I am not a portrait photographer and so I’m not really set up to take portraits. It’s not that I don’t have the technical know-how, I consider myself at least slightly above average in that regard. It’s just that I don’t have an actual studio setup. So when I do my occasional studio style portraits, I’m faced with an interesting challenge: How do I manage without the necessary gear. It’s not the flashes or the modifiers I’m longing for – I have enough of that stuff to get by. It’s really the backdrop. I don’t have one. Oh, but I have a trick up my sleeve…
The Inverse Square Law.
Except that I like to call it the Fake Black Backdrop Trick, because that sounds far less technical and certainly less intimidating. Taking advantage of the Inverse Square Law – Er, I mean the Fake Black Backdrop Trick – it is possible push the camera to essentially ignore everything in the background and record it as black. It requires a fine balance between the camera’s settings and your flash(es), but it’s an effective way to make isolate your subject and create a flattering portrait.
The Fake Black Backdrop (FBB) Trick takes advantage of an elementary law of physics that I already mentioned: The Inverse Square Law. The law states, in so many words, that the intensity of light is inversely proportional to the square of the distance from the source of the light:
Intensity ≈ 1 / distance2
So, for example, if the intensity of the light source was a value of 1, and we wanted to know what the light intensity might be at 4 meters away, we would use the equation to find the light intensity was only 1/16 (1 over 4 squared which is 1/16). In other words, light intensity diminished quite quickly as you move further and further from the light source. This is in part why portrait photographers may put a light modifier very close to their subjects. But it’s also the basis for why we are able to black out the background: It is simply too far from the primary light source.
Putting It To Use
The precise calculation doesn’t really matter that much and we really don’t need to know exactly how much intensity the background will receive. All that we really need to do is make sure that we are overpowering the ambient light and that there is enough space between the background and the light source so that we can turn it black. To do that, we will need some basic gear:
- A camera with manual controls
- An off-camera Flash (or more)
- A subject
An off-camera flash is very important as you won’t be able to accomplish similar results with the camera’s built-in flash. It is simply not powerful enough and it cannot get any closer to your subject than your camera can. In order to take advantage of the FBB trick, you need to move the light source as close to the subject as possible. In the examples I have here, I’m shooting with an umbrella (shoot-thru) and they are just barely out of frame. This had a bonus advantage of yielding much softer light, but we’re doing this so that the light intensity is at a maximum on the subject. Then we make sure there is plenty of room behind the subject so that the offending background clutter receives the least amount of intensity. In the space available, I will put the subject as close as possible and facing one wall. I may even have my back up against the wall if necessary. This puts much more space behind the subject, giving the inverse square law room to work its magic.
As for the camera and flash settings, it’s pretty basic; there is no balancing of light here, we’re trying to overpower the ambient light. We’ll want to work in manual mode so that we can take advantage of the high-speed sync. This allows us to have much shorter (darker) exposures. I’ll work with my key light at full power or maybe half power, supplementing as necessary with a second flash or a bounce card. Then to get dialed in, you’ll want to do a few test shots (maybe with a stand-in so your subject doesn’t get annoyed). I have a pretty good knack for it now, but finding that sweet spot where your subject is well lit but the background is almost black might take some trial-and error. Control the background intensity by increasing the shutter speed or closing up your aperture. Just be aware that at some point (usually 1/200 or 1/250), your flash may no longer be able to sync and the flash won’t even register in the shot. For fine tuning, if there’s too much light on the subject, you can simply pull back the umbrella a little to decrease the intensity on the subject.
Don’t worry too much if the background isn’t entirely black. Metallic and light colored surfaces may just barely register in the background. Maybe they won’t harm the portrait if left alone. But you can make the background even darker in post-processing.
Nothing is going to replace a good background or an interesting background. But the Fake Black Backdrop trick is one that I frequently use effectively. It’s nice to learn for that pinch-hitting moment when you have no other options. But it’s also a great technique if you’re looking for that dramatic style of portrait. I also believe that this is a great technique for photographers new to the world of off-camera flash as it’s simple enough to learn, but it teaches you so much at the same time. The FBB trick is certainly one worth keeping in your back pocket. You never know when it will come in handy.