Been spending a lot of time on here!
- Nov 27, 2011
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We use two different terms; angle of view (infinity focus measured in degrees) and field of view (calculated for the specific object distance closer than infinity measured in width of the subject).As I continue to search/read online, I have now reached a better understanding of the behavior of a theoretical single ideal lens camera.
There, focus is achieved by the ratio between the actual distance lens to sensor distance and the focal length of the lens. When that ratio is 1 the camera is focused on infinity. The viewing angle is determined by the actual distance from lens to sensor, not by the len's focal length.
So in that idea single lens camera, the distance you focus at affects the viewing angle.
Don't see why that would be for a fixed focal length lens.That seems to be less true of modern cameras.
Only with a zoom lens because a zoom lens can change it's focal length -- it zooms. In that case it would be possible to establish a fixed FOV and maintain that for different object distances by zooming the lens to change focal length.To what extent is it untrue, and why, especially with a zoom lens?
For example a 35mm lens on a FF camera has an AOV of 54 degrees. Focused at 10 feet the FOV is 10.3 feet (width). A 50mm lens on a FF camera has an AOV of 39.6 degrees and focused at 14.3 feet the FOV is 10.3 feet (width). A 24-70mm zoom can be set to 35mm or 50mm focal lengths.
Angle of view changes with focal length and format size. To the extent that a zoom lens can change focal length it can change AOV.In other words, to what extent is the viewing angle stable for a lens focal length (as set or reported for the zoom lens)?
I tried the simple experiment: at the max zoom, focus far away, point without refocusing at something just a few feet away and try to see the edges of the blurred image, then refocus, seeing those edges go slightly out of the field of view. So at a fixed lens focal length, the viewing angle is reduced by focusing on a nearer object.
So I need to take that effect into account as well, as I try to understand the relationship between lens focal length, object distance (from sensor) and object width.
I hope/expect that effect is smaller than it would be in a theoretical single lens camera. But I'll need a lot of measurement to find out if that is true. I'm trying to figure out what experiments would measure such things without much error. Any suggestion?
I still want to know where the apex of the viewing angle is. Finding both the viewing angle and its apex (given both unknown) requires test photos at two different distances. If those distances are large and not very different from each other, then the change in viewing angle would be trivial, but any measurement error would be magnified in the process of converting to location of the viewing angle apex. If those distances were close then ordinary measurement error has little impact on finding the apex, if the angle were either constant or known. But the angle isn't constant when focusing at a short distance.
BTW, in that single element lens, the apex of the viewing angle is the center of the lens. My testing hasn't yet been far off from the theory that the apex of viewing angle in my camera is the center of the frontmost (furthest from the sensor) element of the lens. That certainly was not near true of my previous camera. But if it works as a decent approximation for this one, that will simplify a lot of what I'm trying to figure out.