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If I do a handstand, everything looks upside down (which is originally what we see if we’re upright but the brain switches this). I assume this is because the change in how light rays enter our eye.

Now if I use my camera and turn it upside down, the image on screen is still upright.

Why does the same principle not apply to cameras?

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    $\begingroup$ Uh, because the screen you're using to view the picture is turned upside-down too? $\endgroup$ – knzhou Apr 5 at 13:58
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    $\begingroup$ Have you tried this with a camera that doesn't have a built-in accelerometer (i.e. not a phone camera)? $\endgroup$ – probably_someone Apr 5 at 13:59
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    $\begingroup$ @probably_someone It has nothing to do with an accelerometer. The OP's question is essentially the same as asking "if I turn my reading glasses upside-down and look through them, why don't I see words upside-down?" $\endgroup$ – knzhou Apr 5 at 14:08
  • $\begingroup$ @user35594 Please clarify what class of camera you mean, and whether you mean an inversion of the acquired photograph or of the image as seen through the optical eyepiece or on a screen on the camera body. $\endgroup$ – Emilio Pisanty Apr 5 at 20:10
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If I do a handstand, everything looks upside down

Yes, because you are upside down.

Now if I use my camera and turn it upside down, the image on screen is still upright.

Yes, because you are rightside up.

Why does the same principle not apply to cameras?

The principle is the same in both cases, you interpret the image, so the only thing that matters is your orientation.

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First, let's define "up" and "down." If you define "up" to be opposite the gravitational field vector, then when you turn your head down, your eyeball is inverted. The image on your eyeball is inverted from where the top of your head was originally, so you interpret that as "up-side down." If you were to use a film camera and you took two pictures, one with the top of the camera up and another with the top of the camera down, you would find that the images on the film are, indeed, rotated 180 degrees from each other (along the axis perpendicular to the film plane), just like the image on the retina of your eye.

If you use a phone camera or even most digital cameras, they have inverting software that correct for rotations like that. Often you can turn them off. Given that, the actual pixels in the camera sensor which receive the light are flipped over, but that is hidden from you by the software.

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