I am thinking of buying a softbox for a speedlight to step up my lighting, while staying within my budget. I have a Yongnuo YN-568EX II speedlight that O intend to use in the softbox. My question is which softbox would be suitable for me, if I wanted to take full body shots? Or can I even do that, given that I have a speedlight, not a strobe and full body shots require large softboxes. Is my speedlight powerful enough to support a large softbox of, let's say, 80x80 or should I go with 60x60? I'll be highly grateful if you help me out. :)
You can use a small 9 inch softbox which fits on the flash head or you can also use spedlight diffusers. You can use your speedlight at full power inside a large softbox also, the only constraint in doing that is the distance or the guide number of your speedlight. In a large softbox, the light is very nice and soft depending on the power of the flash fired and it is spread evenly.
If your speedlight has zoom feature, a small softbox can be very useful.
You need to know your flash. I'm making a generic answer to be usefull in all fhash models and brands.
1) The hotspot
Grab a sheet of paper, with some grid on it, for example draw some lines every 5 inches. Hold it on a wall.
Put the flash pointing at the paper, lets say 10 inches away. Use a tripod or stand if you have one.
Put the flash zoom at the lower focal length, lets say 28mm. and shoot it to the paper.
Take a picture and measure the hotspot. It will cast a rectangular hotspot.
You can repeat for example now 20 inches away, then 30 to confirm your findings.
Now you have an idea of the softbox size you could have. Not only the box size, but also the internal distance to the flash needed.
Normally a speedlight shoots a rectangular beam (more eliptical), not a square one (or round).
2) The power
Shoot the flash at a white wall, acoeding to some estimates you made at the previous point. For example with the flash 2-3 feet away from the wall.
Use this bounced light to iluminate a test subject. A standing person.
Do not use full flash. Use 1/2 or 1/4 and experiment with ISO 100-200 probably 400.
Depending on the results now you have an idea of the power of the flash.
There are diferent kind of softboxes.
Some shoot the light directly to the white silk, some shoot it to a "pre difusser", and others bounce the flash to the silver interior.
A pre difusser and a bounced softbox help to even the light, but also lowers the total output, probably 1 stop.
Some softboxes allow to put a head holding not one but 2-4 speedlights.
Some softboxes allow more freedom changing the tilt angle than others.
Go for a softbox designed for a speedlight. But the point is that you now have some more information to make a decision.
A quick test I did
With a 560III passing thru a big "photopgraphic" silk:
At 30 inches from the silk
Cast a hotspot aprox 35 x 30 inches
Gives an f/8 at 1/2 power at 40 inches away.
But at the end
Buy the biggest softbox for speedlights you can find. a 80x80 octobox with an internal difusser will cast a nice light on a subject. You always can play with the iso or aperture. :o)