Definition of focal plane?

Hi everyone, I am just looking for a layman’s definition of focal plane. Because, when I look it up it has too many science words for me to actually understand. I think a focal plane is kind of just “how you are seeing now” - like if you look at your eye chart and it comes up 20/40 both eyes, that’s how you’re seeing now. Is that the focal plane? Thanks for any help!

I could be wrong, but focal plane just means how far you can see until you see blur.

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It’s where the images projected into your eye are in focus. You want that to be the same place as your retina and that is our goal. If you are switching between two sets of glasses that have different prescriptions, they’ll each project the focused image into different places.

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Thank you! So when we use normalised and differentials, that’s 2 different prescrips. But because they are for 2 different distances, they each project the image in the same place. Is that right?

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@Meri Focal plane is indeed a hard thing to grasp… I’ll see if I can relate it to the snellen chart measurements somehow:

That vertical line labeled “Principal Focus” is the focal plane.
Notice that it is the line where rays coming in are focused.

Our eye has an adjustable lens that moves the focal plane back and forth in the eye. (accommodation) It can be in front of the retina, on the retina, or behind the retina. If the lens is incapable of putting the focal plane on the retina and instead it falls somewhere in front of the retina, you have myopia.

The closer the lens puts the focal plane to the retina, the more clearly you’ll see. So if you are seeing 20/200 the focal plane is not very close to the retina. If you are seeing 20/40 the focal plane is much closer. If you are seeing 20/15 or 20/10 then the focal plane is very close or maybe perfectly lined up with the retina.


Putting on minus lenses brings the focal plane further back. If, because your eye lens can’t adjust any farther, your focal plane is in front of the retina then the minus lenses will bring it back onto the retina. That is why minus lenses helps us myopes see clearly, although we know there is a price that must be paid for that :slight_smile:.


One last thing: When Jake talks about “multiple focal planes” with multiple pairs of glasses he means that each pair of glasses puts a unique “shift” on the focal plane that your eyeball lens would naturally create. It seems that the brain tolerates two or maybe three “shifts” in the focal plane, but it has to learn each of them.

It can tolerate a shift that lets you see far, normalized glasses. It can tolerate a shift that lets you see near comfortably, differential glasses. It can also tolerate naked eyes if you have low diopters (< -2) for close work. But more than two shifts (more than two glasses) and the visual cortex can become “confused.” It can cause headaches and other problems. Since each shift must be learned, that is why we change one plane at a time: normalized or differentials. (except for a few special cases)

I hope that helps clarify things. (pun intended)

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That’s the gist of it!

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I’m getting this better now. I’m not a photographer but, it sounds like a real camera where you move a wheel to get the image clear and the front of the camera’s lens moves back and forth till the view is sharp. That clear spot must be where the focal plane is. (If only our eyes had a manual wheel when auto-focus isn’t working)

I think naked eyes work good for the place below our centimeter measure. It’s just a smaller bubble so you can’t use it very much depending on how far is your distance to blur.

This explains why I get a eye headache if I forget to put on my differentials or sit at the wrong distance.

In the diagram, the optical centre is the thing you are looking at? and the retina is the same place as the focal plane (or parallel to it, front / behind, if you’re not in focus).?

Thank you much!

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Your camera analogy is good! The only difference is that in a camera the sensor is stationary, the lens moves forward and backward, and the lens stays the same shape; in your eye the sensor is stationary, the lens doesn’t move, and the lens changes shape.
So a camera moves the focal plane (the clear spot) onto the retina/sensor by moving the lens back and forth, your eye moves the focal plane onto the retina/sensor by squeezing the lens and changing its shape. But the goal is the same, move the focal plane onto the sensor.

Beyond the distance to blur, your lens can’t be un-squeezed any more (it is fully relaxed) so it can no longer move the focal plane onto the retina. The retina is too far back (myopia). That’s why it gets blurry.

In the diagram, the optical center is the lens. The thing you are looking at isn’t pictured in the diagram, but the light from the thing you are looking at is drawn as the parallel rays coming in. Yeah, when you see clearly, the retina is the same place as the focal plane.

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Yes! I restudied the very first page of EM with the diagrams and the video there and -now- it’s making sense to me. Thanks again and I hope this helps others too!