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Focal length...in plain language?
Anyone able to explain focal length in plain language?
JB |
#2
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Focal length...in plain language?
On Jul 24, 7:31?am, "Jackson Bryan" 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 |
#3
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Focal length...in plain language?
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 |
#4
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Focal length...in plain language?
"Jackson Bryan" wrote in message ... 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 |
#5
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Focal length...in plain language?
On Jul 24, 6:31 am, "Jackson Bryan" 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. |
#6
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Focal length...in plain language?
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 |
#7
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Focal length...in plain language?
The distance between a lens and the focused image it creates.
JJ |
#8
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Focal length...in plain language?
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 |
#9
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Focal length...in plain language?
On Jul 24, 6:48 am, WalkingMan wrote:
On Jul 24, 7:31?am, "Jackson Bryan" 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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