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.
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.
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.
Learn to use Photoshop and Lightroom
If you want to learn how to use or improve your knowledge of Lightroom Photoshop, Spéos offers different professional photography courses in 1 year, 2 years or 3 years.
The long courses to become a professional photographer allow you to master all the photographic techniques and vocabulary (blurs, hyperfocus, sharpness zone, depth of field, backlighting, focal length, shutter release, autofocus, wide-angle, rule of thirds, etc.), but also all the stages of shooting and image processing.
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.
Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos