Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

Focal plane usually refers to the place that the lens focuses it's image projection inside the camera. It used to be the film plane, but now it's the sensor plane.

However, your thoughts on focus distance are somewhat correct - for a simple uncorrected lens, yes, its a sphere. But most complex lenses used in modern cameras are somewhat corrected for field curvature and can focus to the center of the field as well (or nearly as well) as the edges and corners. Of course, using a wide aperture will reveal flaws in design, and using smaller apertures will hide them - thanks to the differences in depth of field.

Some lens designs are highly corrected to have a flat field - macros and lenses used to copy flat subjects have very little focus difference from center to edge.

DirtFarmer Loc: Escaped from the NYC area, back to MA

A lens focuses a point in front of the lens at a point behind the lens. It could work the other way also. Think about the image on the sensor projecting out into the world. A point on the sensor will be focused at a point somewhere in front of the lens.

The design of the lens has a lot to do with the relationship between the point on the sensor and the point out in the world. Assuming the sensor is flat, most people consider that the array of points in focus out in the world would also be flat. But The relationship between the point on the sensor and the point out in the world may depend on the angle from the perpendicular from the sensor out to the world. It is possible that a plane sensor would result in a curved surface out in the world containing the points in focus. Looking at it the other way, a plane (flat) surface out in the world might focus onto a surface at the sensor that is not flat.

One of the challenges in designing lenses is to make the focal surface as close as possible to a plane (flat surface) on both sides of the lens. I am not a lens designer but I suspect that that challenge is most difficult with wide angle lenses.

Since the sensor is flat, most lenses give an image that is in focus from a plane surface in front of the camera. This is an approximation and the surface containing the in-focus points is probably not perfectly flat, but is most likely close enough to flat for all practical purposes. There could be exceptions for ultrawide lenses. At any rate, there is no real reason why the in-focus surface is spherical, mainly because the sensor is not spherical.

Took me a while to type this so Gene51 beat me to it.

Longshadow Loc: Audubon, PA, United States

Longshadow Loc: Audubon, PA, United States

Last I knew a plane was a flat surface.

The fact that one has multiple "focus points" in an image, those points are simply identifying multiple planes of focus, either in front of or behind other points (planes). Think of them as focusing "layers".

A focal plane is simply a plane through a focal point.

brontodon

Which terms am I confusing?

Longshadow Loc: Audubon, PA, United States

Re-defining physics to one's understanding of it and terms used, correct or not?
That'll ad a bit of ambiguity.

IDguy Loc: Idaho

Yes, but that is not what this discussion is about.

And what you are thinking is actually in the camera, not the lens. It is the sensor plane, which some also call the focal plane. It gets confused with the focus distance.

Here is an example confusing the use of focal plane: https://mastinlabs.com/blogs/photoism/understanding-focal-plane-in-photography

IDguy Loc: Idaho

The real reason, which I described in the opening, is that a sphere represents equal distance from the camera and thus sensor.

A spherical sensor would be a black hole.

IDguy Loc: Idaho

Image correction is for distortion. It can't fix focus. The lens has one focal distance at a time.

IDguy Loc: Idaho

That is the correct use. But not what is commonly used. e.g. : https://mastinlabs.com/blogs/photoism/understanding-focal-plane-in-photography

Longshadow Loc: Audubon, PA, United States

Addendum:
And each of the planes are parallel to each other.

Longshadow Loc: Audubon, PA, United States

The plane in which the image is PRIMARILY focused, <at> the focal point.
Depth of field is the area AROUND (in front and back) of the focal plane that is also in focus.

JD750 Loc: SoCal

Definition:
In mathematics, a plane is a flat, two-dimensional surface that extends infinitely far.
Source: wikipedia Plane_(geometry)

IDguy Loc: Idaho

Yes, we know that. Your point relative to this thread?

Once again noting DOF is actually the distance in front of and behind the sphere (not plane) of focus which is the point of this thread.

Longshadow Loc: Audubon, PA, United States

Just stating it a different way....

Sphere???