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Background Blur vs Bokeh

Published June 12th, 2012

An image will have areas which are in focus and areas which are out of focus. The focused areas are measurable and they are referred to as "Depth of Field" (DOF). The Defocused Areas are also measurable and referred to as "Background Blur".

Bokeh on the other hand is not measurable, it is an aesthetic quality based on how smoothly the de-focused transitional areas are rendered by the lens.

To illustrate the difference between the two I shot the following samples using two lenses. Lens (A) is a zoom lens Nikkor 80-200mm F/2.8, Lens (B) is a prime lens Nikkor 180mm F/2.8. Both lenses were shot at 180mm F/2.8. The background Blur Reference shot is below.

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Background Blur - Reference Shot

The above shot is the reference shot for background blur. To recreate the most background blur possible, take your lens and shoot it as close as possible to your subject ( MFD Minimum Focus Distance). The closer you are to your subject, the larger the quantity of background blur.

The image below is a side by side sample of background blur produced by the 2 lenses.

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Image Above- Background Blur Side by Side

Center crops. One image was shot with the zoom the other with the prime , both at 180mm F/2.8. The background blur is virtually identical.

Now let's take a look at the bokeh, starting with the Bokeh Reference shot below.

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Image Above - Bokeh Reference Shot

For the bokeh reference shot we backed away from the tree at a similar distance where a 1/2 body shot would be taken at 180mm. As you can see the quantity of background blur decreases, and more of the background is in the frame. Now we will compare how smoothly each lens renders the out of focus areas, the side by side samples will be crops of the bushes on the lower right hand corner.

The image below is a side by side sample of bokeh produced by the 2 lenses.

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Image Above - Bokeh Side by Side

Contrary to background blur, here we have a visible difference between the lenses. The prime lens renders the bushes with a smoother transition, where as the zoom renders them harsher.

The image below provides a side by side sample of all 4 crops.

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Image Above - Side by Side All 4 Crops

This is a combination of all 4 crops, As you can see, background blur is similar but the Bokeh from the prime is superior.

Bottom line, do not confuse background blur with bokeh; when evaluating lenses for bokeh do not stand too close to your subject, look for a busy background, stand back to allow more of the background into the frame and then compare items in the defocused areas to evaluate how smoothly the defocused areas are rendered.

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