Field of viewhuman eye

Focal length is NOT at all about the mounting flange distance. The internal focus node can be moved by design. A "telephoto" lens means that node is slightly in front of the front lens element (lens is shorter than the focal length). A retro-focus (wide angle) lens places that focus node behind the rear element, to create space behind the lens. But focal length is to that focal node.

In photography, it's the accepted norm to say that if you know the sensor size of your camera and the focal length of your lens, then you know the field of view of your system.

Field of view vs focal lengthreddit

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However, in optics, it's well known that for a fixed sensor size, the distance from the lens to the image plane (which we can equate with the flange distance) also affects your field of view. Everyone who mounted his\her lens on a macro adapter had seen that increasing the flange distance will narrow down your field of view, and they will end up with a macro lens.

FOV tofocal lengthcalculator

The very good reason for cameras is that field of view also strongly depends on the sensor size. A small sensor captures a more narrow view than a larger sensor can.

Field of view vs focal lengthcamera

The focal length marked on the lens is when focused at infinity. Some imagine that is the definition of focal length, but it is merely one setting for it. Zoom lenses also vary focal length.

When focused closer than infinity, the lens is extended forward (the frontal elements are, or possibly only internal elements) so that the internal focus node in the lens is further from the sensor plane. If we instead assume focal length is the distance from this internal node to the sensor plane, then of course the focal length is a bit longer when focused closer. That longer focal length changes things, like f-number, which can affect exposure, so regular lenses don't allow distances shorter than some nominal close distance, typically at around 0.1x magnification.

These two properties are completely independent of each other, but changing either of them will change the effective FOV - this is a fact of geometrical optics.

This question is predicated on a misconception. The flange distance is included in the focal length in lens labels — it is the distance from the optical center of the lens focused at infinity to the imaging medium. This includes the flange distance. (See What is the reference point that the focal length of a lens is calculated from?, and What exactly is focal length when there is also flange focal distance?)

This means that for a given sensor size, lenses made for different mounts are still comparable — a 24mm lens gives (approximately) the same field of view regardless of the mount distance of any given system. So, focal length does correspond to field of view.

Please try to answer my original question instead of trying to educate me about optics - that part of the conversation I got down pretty good :)

Field of viewcamera

Field of view vs focal lengthcanon

My question was: given that both parameters affect the FOV, why is it then that we attribute only the focal length to the FOV of the lens?

What isfocal length oflens

I realize that it could be the case that it was just an arbitrary historical decision, but I'm curious whether there's justification for that which I'm missing.

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The lens focal length does provide some field of view, which the sensor size crops to capture possibly a lesser amount of it. Cameras with tiny sensors only capture a small field, so they have to use a much shorter focal length lens to compare to an expected "regular picture" view seen by other cameras with larger sensors. Crop Factor compares that sensor size view to the historical 35 mm film frame size view that so many of us are very familiar with.

In the link above it can be seen that the angle of view depends on S2. If S1 is large, F and S2 are approximately the same, and one can say that the field of view is related to the focal length. In macro photography the subject distance S2 is not very large, therefore it has a significant influence, and the field of view is no longer related to the focal length of the lens. Because we are used to relating the field of view to the focal length, we define the effective focal length f, as the image distance. In that case the field of view is related to the effective focal length, which depends on the subject distance.

Focal length and lens FOV are inherently the same thing. The variable is how much of the lens' FOV is utilized (i.e. extension tubes/TC's/crop sensor/etc). Why would you specify a lens' characteristic by something that may be variable?

Field of viewcalculator

From the physics POV of it all, what determines the effective FOV for a given sensor is the effective focal length of the lens and the distance between the back principal plane of the lens to the sensor, which is determined unambiguously by the distance between the principal plane and the flange of the lens plus the distance between the flange of the camera and the sensor.

Unlike the angle of coverage, the field of view, or angle of view are not lens properties. They depend on the focal length of the lens, the size of the sensor and the distance between the rear focal point and the sensor. When a lens is focused at infinity, the distance between the rear focal point and the sensor is zero. When the lens is focused on something closer, the sensor is placed behind the focal point. The location can be determined with the thin lens formula:

Lenses are characterized by their optical and physical properties. Field of view is not an optical property, but a consequence of the camera system to which the lens is attached. The same 50mm lens has different fields of view when paired with differently sized sensors, such as crop frame vs full frame.

1/F = 1/S1 + 1/S2, Where F is the focal length of the lens, ans S1 and S2 are the subject and image distance respectively. https://en.wikipedia.org/wiki/Angle_of_view

You note that using a macro adapter affects field of view. In fact, they also increase the focal length, although since macro extension tubes remove the ability to actually focus at infinity, we leave practical reality and get into the realm of theory.

To further clarify by an example: you can have a 50mm and a 40mm lenses both with the same distance between the principal plane and the flange of the lens. In this case, if you place the 40mm lens sufficiently further from the sensor (compared to the 50mm lens) you will have the same effective FOV.