How much blur? - A visual lens calculator

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One of the most returning questions on photography forums is which out of two different lenses has the smallest depth of field (DOF). However, what many people actually mean when they ask this is which lens has the best ability to blur the background in their shots. The answers given are not always very clear. However, they could be just that. All it takes is some simple math. This tool will give you the answers you are looking for. Add some lenses, select your subject size, and hit update to see the results in a graph!

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I want to take a shot in landscape orientation, which contains someone's head and shoulders (0.9 x 0.6 meters), a full person (3 x 2 meters), or something completely different, which has a width of meters.

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Background blur versus background distance

Theoretical blur disk diameter as percentage of image width [%]

Generated by

Distance between subject and background

With this slider you can set the distance range of the graph:

It is important to intepret the results correctly. The first obvious observation to make is that the background blur increases when the background is further away, regardless of focal length or aperture settings. In order to give meaning to the results, think about what kind of shot you want to make, and what distance there will be between the subject and the background.

As you probably have seen by now, the relative blurring ability of lenses is very much dependent on the specifics of your shot. As a rule of thumb it can be concluded that a wider aperture is more important for blurring closer backgrounds, whereas the focal length comes more and more into play when the background is further away. Please note that there are two additional effects which determine the amount of background blur in your shot:

  1. Each lens has its own bokeh characteristics, which can make background blur appear more or less smooth.
  2. A shorter focal length will have a wider field of view, and therefore it is likely that there will be more objects in the background at a closer distance.

You can always link back to this specific comparison by using the following url:

This is version 1.2 of how much blur. Source can be found here. Hosted with permission by Gijs de Koning