Numbers: Show Me the Numbers..

I like numbers. Good numbers, like 8, and it's just so that I should get to know some more numbers when using my flashes/light modifiers.

Over the years, my training and job have made me a very cautious type - I want to know in advance what I am doing and so we can mitigate risk appopriately. This fits with my use of flash and light modifiers (softboxes/umbreallas etc) and whilst I will still meter each light, I still want to have a very close approximation for possible power output given a particular distance and particular modifier to know if what I am doing is feasible.

And here's my bunch of numbers for a (very limited) set modifiers based on the output from a SB80dx: the SB unit has a given guide number ISO100, 35mm, 125ft/38m.

The light is measured by a Minolta IV F, set to IS200, at a distance of 6ft.

For reference, the SB80dx full output at it's different zoom settings are provided as the baseline for comparison:

24mm	28mm	35mm	50mm	70mm	85mm	105mm
16 +3/10	16 +3/10	16 +6/10	16 +9/10	22 +3/10	22 +4/10	22 +5/10

Notice the gain of 1 stop from widest to tele - whilst usually not applicable with modifiers, the extra stop will be useful when used bare during high ambient (ie noon/sunny 16) situations.

All comparisons below are made with the SB80dx at the full power/24mm zoom setting:

Westcott 43" white convertable bounce umbrella: umbrella at 6 ft			8.0 +1/10	-2 stops
Westcott 43" white shoot through umbrella: umbrella at 6 ft			8.0 +4/10	-2 stops
Westcott 43" white shoot through umbrella: flash at 6 ft			8.0 +6/10	-2 stops
Westcott Apollo 28 softbox			5.6	-3 stops
Ezybox (clone) single diffusion softbox			11 +4/10	-1 stop

At a more conservative working distance of 3ft, we can roughly gain an extra 2 stops over the numbers presented above (ie bare 24mm at 3ft ~ f/32 +3/10).

Realistic Working Apertures

One important thing to note is the numbers provided above are the readings are taken from the centre of the modifier - this reading, whilst providing the max output, will unlikely be the working aperture due to the positioning (ie feathering) of the modifiers during use.

It is very seldom that the centre (and hottest) part of the modifier be focused onto the subject and therefore realistic working apertures, in my experience and with my usage, are approximately 1 to 1.5 stops down from the numbers above.

For example, whilst our numbers show that the shoot through umbrella at 3ft will give us a max working aperture of ~ f/16, experience has shown that I am realistically only able to acheive ~ f/11.