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

Question

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 1

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).

Answer 2

Just to add to what Nlr was written:

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

Instead of "very powerful", what should actually be said: two stops brighter than the exposure of your subject.

So for a given ISO and shutter speed, and (say) f/5.6 for the subject of your photo, you need to have your background at f/11. If your subject is at f/4, then your background has to be at f/8 to be blown out. (On the opposite end, if you want your background black, you need it to be exposed four stops below your subject.)

Most dSLRs (assuming you're using one, and not medium format or film), have about a six stop range. The middle (18%) grey that the camera meters for is most of the way up in that range on all dSLRs that I'm aware of.

So saying "very powerful" is a bit meaningless, and depends entirely on what you are exposing the subject at.

If your subject is at f/2.8, then reaching f/5.6 should be unreasonable for many flashes. If your subject is at f/11, then you'll definitely need a lot of photons to hit f/22.

This introduction by Joey Quintero for B&H is pretty good intro on studio lighting and light ratios:

http://www.youtube.com/watch?v=-ibFanS5GC4

Answer 3

The right way to do this is to light your subject and your background separately.

Ideally you would use two (or more) flashes to light the white background, with one on each side. However, with enough separation and a tight enough shot, one flash for the background can be enough to make this work. Set the exposure for this with a few test shots and refer to your histogram. You should try shooting as metered, refer to the histogram, add some "+" exposure compensation, then try shooting again. You want to get the white to just into the highlights. You do not want to blast them with all of the power in the flashes and then consider yourself successful. Blasting it with so much light will cause the background to reflect light and become a light source itself, causing the light to spill from the back onto your subject.

Once you've got the background set, move on to getting the subject exposed correctly.

Answer 4

@cm49, know that this is hard to do and that you may find touching it up a bit in post is easier than getting the perfect white background. @dan's reply is spot-on. You need your background 1 to 1.5 stops brighter than your subject.

And of course, depending on the distance between subject and background, you may see some bleed from the background being a light source (as @dan mentioned). That's one reason it's hard. So get the happiest medium you can and handle the rest in post.

Good luck!