And then I have this distance over here, so I'm taking any point on that ellipse, or this particular point, and I'm measuring the distance to each of these two foci.

You don't really have to consider them.

Set here equivalent focal length assuming a 35mm film camera, in mm. A value of 0 means the focal length is unknown.

Let's say it's x squared yz.

Focal length

Focal

Focal Length

Focal blur

Focal point

First Focus

Second Focus

Focal Length

Focal length

And this is f2.

And it's for focus.

Focal length of the lens

Well, we just used the distance formula.

Or they can be, I don't want to say always.

We have focused on the worst case scenario.

That's our directrix and it has the equation y is equal to k.

Ruler Select starting planet.

Ruler Select ending planet.

And what we want to do is, we want to find out the coordinates of the focal points.

Ellipse by Focuses Point

I won't revise this equation, but this is the fundamental equation that governs when things are in focus, for a lens.

A parabola defined by its directrix and focus

35mm equivalent focal length

That's this distance right there.

And that's what we set out to figure out in the beginning.

And you enter that space, and you feel like, Wow!

Requires great focus.

Just focus.

Bifocals.

Your focus,

We say we have a parabola, and we're saying that this parabola is the set of all points that's equidistant between some focus and some directrix.

Hyperbola by Focuses Point

I'll call that well, we'll call that a focus because it will be a focus.

But where's the focus point?

So you know that a is equal to x1.

So let's solve for the focal length.

Focal Length

Telescope focal length

Eyepiece focal length

So let's call that a directrix.

Let's now go to a difficult challenging case where I'd like to recover the focal length f from measurements of the type z, delta x, and B.