Before the rush

Before the rush
by evan-pak

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What happens when a photographer with an old photograph machine turns the objective clockwise or counter-clockwise?

Is the lens turned too? Or does it change lens?

I'm studying optics right now and I'm just curious

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up vote 4 down vote accepted

Depending where you turn, you can:

  • Remove the lens. Newer lenses attach with a bayonet mount and a release may need to be pressed before it can be turned this way. Older lenses used a screw-mount and turning simply unscrew them.
  • Changing focus. Turning the focus-ring change the distance at which a lens is focused. This is not possible on all lenses and some have to be set to Manual mode first.
  • Changing aperture. Some modern lens and most old lenses have an aperture ring. If you turn it, it may directly change the aperture or set the aperture to be stopped down to when a photo is taken. If there is an A position, it lets the camera select the aperture.
  • Zoom: The lens has a zoom ring, then turning it changes its focal-length. Some older zooms use a push-pull mechanism instead of turning.
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oh.. is that what he meant? – Michael Nielsen Mar 12 '13 at 20:23
Yes Michael, but you gave me a useful answer too! Thank you both!! – John Smith Mar 12 '13 at 20:52

The following happens when you turn the lens (objective) on a camera (photograph machine):

  • The buttons and prints, focus and aperture information will turn the wrong way
  • if available, the electronic contacts wont work
  • it will be in danger of coming loose
  • The principal point which is the true image center at which barrel and pincushion distortion is centered and increase radially will move with it.

In conclusion, if you discard the annoyances in bullets 1-3, then if the lens has noticeable distortion you will get a notably different image as well.

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