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what does the focal distance actually mean?



 
 
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  #1  
Old June 30th 04, 10:36 PM
scott
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Default what does the focal distance actually mean?

When you see a 200mm lens, what is the 200mm actually a measure of? I
assume it's something to do with how much "zoom" there is, but surely
measuring the angle that is shown by the image would be better? Eg a
wide-angle lens would be a 90 degree field of view but a telephoto one would
be maybe 5 or 10 degrees.

Is it possible to work out the field of view in degrees from the focal
length?

TIA

Scott


  #2  
Old June 30th 04, 11:04 PM
Martin Francis
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Default what does the focal distance actually mean?

"scott" wrote in message
news:[email protected]
When you see a 200mm lens, what is the 200mm actually a measure of? I
assume it's something to do with how much "zoom" there is, but surely
measuring the angle that is shown by the image would be better? Eg a
wide-angle lens would be a 90 degree field of view but a telephoto one

would
be maybe 5 or 10 degrees.


In another dimension (pardon the pun) perhaps lenses could be referred to by
their angle of view rather than focal length... it's probably a male thing.
Anyway, nowadays everyone is accustomed to the mm scale, even nations so
stubbornly Imperial in their measurements, that a change to a figure in
degrees would be greeted with a reception akin to a sudden change of
currency.

--
Martin Francis http://www.sixbysix.co.uk
"Go not to Usenet for counsel, for it will say both no, and yes, and
no, and yes...."


  #3  
Old June 30th 04, 11:18 PM
Roland Karlsson
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Default what does the focal distance actually mean?

"scott" wrote in news:[email protected]:

When you see a 200mm lens, what is the 200mm actually a measure of?


The focal length of a thin lens is the distance from
the lens (or rather the centre of the lens) to the
picture plane if the object you are imaging is at
infinity.

The focal lenghth of a thick lens (such as you find in
your camera) is defined as the focal length of an imaginary
thin lens that gives the same size picture in the focal plane.

And yes - angles would be nice. But - to define the view angle
you must also provide the sensor/film size. So - the actual
meassure for the lens is the focal length and not any angle.

I agree though that it would be nice, to be able to compare
cameras easier, that the max view angle would be given instead.
But evan that is not perfect as the sensors migh come in different
shapes, e.g. 1:1, 2:3, 3:4, ...


/Roland
  #4  
Old June 30th 04, 11:21 PM
Bart van der Wolf
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Default what does the focal distance actually mean?


"scott" wrote in message
news:[email protected]
SNIP
Is it possible to work out the field of view in degrees from
the focal length?


Yes, if you know the sensor size, at infinity focus it's:
ATAN( sensor_size / 2 / focal_length) * 2 = FOV

Bart

  #5  
Old July 1st 04, 12:19 AM
David Dyer-Bennet
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Default what does the focal distance actually mean?

"scott" writes:

When you see a 200mm lens, what is the 200mm actually a measure of?
I assume it's something to do with how much "zoom" there is, but
surely measuring the angle that is shown by the image would be
better? Eg a wide-angle lens would be a 90 degree field of view but
a telephoto one would be maybe 5 or 10 degrees.


Focal length is a real optical property, basic to the lens. Angle of
view isn't; it's the result of cropping the image circle to some
particular film format -- and assumes that the lens throws an image
circle big enough for that film format. (The image circle isn't
simply a function of the focal length, for real-world complex lens
systems like what we actually use on cameras.)

The focal length is needed to calculate aperture, the depth of field,
and a number of other things. So going just by angle of view isn't
really enough.

Is it possible to work out the field of view in degrees from the focal
length?


Yes, for some particular given film format.
--
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RKBA: http://noguns-nomoney.com/ http://www.dd-b.net/carry/
Pics: http://dd-b.lighthunters.net/ http://www.dd-b.net/dd-b/SnapshotAlbum/
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  #6  
Old July 1st 04, 02:24 AM
Bob
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Default what does the focal distance actually mean?

On Wed, 30 Jun 2004 22:36:49 +0100, "scott" wrote:

When you see a 200mm lens, what is the 200mm actually a measure of?


Focal length.

You know when you take the lens off of your camera and use it burn ants on the
sidewalk by using light from the sun?

The focal length is the distance from the center of the lens to the ant...



