Why do out-of-focus point lights look like bright discs instead of normal blur?
Asked 11/8/2013
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In intentionally out-of-focus photos, such as night street scenes, bright lights often appear as distinct discs or shaped highlights rather than the soft, Gaussian-like blur seen in darker or lower-contrast areas. Why do these point lights stand out so much, and why can their edges look sharper or even brighter than their centers?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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They stand out because they're so much brighter than the surrounding areas.
The out-of-focus blur is not gaussian; every out-of-focus point in the scene projects a fairly evenly-shaded disc, like what you're seeing, onto the sensor. These discs overlap and additively blend so that, in areas without much contrast, the result is a more-or-less smooth blur. But light sources can be much brighter than their surroundings, so the discs they project contribute almost all of the light hitting any given pixel in those areas, and thus stand out.
The reason the borders of the discs are often brighter than the centers has to do with spherical aberration. Lenses vary greatly in how they handle this optical distortion; it is closely related to how smooth and "creamy" bokeh they produce.
Originally by user14547. Source · Licensed CC BY-SA 4.0
user14547
12y ago
0
Generated from our catalog & community — verify before relying on it.
Out-of-focus blur is usually not a Gaussian blur. A defocused point in the scene is projected by the lens as a blur disc shaped by the aperture and lens aberrations. In most parts of an image, many nearby tones overlap, so these discs blend together and look like smooth blur.
Bright point lights are different because they are much brighter than their surroundings. Their blur discs dominate those pixels, so you can clearly see the disc shape instead of a smooth average.
If the disc edge looks unusually defined, two common reasons are:
- the lens’s optical behavior, especially spherical aberration, which can make the disc brighter at the edge than in the center
- image sharpening, which can increase contrast around edges and make the border look harder
So the effect is mainly a combination of defocused point sources producing blur discs, high contrast making those discs visible, and lens/rendering characteristics affecting their brightness distribution and edge shape.
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