Why can my eye still see a lens image in focus at different distances when a screen only focuses at one plane?
Asked 1/1/2019
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With a single convex lens, a sharp projected image on a screen only appears when the object distance and screen distance satisfy the lens equation. But if I remove the screen and look through the lens with my eye, I can still see the inverted image while standing at different distances behind where the screen was. Why does a screen require one exact image plane, while my eye can still see the image in focus from several positions?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
4
You don't really see a real image. You always see a virtual image, just an upside-down one. When you say you see a real image, this just means that there is a focusing plane where you could place a translucent screen and still get an image.
Now your premise is that you can place your "eyeball" in the focusing plane and see an image. That's actually exactly when you can't see an image since you cannot focus on your eyeball (and a good thing you can't). The lens in your eyeball allows you to focus on virtual images before your eye by creating a real image on your retina from them. But you cannot look at real images that would appear on your eyeball itself.
In fact, when the real image passes your eyeball and consequently the virtual image flips upside down you only see an indiscriminate blur (the image of a single point that cannot decide whether it's upside down or not).
Originally by user80980. Source · Licensed CC BY-SA 4.0
user80980
7y ago
0
Generated from our catalog & community — verify before relying on it.
A screen and an eye do different jobs.
A screen records a real image only at the exact plane where rays from each object point physically converge. Move the screen away from that plane and those rays no longer meet at single points, so the image blurs.
Your eye does not work like a screen placed in that beam. It takes whatever rays enter your pupil and uses its own lens to focus them onto your retina. So when you stand behind the lens, you are not “seeing with your eyeball at the image plane”; you are viewing the light field coming from the lens and your eye refocuses it.
The real image formed by the convex lens is the place where the rays cross. A translucent screen there would scatter that light so you could directly see the image on the screen. Without the screen, the rays keep traveling, and your eye can intercept some of them from different positions and focus on them.
So the image plane is still unique for projection, but your eye can accommodate and form its own retinal image from the rays before or after that crossing point. That is why the projected image is only sharp at one distance, while your eye can still perceive it from a range of positions.
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