Lighting Ratios And Controlling Contrast In Flash Photography

Studio lighting using flash mono blocs requires that the photographer masters the use of a flash meter. While it might be possible to guess the strength of flash light and then check the results on your camera's LCD, even a simple flash meter will help reduce the amount of time you spend in working out the individual settings for, and exposures  from, each of your flash units, the ratio between their brightnesses, and the final camera settings that you use to make your picture.

The situation is made more complicated by the fact that all photographic prints and drawings have a tonal range that sets the limits to what can be done. You cannot make any part of a picture lighter than the bare white surface of the paper. Similarly it cannot get any darker than the deepest blacks that can be printed. Somehow you have to measure the light emitted by the flash units, add in any daylight or other ambient light, and work out settings that control the contrast of your picture, so that it doesn't exceed the tonal range possible in your print. Only a flashmeter can do this quickly and accurately.

The contrast of a photograph can be expressed as a lighting ratio, which is the difference between the amount of light falling on the the brightest parts of the subject compared to the amount of light in the darkest shadows.

Many flash meters come with a flat light measuring disk designed specifically for working out the lighting ratio, or, like the Sekonic L-358 flash meter I use in the studio, have a retractable dome, sometimes referred to as an invercone, which can be used to measure both the lighting ratio (while retracted) and the overall exposure (while extended). Other models may have a clip-on dome or disk, or even a sliding dome.

The way I set up lighting is to point the flat disk (or the retracted dome) towards the main light and measure the strength of that flash, then point the disk towards the secondary or fill-in lights one by one. Working round the lights one by one, I adjust each to the desired strength. When all of the lights are balanced, the invercone (extended dome ) is pointed at the camera from the subject's position and the cumulative exposure given by all of the flashes measured: the camera is then set to this reading.

Let's look at some examples of different lighting ratios, starting with very flat lighting. 

This is good for showing detail, texture and subtle colour effects. A good example would be the type of picture used by high street fashion stores to show examples of their clothing. The contrast ratio would be 1:2 or one stop difference between the brightness of the main flash light and the fill-in light or reflector. If you look at the shows cast by the figure on the floor you can clearly see that they have very little density. This type of lighting reveals all the surfaces darkened by subtle shading.

 Now lets increase the contrast to 1:3 ratio (1.5 stop difference between the main light and the fill-ins). This gives the subject a little more presence. It would be achieved by lowering the the power of the fill in lights or reflectors. Either turn the fill-ins down or move them slightly further away from the model.  The effect is to reduce their strength thus making the shadows a little darker. The whole picture becomes a little more dramatic and the figure/object appears to have more substance.

As the contrast gets greater the image grows in strength and the shadows change from being transparent to being superficially solid. The shadows start to play an important role in the picture itself. When you look at the original, rather than the small sized jpeg shown here, it's still possible to see detail in the dark shaded areas of the photo even with a contrast ratio of one to four (two stops) The nude life model is powerfully drawn with light and shade.

Now, let's look at 1:8 contrast ratio (3 stops difference between the main light and the fill-ins) which is about as far as you can go without rendering all of the shaded areas as solid black without any surface texture. Overdone, black areas can look very two dimensional.

Contrast ratios are closely connected with controlling light bouncing around the studio. This last picture was done on the same white background as all of the previous photos. There is just more control over where the light is allowed to fall. In this picture, no light gets on to the background at all, rendering the white surface as a near black.

I believe lighting is best learned by starting very simply with one light and a reflector, and then adding extra lights until the desired effect is achieved. Understanding flash measurement enables the photographer to achieve the desired amount of contrast to construct the  picture. This forms the basis of my teaching on the one day lighting courses I run.

(The model in these pictures is Rosemarie Orwin, art nude and photographic model, who I found to be a pleasure to work with. Highly recommended.)