Screen too far back: the light from each point came together in front of the screen and spread out again, so each point makes a patch, and the picture is blurry. Slide the screen forward.
- Object distance
- 25 cm
- Sharp picture behind the lens
- 16.7 cm
- Screen behind the lens
- 24 cm
- Each point blurred over
- 53 mm
What’s happening
A pinhole camera is sharp only with a tiny hole, which lets in very little light. A lens fixes that: put it in a big hole, and it bends all the light from each point of the object so that it meets again at one point behind the lens. Put the screen exactly there and you get a picture that is sharp and bright, upside down as before. Put it anywhere else and the picture is blurry, so a camera has to be focused.
Try this
- Slide the screen until the picture is sharp. Then move the object. Is it still sharp?
- Bring the object close. Do you need to move the screen forward or back?
- Close the lens down to “Narrow”. What happens to the blur, and to the brightness?
- Can you find a spot for the screen where the picture is exactly as big as the object?
- At home: hold a magnifying glass between a bright window and a white wall, and move it until the window shows on the wall, upside down. Now try the same with a lamp nearer by. Do you have to move the glass?
Learn more: focusing, the lens opening, and the camera in your eye
Where the picture is sharp. The lens equation says how far behind the lens the light comes together, where is the object distance, the distance to the sharp picture and the focal length:
Far-away things make a sharp picture just beyond the focus F; close things, farther back. So focusing means changing the distance between the lens and the screen. This camera slides its screen, an old bellows camera slides its lens, and a phone camera moves its tiny lens with a little motor. Your eye can’t move its lens, so its muscles squeeze the lens fatter to look at things close up.
The lens opening. With the screen at a distance , light from each point is spread over a patch whose width grows with the width of the opening, :
Closing the lens down (photographers say “stopping down”) makes the patches smaller, so more things, near and far, look sharp at once: a bigger depth of field. But less light gets in. Close it all the way to a tiny hole and you are back to a pinhole camera. Cameras write the opening as an f-number : a smaller f-number means a wider opening.
Real lenses are not quite ideal: the edges of a simple lens bring light together a little closer than its middle, and blue light closer than red. Camera lenses use several pieces of glass that cancel each other’s errors.
- Your eye: the cornea and lens throw an upside-down picture on the retina at the back, and your brain turns it the right way up. Your pupil is the opening: small in bright light, wide in the dark.
- Phone cameras: a lens a few millimeters across focuses the picture on a light sensor.
- Projectors: the same thing backwards: a bright small picture inside, and a big sharp one on the wall, once it is focused.
- Without the lens: the pinhole camera, and the lens itself, in the convex lens.