When indeed. A question that might puzzle the ages - it certainly has received the attention of the manufacturers. Some of their answers are worth looking at.
We all know what a standard soft box looks like - we get the medium size square ones when we buy an Elinchrom D-Lite 4 or D-Lite 2 kit. Four metal rods, a funny-shaped ring, a black fabric pyramid , and a diffuser screen for the front. Clap it together and it does the soft-box look beautifully.
Big ones from this same manufacturer can come in octagonal, rectangular, or narrow strip configuration - the all have their own characteristics and they are all needed for certain classes of photography. You can even get generic ones from other makers that will fit the Elinchrom and some of them have umbrella-like self erection. Great for travelling with it.
Really huge ones are also made, but photographers contemplating putting these together need a will of iron, muscles of steel, and the language of a stoker on a bucket dredge. Once erected, they provide a buttery-soft light but when they are no longer needed they are better abandoned derelict rather than dismantled.
The subject of this column is the small versions that are intended to go on speed lights. Basically they are the same as the big ones only tiny in size...and to some extent that means they don't do what big soft boxes do. There just isn't enough light radiating surface to envelop the subject. If they do get that big, they are too big to handle. Catch 22.
All is not gloom - what they do is let out a blast from the speed light in a flat manner - no concentrating ridges on the front of the speed light lens and fewer hot spots. They are particularly good for catchlights in the eyes - not the sharp pinpoints of a bare flash and not the alien zombie look of a full-sized soft box. Now that you can get them from Metz in octagonal form you get much nicer eyes.
You fasten them on with elastic straps and velcro and they are sturdy enough to run at weddings or fashion show. There are rectangular styles as well as the octagonal and several sizes are made. Note the neat pack it all folds down into for transport. Try doing that with a 2mtr by 2.5mtr box...
