Here’s a little tidbit (but an important one) in lighting. It is usually far easier to add light in an area of an image that needs light, than it is to take light away from an image area that has too much of it. You might be asking – “Why would you even give any thought to this?” Because keeping this tidbit in mind will make your life easier as you set up your lighting, or choose the location that you shoot in. Below are some examples:
1) Be careful about placing your subject (people or still life) too close to your background. If your subject is too close to your background, then your subject light will spill over and contaminate your background. Obviously, you can indeed use one light to illuminate your subject and background together, but you will have more control if you light the two image elements separately. If your subject light does not strike your background, then you will have a clean canvas to work on – in other words, your background can be anything from pure black, to pure white and every tone and colour in between. If your subject light spills onto your background, then you will have already partially lit the background – which means your lighting options are limited. It may be that the spill over lighting has caused your background to be lit to the equivalent of 20% black – that means your choices range from dark grey to white.
Certainly, you can use a gobo or flag to block off the subject light from hitting your background, but that is more work than simply placing your subject farther away from the background. If you are shooting in a small room with white walls, even a flag or gobo will only have limited effect. If you have to try and take light off your background to make it black or darker, it will potentially be a lot more work than making a background with no light whatever tone you want – adding light is easy, and this is an error that many new portrait and people photographers make.
2) It is one of the reasons that many studios (at least small ones) have grey walls and dark floors and ceilings – it makes the studio less efficient and creates fewer reflections. Large white studios can be beautiful to work in, especially if you are a people photographer and the studio has north facing windows. As long as the studio is large, light fall off will ensure that you won’t have too much problems with light bouncing off the walls and lighting areas that you do not want lit. But if the studio is small, then white walls can cause a real problem – light will tend to reflect off the walls and bounce back in on your shot. Let’s say you were using a grey background and trying to create a subtle gradation in tones to your background from dark grey to black – you could find that you won’t be able to reach black because of the light reflecting off your white walls.
3) It is the reason that subtractive contrast control (or key shifting up) can sometimes be much more challenging than additive contrast control (or key shifting down). Key shifting is a lighting technique where you add light to your subject in order to overpower the ambient light, make your background scene go darker (additive contrast control). Alternatively, you can take light away from your subject, such as putting a scrim between your subject and the Sun, in order to make your background scene go brighter (subtractive contrast control). When you use subtractive contrast control, and you key shift the background up by say 2 stops, then as far as your camera is concerned, the world just became 4 times brighter. This will look beautiful, but it can also wipe out the shadow contrast on your subject. This means that you will need to have black scrims, or black fabric that you can put on your subject’s shadow side to try and increase the contrast, or darken your shadows.
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Bluesky is a photography tutorial website created by veteran commercial photographer and college instructor Greg Blue. This site advocates an approach to learning studio & location photography that focuses first on light theory (these are free tutorials). Lighting, camera and post-production techniques are equally important, but should follow the study of lighting theory (these tutorials are also free). The site also offers a host of specific image tutorials that take a mile deep look at theory and technique from the perspective of individual images (these tutorials are members only).