Black Background

The second exercise in the Lighting 102 course at http://www.strobist.blogspot.com/ is concerned with the ratio of flash to subject distance and flash to background distance. Control of this ratio allows the relative darkness of the background to be fully controlled. All of these images were shot at ISO200 with a shutter speed of 1/200th second. The subject is 1.5m from the background and the flash is at approximately 45 degrees to the right. The starting aperture was f5.6 with 1/8 power on a Nikon SB800 speedlight. As the flash to subject distance was decreased the power and aperture were reduced as required to achieve a good exposure.

The first image was taken with the flash 5m from the subject. Clearly enough light is hitting the background to make it appear similarly exposed.
The flash is now 2.5m from the subject and the background is getting darker. This happens because the fall off of light intensity is proportional to the inverse of the square of the distance.
The background of the third image is getting quite dark (flash 1m away from subject). Notice how the nose shadow is also darker. This will be because in the earlier images enough light was being reflected back from a wall to the left to soften the shadow. The 'Depth of Field' of the light is now so shallow that this no longer happens.

Now we're talking! The background is now really dark (flash 45cm away from subject) and the fifth image (below) which is taken with the flash 30cm away from the subject is nearly black.


I couldn't quite achieve black so I think I need my light even closer or my subject a bit further from the background. The ambient contribution is non existent at these settings (ISO200, 1/200th, f11) so any light is from the flash. This exercise has been a real eye opener for me as I have often wondered how to control this aspect of flash photography. The 20 minutes spent doing this exercise has taught me so much. Blasting away with TTL flash systems can be handy at times but this stuff is much more fun.
However, for the next exercise I will probably be using an inanimate object. Triggering the self timer and dashing round to get in the shot is likely to end in a knocked over lightstand.