@otterlove extra bit of detail: the center shape must fit within a circle the size of the aperture you want to use.

So, for example, if you’ve got a 50mm f1.7 lens, your aperture’s diameter is 50mm/1.7 = 29.4mm. If you draw a circle that size (i.e. with a radius of 14.7mm), you can cut out a shape within that.

Lastly, make sure you use the *actual* focal length of your lens for this calculation, not the 35mm equivalent.