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.