Getting a white seamless look with one light.

[CameraMonkey]

I'm well aware I'm asking a lot here, but I'm going to do it anyway apparently.
So I really like white seamless backgrounds. A black and white portrait on a white seamless background is a really classy look, imo. Problem is, I don't have two lights to stick on the background and another for my subject. I have one flash gun and a shoot through umbrella. I've tried two methods to try and achieve the look, one was to put the flash behind my subject on the background and use a reflector to catch light from the background and light my subject. That sort of worked, the background popped nicely to white, but in order to light the background and my subject I had to up the power on my light until it blew out the edges of my subject's shoulders.
My second attempt was to have my light directly behind me, pointing straight at my subject. I had to work on it a bit in lightroom, because to get the background white I had to over expose the subject, and in the end I didn't really get either right.

So am I missing something, does anyone have any ideas or techniques? Is it even possible?

(For reference, this is the second attempt. I don't think it's a terrible image, but it's not quite what I was aiming for)

 

[andyscott]

Light your background with your one flash and the subject in natural light

 

[Phil V]

It's called the Inverse Square Law, and as such it's a Law.

As light travels it's power lessens, you acknowledge in your post what the ideal setup is, and in your first attempt you tried to overcome it.

There are two methods though... move your light source so far away that the difference between subject-light source and background-light source is so small that the light hitting them is almost the same (like on a bright sunlit day - the sun is a long way away), the other is to put your subject right against the background (the same maths applies) which can work for some fashion high key, but is less useful for straight portraits.

Without 2 other flashguns though? a single cheap manual flash with an optical slave is cheap, stick it on it's widest setting and directly behind your subject and it would be usable.

edit: or what Andy said

 

[Kodiak Qc]

•
If you use a wide umbrella or soft box that you will keep close to your subject,
you will get no shadow and, depending how far all this is to the background,
a with white seamless may turn onto a black one!

 

[Garry Edwards]

For the reasons given in the first answers, it simply cannot work - there are 2 separate subjects here, I call them front subject and rear subject, and they need to be lit separately.

There are some people who are happy with using just one light on the rear subject, but I'm not one of them because a white background needs to be lit evenly, and this requires a minimum of 2 lights IMO. When done with just one, the power needs to be much higher in order to make the less well-lit areas photograph as white, which means that there is dramatic overexposure of the area actually lit by the single light, which causes severe edge degradation, flare and low contrast.

 

[Kris]

It can be done. At least for a head and shoulders.

It involves

- One light on a stand to subject side
- 'shading' the subject using a two stop scrim (photographer speak for a translucent panel) for a soft light effect,
- letting the light fall unhindered (i.e. passed the scrim) onto the background which makes it two stops brighter than the subject (and therefore appear white). Twisting the bg slightly toward the light to even it out if necessary.
- and you can even carefully angle a mirror from the opposite side to give you a hairlight.

Though another flash is going to make your life a hell of a lot easier.

It's a common puzzler / task that is set for certain lighting courses at degree level photography. Makes the students think about light and how to make one light look like three.

Try it. Experiment. You'll certainly learn along the way.