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Blurring Out Sharp Backgrounds

Background blurs sometimes show sharp transitions due to a blooming effect. Local blurring of these areas brings visual coherence to the image.

A film and a sensor record highlights in a different way. This is especially noticeable with very bright areas next to dark areas. On film, the light diffuses. The highlights spread progressively in the emulsion.


Overexposure on digital sensors creates a blooming effect. The photosites that are saturated by too intense lighting generate excess electrons, which overflow on the neighboring photosites. The halo effect produced by the excess of light stops more clearly on a sensor than on a film.

This is because of the discontinuous structure of a sensor compared to a film whose silver halides constitute a continuous tangle. Blooming is a parasitic artifact that creates an area of sharpness inconsistent with the surrounding blur. It is distracting.

Blooming Correction in Post-production

To compensate for this blooming phenomenon when shooting, diffusion filters have been used for decades in the movie industry. They create a progressive halo on the specular light sources of a scene, reducing imperfections and wrinkles on faces.

In post-production, Lightroom or Photoshop offer tools with similar effects to generate halos, blurring a specific area. But when the image has more or less noise, blurring smoothes it. A discontinuity of structure appears between the blurred part and the rest of the image. The addition of noise in Lightroom’s local settings is ineffective on a previously blurred area. This adjustment, if it had been possible, would have allowed the blurring to blend into the rest of the image. Photoshop is the solution.

Gaussian Blur

In Photoshop, several possibilities allow you to blur just a part of the image. Our preference is to duplicate the background (Cmd+J on Mac, Ctrl+J on PC, while clicking on the background). In order to be able to change the amount of blur later on, the layer is first transformed into a dynamic object (Filter>Convert to smart objects), then the Gaussian blur filter is applied (Filter>Blur>Gaussian Blur…) which will blur the whole image. Adding a mask on the layer (Layer>Layer Mask>Hide All) hides the blur effect. Simply paint in the mask on the area that is too sharp so that the adjustment only appears there.

Add Noise

If the starting image has noise, the smoothing becomes artificial. The effect of smoothing is visible, for example, on large format prints. By applying a little noise to the smoothed area, the blurred parts blend in perfectly with the native noise of the image. The Filter>Noise>Add noise with Monochromatic Gaussian Distribution. The noise appears only on the blurred area thanks to the mask. Its appearance must match the noise present in the rest of the image.

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The light globes in the background show sharp edges while the immediate environment is blurred due to the wide aperture. Blurring is necessary to bring coherence to the entire background. Leica M9, Summarit 50 mm f/2.5 ED, 1/350 s at f/4, ISO 800.
1/ Sharp Area
The blooming effect generates sharp contours at the periphery of the background lights, yet located in a blurred area, due to the shallow depth of field caused by a close focus and an aperture of f/4 (50 mm lens).
2/ Blurred Area
If we look at the background next to the lights, a higher level of blur shows. The panel “A LA RENOMMEE” can be taken as a reference area to consider the blur that should be brought to the lights located in the upper left corner.
3/ Background Copy
The blurring is made on a copy of the background. A mask will allow the blurring to be applied to a specific area.
4/ Convert to Smart Object
The layer is transformed into a dynamic object (Filter>Convert for dynamic filters or by right-clicking on the layer). It will therefore be possible to return to the various filter settings at any time.
5/ Gaussian Blur
A Gaussian blur is the ideal filter to bring the appropriate blur to the lights (Filter>Blur>Gaussian blur…). A noise smoothing effect is however inevitable. We will correct it in the last step.

6/ Mask
The blur should appear on the globes of light only. A black mask is associated with the layer (Layer>Layer Mask>Hide All) to cancel the effect of the blur on the whole image. With the brush, white is applied in the mask at the place of the globes to reveal the blur.

7/ Add Noise
The smoothing produced by the blur generates an artificial flatness with regard to the native noise of the ISO 800 of the original file. Filter>Noise>Add noise with Monochromatic Gaussian Distribution restores the initial noise. Its amount is adjusted to match the noise located at the periphery.

Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos

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