How does photography (as opposed to physics) define "focal-plane"?

Question

This wikipedia article concerning focal-plane shutters opens with the following statement (bold emphasis added by myself):

In camera design, a focal-plane shutter (FPS) is a type of photographic shutter that is positioned immediately in front of the focal plane of the camera, that is, right in front of the photographic film or image sensor.

Other references concerned with photography almost universally seem to equate focal plane and film plane. This statement is from the wikipedia article concerning focal length (bold emphasis added by myself):

When a photographic lens is set to "infinity", its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane.

In both of these references the meaning of focal plane and image plane appear to be interchangeable when the distance from the lens' rear nodal point to the film/sensor plane is equal to the lens' focal length.

Yet in physics references that discuss the field of optics (such as this wikipedia article linked from the article cited above) the focal plane is defined differently. It seems the front focus plane is defined as what photography refers to as the plane of focus. The back focus plane seems to be defined as the point behind the lens where the aperture is ideally situated. Yet the charts and diagrams on the same page seem at times to use the terms rear focal plane, back focal plane, and image plane interchangeably. Sometimes it seems they are referring to the plane where the image is brought into focus (such as this diagram and the text beneath it in the article). At other times it seems to be referring to the crossover point between the lens and the image plane (such as this diagram and the text next to it in the article).

When speaking in the context of Photography that uses film or a digital sensor to record an image projected onto that film or image sensor is it legitimate to refer to the film or sensor as occupying the Focal Plane? Is there a nomenclature that allows one to distinguish between the plane occupied by the objects depicted as in focus in the image (plane of focus?, front focal plane?) and the plane the in-focus image is projected on (rear focal plane)?

Answer 1

EDIT: I have studied the focal plane vs. image plane issue more. The description below the === line is true in optics, and sometimes in specialized photography.

In generic use,

• (optics) image plane = (photography) focal plane (e.g. see Nikon's focal plane mark or this article). This is the sensor plane, this is the plane where if you put a piece of paper, the image is considered "in-focus", sharp.

_{From page 98 of the Nikon D7200 User Manual}

• (optics) focal plane/ back focal plane = has no equivalent in generic photography. Probably because your diaphragm/shutter is sitting there, you cannot mess with it, so a term was not needed.

Notice also the two definitions here.

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Let me walk you through the focal plane and image plane definition in optics (!!), where these are two distinct planes.

Let's start with: http://upload.wikimedia.org/wikipedia/commons/thumb/e/e3/Cardinal-points-1.svg/500px-Cardinal-points-1.svg.png

You can see two focus points, F and F'. The function of those: if you emit light from either, let's say F', you will see parallel light rays on the other side of the lens. Conversely, an object that is infinitely far away, will emit rays that are parallel and all those will go through point F'. Now, that's great. Why? Because all those rays go through a concentrated, small space, so you can put a focal-plane shutter there, you can control aperture using a diaphragm , and since all rays go through that point F', you do not have to worry about how big lens you have (maybe 77 mm? maybe 1 m?), it is very easy to let rays pass through there or stop them from propagating.

Okay, so that is point F'. You use only one of the focal points, obviously, so you do not care about F.

A focal plane is just the plane that is perpendicular to the optical axis of the lens, and it goes through point F'. You basically put the diaphragm, the focal-point shutter directly in the focal plane.

Please note:

• parallel light rays, that are also parallel to the optical axis, will converge to the focal point.
• parallel light rays, that are in angle with the optical axis, will converge a point on the focal plane.

(see animation - sometimes just a static image...)

The film or sensor is in the image plane. This is NOT the focal plane at all in optics.

If you put the film or sensor in the focal plane, you would ideally have a single tiny white spot on the sensor. Since the lens is not ideal, you would see a very blurred small spot, that's all.

Now, I hope this is all clear, sorry for my English.

If you understood and agreed with what I wrote, here is another link: http://en.wikipedia.org/wiki/File:BackFocalPlane.svg

The rays going vertically from the "object" are the parallel rays. There are two red ones and the black axis. All these go through the intersection of the back focal plane and the optical axis (vertically, the symmetry axis).

If the object is not infinitely far away, you will see the other lines emanating from the object. Those go through the back focal plane, and you can see, they are close to the focal point, yet those light rays create a bright spot on the focal plane.

On the image plane, all rays, that depart from the same object point, arrive in the same image plane point. This is important: this is the requirement for an image to be in focus, to be sharp. (It is kind of obvious: if a point on an object created multiple image-points, e.g. a tip of a needle creates a big spot - then that is obviously out of focus).

I hope my description is clear.

Answer 2

When speaking in the context of Photography... is it legitimate to refer to the film or sensor as occupying the Focal Plane?

I would say, yes. Even though they are different things.

After here, sensor will mean digital image sensor or film.

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Referring to my answer here, the position of the sensor matters! You can focus on something by moving the sensor back and forth, while having the lens stationary. You are adjusting the focal points by moving the focal plane, in which case can be the sensor (or image plane).

As an example, most prime lenses for film cameras, turning the focus ring will move all lens elements away/towards the sensor (or image plane). So, moving the image plane (or sensor) away/towards the lens elements will have the same effect.

I've actually done this on my OM-D and an OM 50mm f/1.4 lens. I turn the focussing ring over to it's minimum focussing distance, cup one hand over the back of the lens an hold it down on the table while moving the OM-D back and forth with my other hand.

There I saw it. It had the same effect! (As focussing the conventional way) Although, not very practical.

EDIT: But, I guess in terms of photography, we adjust the focal plane with the lens. We don't adjust it with the image plane.