It’s common to shoot business portraits against seamless background paper. It’s also common practice for a photographer to create a tonal gradient on that background rather than having a flat tone. A gradient adds a bit of depth to the photo by separating the subject from the background.
The problem with a tonal gradient is that when the finished photo is saved as a jpeg, the file compression can cause the gradient to posterize. The amount of posterization also depends on how “steep” the gradient is.
A small amount of posterization won’t be noticed by most customers nor will it be visible in print or online. But many photographers want to deliver the best possible image to the customer.
At low jpeg settings (i.e. high compression), posterization can be quite visible. The highest jpeg setting virtually eliminates posterization but if the customer needs a reasonably small file size, then using a jpeg setting of 11 or 12 may not be possible. For example, with a business portrait I did yesterday, a jpeg 12 setting produced a 4.8MB file with almost no posterization and a jpeg 9 setting produced a much smaller 1.2MB file but with perceptible posterization.
One way to minimize posterization of a tonal gradient is to add noise to the photo just before converting to jpeg. In Photoshop, adding around 1% to 3% noise can hide most of the posterization and it won’t usually be noticeable in the photo. If it should be visible on the portrait subject, the added noise can be easily masked out.
Here’s the histogram of a fully edited psd file, (a portrait of someone dressed in black and standing in front of a dark gradient background), before converting to a jpeg:
Converted to a jpeg at a setting of 9, the spikes in the histogram show the posterization:
But when noise is added to the psd file before it’s saved at a jpeg setting of 9, the final image shows much less posterization:
