To help me learn what happens to light from flash units at different distances I recorded the aperture I would need to use to correctly expose a shot every 2 metres. I set my shutter speed to 1/25sec and laid out a tape measure along the floor. Unfortunately the cable for my flash meter is only 8 metres long and therefore could only take light readings up to this distance. Pointing the flash unit directly towards me and holding the meter towards it, I began to take readings at at ISO 100 at 1m then 2m, 4m, 6m and finished at 8m. I decided to take readings at different ISOs (100, 200 and 400) to compare the differences. My results are shown below.
Table of results
Graph showing how the distance from a flash unit effects exposure with the shutter speed locked at 1/20sec
From the graph we can clearly see that the closer the flash unit is to a subject, the smaller the aperture needs to be in order to correctly expose a shot.
I have also added two extra lines to the graph; at midday on a sunny day and at midday on a cloudy day. These lines are perfectly straight across the graph because distance doesn’t make any difference to how bright the sun is as it is already very far away.
Opening the aperture by one f-stop will let twice as much light into the lens. At twice the distance from any point near the flash unit the light level will be four times less so to compensate for this the aperture will have to be opened by two stops.
