Making your digital image appear more human

The retina consists of two channels, a vertical one which detects and relays the light it receives along discrete channels to the visual centres of the brain for reconstruction, and a second which runs horizontally and serves to enhance the contrast and image of light which the retina receives. These two systems are illustrated in the inset diagram, the flow of light information in vertical system is shown by the violet arrows and in the horizontal system by the red arrows. Light is detected by a specialised layer of light detectors called photoreceptors which capture the light they receive and convert it into electrical information, which is then transmitted 'vertically' straight onto the ganglion nerve cells via layer of bipolar relay neurons which help to create an impression of contrast. Contrast is the magnification or amplification of differences between regions of darkness and light in the field of view. One of the reasons for the high level of performance of our visual system (acuity) is our ability to contrast between areas of relative darkness and light. This capacity is further enhanced by the horizontal (lateral) processing of light information within the retina. The light signal from the light detecting photoreceptors is not delivered exclusively to the bipolar cell layer, as light information also flows to a layer of horizontal cells which lie between the bipolar and photoreceptor cell layers. Horizontal cells are believed to spread a radiating negative wave to surrounding bipolar cells in the vertical light channel informing neighbours that they are relatively dark. This provides us with an exaggerated impression of contrast between regions of light and shade in our surroundings. In more philosophical terms, where we see black and white the digital camera only detects shades of gray.

It isn’t just black and white which are subject to contrast enhancement. The retina also distorts our impression of colour. Specific wavelengths of light corresponding to our perception of the colours red, green and blue are detected by specialised photoreceptors we call cones, so-called because of their stubby appearance. Colour, as Professor Oliver Sacks once noted, is an invention or creation of the brain, as damage to the higher visual centres of our brain may reduce our vision to mere shades of gray. The artist Claude Monet took the idea one step further, and was renowned for his striking use of contrast enhancement between objects in his paintings, especially through his use of colour. Through an absence of natural shading in his paintings he was able to create a dramatic impression of contrast, in effect imitating the function of our retina.

Enhancing your digital image

Armed with the knowledge that the retina enhances the contrast between light and dark, and also that it enhances our impression of colour, we may start to use an image editing program to restore life to our digital images. Using Adobe Photoshop (6.0) we’ll start with a simple picture taken from a reception area. In the raw image we can see that there are points of relatively extreme light and dark, and that the image was taken in low light during in the evening. By clicking on the topic “Layer” on the upper menu of Photoshop, then selecting “New Adjustment Layer” and finally “Brightness/Contrast” we have the options of altering these two parameters. First we may increase the brightness to compensate for the low ambient light levels in this image, and also by increasing the contrast setting we get a clearer impression of the elements in the image (image 2).

We may then enhance the color in the red channel (which appears to be particularly affected in digital photography) by clicking on Layer > New Adjustment Layer > Colour Balance and increasing the red channel (image 3). Finally we will enhance the lightness and colour saturation by clicking on Layer > New Adjustment Layer > Hue/Saturation (see image 4). Now compare the first and last images. The scene has been brought to life, the colours are more vivid and we can now clearly see the individual sweatshirts on the rack.

Enhancing model images

Now we can move to the object lesson. Making a model project from an image with vitality and colour. An identical set of transformations brings us from the first image of the model to the second, where we can clearly see that we have enhanced both flesh tone and colour, and we have recreated the impression of a healthy glow and vitality.

That was a fairly straight-forward example. Now for something a little more testing. The photographer was looking for an ideal shot, with the model in the foreground and the lights in background. The ambient street light was poor, especially against the background light, causing the model to appear in poor light and her features to be lost in the digital image. The use of a flash would have caused the loss of the background lights and a “retinal reflection” or “red eye”. So the picture was taken (image 1) and this time both contrast and brightness were decreased (image 2). All three colour channels were enhanced before clicking on Layer > New Adjustment Layer > Curves, where the contrast for each colour and the image (for colour select “none”) were enhanced to preference. Finally using the commands Layer > Layer Style > Outer Glow, a soft light aura was added to recreate the warm orange street lighting (image 3). Now the colourful background and the warm and rich colours of the scene may be seen along with the models features (which could of course also have been further enhanced, but here we are looking for the background effect).

I hope that this short article has given an insight into how the human visual system functions, and why there is often a marked discrepancy between what we perceive at the time of the picture and what the camera records...!