What is Edge Burning?
In traditional darkroom printing, especially in black and white, the enlarger systems used often suffered from 'fall off' (the light in the centre of an image was brighter than at the edges hence the centre of the print always had a lighter area around it.) The same is true of cameras; the camera image is brighter in the centre than at the edges. This is most evident with really wide-angle lenses but is present to some extent for all lenses.
This is commonly called 'fall off' and if you want to get really technical it can be calculated using the Cos4θ rule of trigonometry but I digress. As a rule of thumb, most lenses lose about 1/3 to 1/2 a stop of light at the edges of the image compared to the centre.
So, to compensate for negatives and enlarger problems of fall off, most expert printers use edge burning to balance the tonal difference between the centre and edge of the print. Edge burning is simply adding more tone to the edges of the image. In printing it was usual to add about 10% additional exposure time for each of the edges.
Edge Burn and Composition
Now, edge burning has another important effect and that is to do with visual perception and not deficiencies in photography. When you darken the edges of an image it holds the viewers attention into the picture and prevents the eye straying beyond the boundaries of the print or image. This is a subtle technique for concentrating attention inside the image borders.
Inner Glow to Edge Burn in one easy lesson.
One day it dawned on me that the 'inner glow' layer effect in Photoshop produces an 'edge lightening' around the four sides of an image and although this is the exact opposite of the desired edge burn, it got my brain cells sparking so I dove into the Inner Glow dialog to see what gives.
My easy Photoshop edge burn effect just needs you to change the settings for the Color, the Blend mode, and usually the Size values in the Inner Glow dialog and there you have an edge burn. Change the colour to black, the Blend Mode to Multiply, and the Size value for the width of edge burn you want.
The Opacity value can be modified down to around 5-15% depending on how obvious or subtle you want the effect to be. Sometimes I use higher values to make the edge burn very obvious and then it becomes part of the image composition and mood.
You can only apply the layer effect to a real layer (not the background so convert it to a layer) and sometimes you will need to add a new layer filled with white and blend mode set to 'Multiply' at the top of the stack to add the effect to for the best results.
There you have a very easy and controllable method of quickly adding edge burn to an image. Please note, although there is a Photoshop layer effect called 'Inner Shadow' which you may think does the edge burning job; it doesn't! This is because the Inner Shadow effect only applies the effect to two sides of the image to create a shadow and not an edge burn on all four sides.
