Project 55; Light Intensity.

I used a canon speed light for this. The Flash was set on full power and manual mode. The camera was set on 100 ISO, 1/250 second and I used a hand held flash meter to measure the light output at different distances.

I have chosen 1/250 second as this is my flash sync speed on my camera.

There are 5 variables in shooting with a light source, these are;

Shutter speed; This controls the ambient exposure. This can be the sun, lights in a room, candles. Basically what ever the continuous ambient light source is.

Aperture; This controls the flash exposure. If the subject is over exposed the aperture is too large. If the subject is underexposed the aperture is too small.

Flash power; On manual mode this will be constant. 1/1 full power, 1/2 power, 1/8 power, 1/16th, 1/32nd, 1/64th, 1/28th are the setting available. Each change reduces the light available from the flash by one stop.

ISO; again this will be constant with the camera set on manual.

Flash subject to distance; This can be constant in a still life but may change in some situations like a group of people or objects at different distances from the camera. The inverse square law is an equation which covers this it states that doubling the flash to subject distance reduces the light falling on it to 1/4 (4 times). So in effect if I was taking a photo of two subjects with a distance of 1 meter from the flash to the first and two meters from the flash to the second there would be 2 stops in the difference of the light falling on both from the flash. This could be particularly useful when you want to separate a subject from the background.

Below is a graph showing the fall of in light from the flash with the lines showing the max aperture found earlier on both midday on a sunny day and a cloudy day.