backdrops - How can I get a pure white background in studio photography?

I am a natural light photographer that is venturing into studio/backdrop portraits at the request of some of my clients.

I have a portable studio for my needs with a white muslin backdrop.

I am using one (maybe two) Speedlite 580 EX flashes with shoot through umbrellas as lighting.

In my practice shots, I cannot seem to get a nice "blown out" white background without dulling my subjects (decreased contrast/lighter exposure on their face and clothes).

I am not a fan of seeing the muslin draped or crinkled in the background... although I know that's definitely a style... I will probably iron it but I can never get it perfectly flat and plus I need to transport it so it will get some wrinkles.

Also, what can I do to avoid any shadows against the backdrop itself (shadows created by the subject). I do have them placed slightly away from the backdrop but kids sometimes move around. Also if I am doing little children, I don't think having a small backlight would be practical (not that I have one... )

Answer

To get the "blown out" white background you have to overexpose the background.

You have no choice, in order to over expose the background you need a very powerful light aimed at the background.

If you have two flashes place one of them behind or to the side of the subject aimed directly at the background behind the subject (unmodified, without an umbrella), you want to set the power to the minimum that will overexpose the area around the subject (any area not touching the subject can be easily made white in post).

Placing you subject as far as possible from the background helps because it minimizes the reflection from the background on the subject.

You then use the second flash as the key light and ambient light as fill (obviously that means you start by metering for the ambient and set your flashes power based on it).