Saturday, 5 September 2009

Distance Matters, No Matter the Size

One thing that controls the amount of strobe light is the apertrure (the other is distance) so when someone posed the question, "My (AC powered) strobes, through my softbox, on the lowest power is still too much for my shooting aperture, what's my best options?", I was surprised to hear that people suggesting ND filters as supposed to moving the lights futher back.

The answer to the posed Q was actually "ND filters".

At this point, I thought the Strobist 101 and Strobist 102 was burned into my unconscious. But, obviously not.

The important part that has slipped out of my brain is the section related to apparent size and distance.

The reason why moving the lights (which are already pushing out the lowest power) further back may not help is that this may result in a loss of softness to the light and affect the speed of the light falloff.

I had forgotten that this mattered.

We know that the larger the apparent light creates a softer light, but if we move the light further away, the apparent light size becomes smaller. The sun; that's a big light source, but during the middle of the afternoon the light is hard - the reason is because of the distance.

As for the falloff, we remember that light close in requires less power to illuminate the subject (inverse square law). Therefore, the further the light gets from the point of illumination, the less energy it has. This light falloff can be used to create dramatic portraits for instance; in these situations the aperture and the designed light falloff is important.

With these points re-inforced, we can conclude that we can not simply move the lights further away as it will change the design of the shot.

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