Why does a shaped aperture change out-of-focus bokeh instead of the whole image?
Asked 4/8/2011
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I understand that shaped bokeh can be made by placing a cutout over the front of a lens, but I’m confused about why this only affects out-of-focus highlights. Intuitively, I’d expect the whole image to look like it was viewed through that shape, with black around it, like a keyhole. What is the optical reason a cutout creates shaped bokeh rather than simply masking the entire photo?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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It's complicated but I'll have a go at explaining it. Light from a single point spreads out in all directions and hits the entire front surface of the lens, this light gets focused back down onto a tiny dot by the lens if the lens is focussed at the distance of the point. If not (i.e. the point is part of the background) the light lands on the sensor in a big circle and creates bokeh.
If you put a shape on the front of the lens you block out bits of the spread of light from the point and the result is that instead of a circle, your shape is projected onto the sensor.
Your shape has to be very close to the lens for this, otherwise the light from the point wont have spread far enough by the time you start occluding it.
I would just expect your image to be in that shape (like looking through a keyhole) with the outside of the shape being black.
The reason you don't simply see an image in the shape of your bokeh filter like you suggest is that the filter is very close to the front of the lens and so doesn't block the centre of the lens. Any light which hits some part of the lens front element has a chance to get into the picture, so there are no black edges etc.
If the shape is a little distance in front of the lens, then there will be parts of the scene which are blocked from hitting any of the lens surface, these areas will appear black in your image.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
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Shaped bokeh happens because out-of-focus points are recorded as blurred disks that reflect the lens aperture shape.
A point of light sends rays across the whole front of the lens. If the lens is focused at that point’s distance, those rays converge to a tiny point on the sensor, so the aperture shape doesn’t show up much. But if the point is out of focus, the rays don’t meet at the sensor plane; instead they form a larger blur circle. That blur circle is effectively an image of the aperture.
So when you place a shaped cutout very close to the lens, you’re changing the effective aperture shape. Out-of-focus highlights then take on that shape instead of the usual round or polygonal blur.
It does not normally make the whole image look like a keyhole because in-focus parts are still focused to small points or fine detail, not large aperture-shaped blur disks. The cutout must be close to the lens so it acts like the aperture, not like a distant mask in front of the scene.
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