OK, so is focal length is the point when the image is focused, or is st when the light beams converge?
Have you ever taken a magnifying glass, hold it so the sun shines thru it onto a piece of paper? The bright spot is an image of the sun! And the distance from the center of the lens to the paper is the 'focal length'. Modern camera optics are much more complex that that magnifying glass, but the principle is identical!
I don't see where the OP asked about angle of view. :confused:Practically speaking, the longer the focal length, the narrower the angle of view.
I don't see where the OP asked about angle of view. :confused:
Angle of view is not related to focal length except by general grouping. It's a function of lens design.
- Leigh
OK. I'll go along with the 'illustrations' thing.And for most people, who don't have the advantage of camera movements, format plus focal length determines the angle of view.
Focal length is the distance along the lens axis from the rear node of the lens to the film when focused at infinity.
The rear lens node is at the intersection of the lens axis and the second principle plane, which may be anywhere inside or outside of the lens. It's a design feature that's usually specified on the lens spec sheet.
The rear lens node is the point within the optics from which all exiting paraxial rays appear to emanate.
In contrast, the front lens node, at the intersection of the lens axis and the first principle plane, is the point on which all incoming paraxial rays appear to converge.
The front and rear nodes are two distinctly different points. Both can be anywhere on the optical axis, and the rear node can be behind, on, or in front of the front node.
In the Nikkor-T ED 270mm/6.3 for example, the first principle plane is 35.1mm in front of the front lens element, while the second principle plane is 9.6mm behind that, or 25.5mm in front of the front element.
- Leigh
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