I assume it's something to do with how much "zoom" there is, but surely
measuring the angle that is shown by the image would be better?
Eg a
wide-angle lens would be a 90 degree field of view but a telephoto one would
be maybe 5 or 10 degrees.

Is it possible to work out the field of view in degrees from the focal
length?

TIA

Scott


  #7  
Old July 1st 04, 06:52 AM
dj NME
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Default what does the focal distance actually mean?

"scott" wrote in message news:[email protected]
When you see a 200mm lens, what is the 200mm actually a measure of? I
assume it's something to do with how much "zoom" there is, but surely
measuring the angle that is shown by the image would be better? Eg a
wide-angle lens would be a 90 degree field of view but a telephoto one would
be maybe 5 or 10 degrees.

Is it possible to work out the field of view in degrees from the focal
length?

TIA

Scott


My understanding of this is that the stated focal length of a camera
lens is the distance a pinhole needs to be from the film plane to
produce an image the same size on 35mm film.
What this means is that as the film (or sensor) size goes down (like
the smaller dslr aps sized sensors and tiny p&s 2/3 sensors) the
realative focal length goes up and as the film size goes up
(120/meduim format and up to large format and beyond for plate
cameras) the realtive focal length goes down.

Just my 2c
  #8  
Old July 1st 04, 04:22 PM
Don Stauffer
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Default what does the focal distance actually mean?

We must be careful to differentiate between zoom and focal length.

A little history of photographic lenses may help here. But first of
all, consider that the longer the focal length is, the larger will be
the image (in physical size) for a given distance for a given object.

Now, I will ignore the original meaning of the term 'telephoto' in
camera lenses as being unimportant for now. But it came to mean a lens
with a longer focal length than a 'standard' lens. So it magnified the
image. Or, in other words, made it look like you were closer to the
object when you took the picture. Sometimes people talked about the
power of a telephoto lens, as the ratio of that lens to a 'normal' or
standard lens (50-55mm for a 35 mm camera).

Photographers used to carry around a whole bag of lenses with different
focal lengths.

Then, they perfected the zoom lens. A zoom lens is one that by moving
various lens elements inside the lens can have different focal lengths.
People talked about the zoom range as a 3X or 4X zoom range. Sometimes
they called this 'power', but that was not really a good idea. It is the
same as the older meaning of the term 'power' if the minimum focal
length for that zoom is the standard lens size (say 50mm for a 35mm
camera).

Today, we may have a 33 to 100 mm zoom lens as having a 3X power zoom,
but that is not the same as saying that in the 100 mm position it is 3X
power (magnification), since the ratio of 100 to 50 is 2X, not 3X.

So today a zoom lens of such and such 'power' means the ratio of longest
to shortest focal lengths, NOT the magnification of the image at max
zoom compared to normal lens.

BTW, for any format, the old meaning of standard or normal lens was a
focal length approximately equal to the diagonal distance of the format
(film frame size or image chip active area size).

scott wrote:

When you see a 200mm lens, what is the 200mm actually a measure of? I
assume it's something to do with how much "zoom" there is, but surely
measuring the angle that is shown by the image would be better? Eg a
wide-angle lens would be a 90 degree field of view but a telephoto one would
be maybe 5 or 10 degrees.

Is it possible to work out the field of view in degrees from the focal
length?

TIA

Scott


--
Don Stauffer in Minnesota

webpage-
http://www.usfamily.net/web/stauffer
  #9  
Old July 1st 04, 08:20 PM
Dave Martindale
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Default what does the focal distance actually mean?

"Bart van der Wolf" writes:

Yes, if you know the sensor size, at infinity focus it's:
ATAN( sensor_size / 2 / focal_length) * 2 = FOV


Of course, the "sensor_size" in the above can be the horizontal width
or the vertical height of the sensor, which gives you horizontal and
vertical field of view. Using the sensor diagonal tells you the angular
covering power of the lens.

Dave
  #10  
Old July 1st 04, 09:02 PM
scott
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Default what does the focal distance actually mean?

Bart van der Wolf wrote:
"scott" wrote in message
news:[email protected]
SNIP
Is it possible to work out the field of view in degrees from
the focal length?


Yes, if you know the sensor size, at infinity focus it's:
ATAN( sensor_size / 2 / focal_length) * 2 = FOV


Thanks for that, and for all the other replies guys. I just need to draw a
few diagrams for myself now to make it a bit clearer!


 




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