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This question already has an answer here:

Simply putting a heart shaped cut-out in front of the lens

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turns this

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into this

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How does that happen? What's the optics behind this phenomenon?

marked as duplicate by Philip Kendall, Paul Cezanne, mattdm, Matt Grum, John Cavan May 9 '14 at 14:20

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  • This is related to why the aperture impacts bokeh shape and is also why you have to use a wide enough aperture. The bokeh shapes comes from whatever portion of the lens limits the size of the entrance pupil (the front of the lens) visible to the sensor. – AJ Henderson May 9 '14 at 14:22

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