Solve a Disagreement, Smart Guys - Is the Focus Plane Actually a Plane?

Hi everyone -

I got to discussing hyperfocal distances over the weekend with a friend who has recently become interested in photography and when I said the words "focus plane" his younger brother, who is too smart for his own good (an electrical engineering graduate as of this spring), decided he disagreed with the term "focus plane" and the entire idea that it would be parrallel to the sensor.

I argued that he was confusing the focus plane at your focusing distance with the shape of the resulting depth of field, but I'm no engineer - optical or other - and I couldn't formulate an an explanation that he could come to terms with. So, my POTN friends, what say you on this matter? Care to educate me on why I'm wrong or how I could help explain it better to him?

Thanks!

so what does he think it is if it's not parallel to the sensor? or is he just the kind of person that says something is wrong, without having any clue how to back it up...

what do you mean by 'the shape of the resulting depth of field'?

I'd also like to know what he thought it was if not a plane.

I knew I read this somewhere before...

https://photography-on-the.net/forum/showthre​ad.php?t=621561

He argued that it was a curve that could be perceived as a straight line as focal length increased, but a curve none-the-less...and more of a curve at wider focal lengths.

MNUplander wrote in post #14320752
He argued that it was a curve that could be perceived as a straight line as focal length increased, but a curve none-the-less...and more of a curve at wider focal lengths.

I think that would be accurate as long as the given lens is not optically perfect.

Well, given that DoF implies three dimensions, one might argue that it's not truly a plane. As a result it may be that it's more of a polyhedron than a plane. Although, I guess you could argue that there is a plane which just happens to exist somewhere near the center of your DoF.

Why he would argue that it isn't parallel to the sensor, I can't guess.

mplezia wrote in post #14320784
Why he would argue that it isn't parallel to the sensor, I can't guess.

He simply meant that in order for it to be parrallel it would have to be a plane, a curve cannot be parrallel to a line.

MNUplander wrote in post #14320787
He simply meant that in order for it to be parrallel it would have to be a plane, a curve cannot be parrallel to a line.

Or in this case a curve cannot be parallel to a plane.

The focal plane is a plane* that is parallel to the sensor.

However, the range of acceptable focus covers an area (a volume, I suppose, in engineering terms) that is thicker than the plane. Anything within that volume may be perceived to be "in focus" even though it does not lie precisely on the focal plane - because the lens, the sensor, and the eye can not resolve the differences in detail (or lack thereof) between a point that is precisely on the focal plane, and a point that is near the focal plane.

The "thickness" of that perceived-acceptable-focus volume depends on aperture, distance between sensor and focal plane, size of sensor.

*I suppose it's possible that, due to optical irregularities in an imperfect lens, the focal plane might not be a perfectly flat plane. For the purposes of this discussion, it's a plane.

imsellingmyfoot wrote in post #14320790
Or in this case a curve cannot be parallel to a plane.

Touche.

Well, first things first, the actual focal plane is absolutely parallel with the sensor (or film) that you're using, except in the case of tilt-shift lenses, etc (in which case, it's still technically parallel, it's just that the lens has changed it's field of view).

I'm not 100% sure what you're asking here otherwise..

Also, it's not a curve. It's a plane that lies parallel to the sensor, not a sphere that is centered around the sensor. I suspect the friend is assuming that it is a sphere.

mplezia wrote in post #14320784
Why he would argue that it isn't parallel to the sensor, I can't guess.

Perhaps he's used a Sigma 10-20

Bob

Check out what Hyperphysicshas to say on the matter.

What I suspect is this: Theoretically it is a plane, but in reality it varies slightly and becomes a very thin volume, which would explain why the centers of lenses are typically sharper than the edges.