Anyone able to explain focal length in plain language?
>
> JB
>
>

For a simple lens (one element, or piece of glass) the lens is pointed
at a far-away object (approaching infinity). Then a flat surface is put
behind the lens and moved back and forth until the point is found at
which the image of that object appears sharpest on that surface. The
distance between the center of the lens and the focusing surface is
measured; that distance (in millimeters, inches, feet, meters--any unit
of length) is the focal length of the lens.

With complex lenses (more than one piece of glass) it becomes more
problematic. The _optical_ center of the lens must be determined. For
fairly simple lenses (not extreme wide-angle, telephoto, or especially
zoom) the optical center will usually be fairly close to the physical
center. For those exceptions, which contain elements with _negative_
focal lengths) the calculations become much more complex, and the
average person is at the mercy of the lens designer or marketer as to
where the optical center is located. The focal lengths of these lenses
is actually "backed into" by the effect the lens has on the image size.
For instance, going back to 35mm days, a lens with a focal length of 50
mm was generally accepted as standard. A lens that produces an image
twice as large as the 50mm is 100mm. Say a lens is marked as 500 mm:
this means the image is twice as large, and the _optical_ center of the
lens is actually half a meter (20 inches) in front of the physical
center. This gets very, very complicated and quite often
confusing--optics isn't a simple study.

I hope this helps you rather than confuse you.

Allen
 
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JJ
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      07-24-2007
The distance between a lens and the focused image it creates.
JJ


 
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Joseph Meehan
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      07-24-2007
JJ wrote:
> The distance between a lens and the focused image it creates. (Of a
> subject really really far away.)
> JJ


--
Joseph Meehan

Dia 's Muire duit



 
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AustinMN
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      07-24-2007
On Jul 24, 6:48 am, WalkingMan <(E-Mail Removed)> wrote:
> On Jul 24, 7:31?am, "Jackson Bryan" <(E-Mail Removed)> wrote:
>
> > Anyone able to explain focal length in plain language?

>
> > JB

>
> Distance from the lens to the film, in millimeters (Simple enough?).


Too simple...that would mean the focal length changes with subject
distance. It doesn't. A more accurate definition would be the
distance between the optical center of the lens and the film plan
*when focused on an object at infinity*.

> Due to the lens configuration, the physical distance is not always
> same as the stated length.


Agreed.

Austin

 
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