How is the term Gaussian blur used?

Question

Is the term Gaussian blur used strictly in post-production, or can it also be used as a term for an out-of-focus area in your image when you-re taking the picture? I'm pretty sure that bokeh is used to describe an area out-of-focus when you're taking the picture, can gaussian blur also be used when you're taking the picture?

Answer 1

It's not appropriate to use the term "Gaussian blur" for the out-of-focus parts of an image, because "Gaussian" refers to a specific blurring function. It's the same Gaussian curve that you may know from the "normal distribution" or "bell curve" in statistics. A bright point that's smoothed by a Gaussian will taper smoothly from a bright center to a dark edge.

Example of bright points with Gaussian Blur, made in the GIMP.

The out-of-focus parts of your photograph are not smoothed in the same way. Instead, an out-of-focus bright point in your image will appear in the shape of your aperture. So if your lens isn't stopped down, it will look like a bright circle. It doesn't taper smoothly from the center to the edge like a Gaussian does. (If your lens is stopped down, you'll get a polygon instead of a circle—for example, a hexagon if your aperture has 6 blades. But the same point applies: It doesn't smoothly taper from bright to dark like a Gaussian does.)

In the above picture, notice that the out-of-focus lights are evenly filled circles, not Gaussian profiles (which would fade gradually from a bright center to a dark edge).

Answer 2

Gaussian blur is a transformation that you apply to (pixels of) your digital image. You can infer that it is an algorithm since the name "Gaussian" refers to the mathematical properties of the transformation that you are applying to your image (which is related to the gaussian density exp(-r^2/2).

Bokeh is the shape of out of focus source of light, depending on the mechanical features of your camera.

As you can see, the two are completely different things.