Alpa Cameras: Image circle and focus

[Mike M]

I'm wondering how the image circle of a lens relates to where the lens is focused in the scene. For example, a lens might have an image circle of 189 at F22 and focused at infinity. But what kind of image circle might that same lens have when focused at moderate distances or close focus distances? Thanks for any insight

 

[dougpeterson]

I'm wondering how the image circle of a lens relates to where the lens is focused in the scene. For example, a lens might have an image circle of 189 at F22 and focused at infinity. But what kind of image circle might that same lens have when focused at moderate distances or close focus distances? Thanks for any insight

Image circle (as seen by the sensor) increases in size at closer distances, though you won't notice a large difference until you get towards macro.

Think of the lens as a fixed element* in the universe - it doesn't move and the the cone of light it projects doesn't change. However when focusing closer than infinity the sensor is further away from the lens and therefore further away from the "tip" of the cone of light and therefore sees a larger image circle.

*no pun intended

Doug Peterson (e-mail Me)
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[Mike M]

Thanks Doug, that's similar to what I was thinking but just wanted to make sure....thanks again