DIY softbox question

[Dmitri]

How much does length matter?

I mean: I know that its better to have a 2'x'2 diffuser on the end of the softbox than a 1'x1' (for larger coverage/more even light), but how much would it matter if it extends 5 inches from the flash or 2' from the flash?

 

[Derrel]

The needed length of the softbox, which is usually called the depth, depends quite a bit on the flash head that will be used. If the flash is capable of bare-tube operation, then the depth does not need to be as great as if the flash puts out a narrower-angle beam of light. So, for let's say a softbox that will be used with a flash head that MUST have a reflector on it, a deeper softbox is usually going to give a more even dispersion of light is the flash head is positioned far enough from either an internal baffle, or the front face of the softbox. If the box is too shallow, and the flash head is not dispersing the light reasonably well and evenly, the softbox might have a rather hot spot in the center, with strong fall-off at the edges.

Using a spededlight flash, let's say one with a 14mm diffuser flipped down, I still do not think a 5 inch deep box will be deep enbough to illuminate a 24x24 inch softbox with good evenness of illumination. However, if the box were 24x24 and 24 inches deep, the evenness of illumination would be more even.

You can try positioning the flash very close to an even-toned wall or fabric, and shoot some underexposed shots to get an idea of how the flash beam disperses, and that might give you a better idea of where your particular flash or flash head works acceptably well.

 

[Dmitri]

Thanks for the info Derrel. I had feared an answer like this lol (both!). But at least I better understand "why" now.