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If you abuse your lens by just stacking a bunch of extension rings one on top of the other, will this start to affect your image circle?

For instance, you have a lens that can just about cover your sensor with the image circle and you add a bunch of extension rings to focus up-close. Will the image circle start to show on your sensor or will it even widen? Or does it even matter?

Also, is it possible to express this relationship mathematically or does it depend on internal lens design too much?

Note: I ask about focused images, so I assume you still can focus on the object with all the extensions.

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You can grab a lens and move it closer and farther from a piece of paper to directly see what happens.

When you move the lens away from the film plane, light has to travel farther. The intensity of light used to record the image will decrease. (See Inverse-Square Law.) Also, the size of the imaging circle will increase because diverging rays of light spread apart as they travel.

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  • Is this the case for all lenses of all focal lengths?
    – lijat
    Jul 23, 2018 at 11:04
  • All lenses will exhibit this behavior except those which are image telecentric. An image telecentric lens, by definition, produces images which are of equal scale regardless of distance from the sensor. The longer a focal length is, the more telecentricity it exhibits. To my knowledge, though, no pictorial lens has ever been intentionally designed to be perfectly telecentric.
    – PhotoScientist
    Jul 23, 2018 at 14:01
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    Note also that although it is outside the scope of this question, the angular resolution of a lens is unaffected by an extension tube such that it's spatial resolution decreases proportionately to the spread of its image circle.
    – PhotoScientist
    Jul 23, 2018 at 14:02

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