Say I have a single convex lens. I set it up like a camera obscura, shine a light on my object and project the image onto a projection plane. Given the focal length of my lens, the projection plane, object and lens need to be certain specific distances from each other for the projected image to be in focus. This is given by the equation:
1/f = 1/s1 + 1/s2
f is the focal length,
s1 is the distance from lens centre to the projection plane, and
s2 is the distance from lens centre to an object.
Now say I remove the projection plane and replace it with my own eyeball. I can see (part of?) the real image through the lens. I know its not the virtual image because the image is upside down (whereas the virtual image created by a convex lens is the right way around). Now I take a step backwards from the projection plane, and the real image as seen through the lens is still in focus to my eyeball. I take a few more steps backwards and the image is still in focus.
Why can my eye see a focused image at many different
s1 values, but a projection can only be focused at a specific