Bokeh and background blur

Amount of blur, not the signature of one lens

This page opens two full-frame 85mm views of the same portrait. The left side is f/1.4 and the right side is f/2.8, so the framing and the working distance match and only the blur changes. A lower f-number throws the background further out of focus. The side view shows the sharp zone shrinking as the aperture opens.

The pictures are a thin-lens simulation. They do not reproduce onion rings, soap-bubble highlights or the swirl of a particular optical design. Two 85mm f/1.4 lenses from different makers can share this amount of blur and still look different in a photograph.

What to compare

Use the aperture presets to put both sides on the same f-number, or open both as far as the controls allow. Match depth of field sets the right-hand aperture so the background blur matches the left. For a change of perspective as well as blur, open one of the focal-length comparisons.

Questions

What is the difference between blur and bokeh?

Blur is how out of focus the background is. Bokeh is the shape and smoothness of that blur. This simulator calculates the blur from focal length, aperture and sensor size.

Why do two lenses at f/1.4 not match here?

The model is a thin lens with a round aperture. It leaves out the iris shape, spherical aberration and the particular drawing of a real lens.

What does a lower f-number do?

It widens the aperture, shortens the sharp zone and blurs the background more. f/1.4 blurs more than f/2.8 at the same focal length and the same subject distance.