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.

Blooming

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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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).