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#1
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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
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what does the focal distance actually mean?
"scott" wrote in message
news:NfHEc.276$WO5.171@newsfe3-gui... When you see a 200mm lens, what is the 200mm actually a measure of? I assume it's something to do with how much "zoom" there is, but surely measuring the angle that is shown by the image would be better? Eg a wide-angle lens would be a 90 degree field of view but a telephoto one would be maybe 5 or 10 degrees. In another dimension (pardon the pun) perhaps lenses could be referred to by their angle of view rather than focal length... it's probably a male thing. Anyway, nowadays everyone is accustomed to the mm scale, even nations so stubbornly Imperial in their measurements, that a change to a figure in degrees would be greeted with a reception akin to a sudden change of currency. -- Martin Francis http://www.sixbysix.co.uk "Go not to Usenet for counsel, for it will say both no, and yes, and no, and yes...." |
#3
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what does the focal distance actually mean?
"scott" wrote in news:NfHEc.276$WO5.171@newsfe3-gui:
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
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what does the focal distance actually mean?
"scott" wrote in message news:NfHEc.276$WO5.171@newsfe3-gui... 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
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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. -- David Dyer-Bennet, , http://www.dd-b.net/dd-b/ 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/ Dragaera/Steven Brust: http://dragaera.info/ |
#6
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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
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what does the focal distance actually mean?
"scott" wrote in message news:NfHEc.276$WO5.171@newsfe3-gui...
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
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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
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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
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what does the focal distance actually mean?
Bart van der Wolf wrote:
"scott" wrote in message news:NfHEc.276$WO5.171@newsfe3-gui... 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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