UPDATE, JUNE 2024: Strobist was archived in 2021.
(Here is what I am up to now.)


Lighting 101: Balancing Flash and Ambient, Pt 1

Abstract: Okay, here we go... so far we have only been using flash as a light source. Now we are going to start mixing flash and ambient light!

F/stop, Shutter Speed and Flash

While f/stop and shutter speed both control exposure, for our purposes it is important to know how they do so differently. Shutter is a time-based control. F/stop is a diameter-of-the-lens-hole based control.

Since the light from your flash is pretty much instantaneous, it really does not care about the shutter—as long as you are at or below your camera's top "sync" speed. Which for most cameras is either 1/250th or 1/200th of a second.

Note that there are fancy, flash-pulsing methods which will allow you to sync at higher shutter speeds such as 1/1000th of a second. But (a) they have their drawbacks, and (b) getting into that now would be needlessly complex. So just set that aside.

Two Exposures Happen at the Same Time

Every time you take a flash photo, you are making two exposures simultaneously. You are making an exposure of the ambient light, and an exposure of the flash's light. Whether you take this into account or not, it is happening every time.

The ambient exposure is controlled by the f/stop and the shutter speed. The flash, being instantaneous, is controlled by the aperture.

The photo up top is a good visualization of the fact that two images are being made at once. The shot of Robert, a soldier in the U.S. Army, was made with a slow shutter speed. But I also included a flash, which happened instantaneously and froze Robert irregardless of the shutter speed:

Think of it as two overlaid exposures: Frozen, instantaneous flash exposure, mixed with a slow-shutter-speed ambient exposure. Both are made at once, and both light sources are additive to the exposure.

So you have two exposures to consider in every flash-lit picture: the ambient and the flash. I like to find my ambient exposure first—nothing fancy, just trial and error. Once I have that exposure (in which, remember, the shutter speed must be at or below my camera's sync speed) I have a starting point for my final, lit image.

Next, I'll "dial down" my ambient exposure. This means nothing more than changing my camera's settings to underexpose the ambient. How much? That's your choice. And it will determine the contrast range in your final, lit picture.

Remember, when you move your flash off camera, the difference in location produces shadows in your image. That's what makes your subject look all cool and 3-D. And the depth of your shadows—your contrast range; your drama— is determined by the underlying ambient exposure.

Let's Give This a Test Drive

Below is a portrait I shot of Jessie, a local social media entrepreneur. We are going to use a second flash here, to light the background. But the light balancing principles are exactly the same. They work whether you use one flash, two flashes or a hundred flashes.

Okay then. Let's get her in some shade first, because it's much easier to balance a small flash indoors or in shade rather than competing with the full sun. (But we'll get to that next post.)

Here she is, exposed for normal ambient light in shade:

The exposure here is f/5.0 at a 1/160th of a second. For the record, we are at ISO 200 on the camera's overall sensitivity setting.

It's okay, but kinda "meh," right?

So before we even add any flash, let's crank her down a little bit and create some "drama" in our final image. I am going to close down my aperture and drop her by a little over 2 f/stops. So I am going from f/5.0 to f/11. Nothing else has changed:

Exactly what you'd expect, right? Everything is darker. But there is still legibility everywhere - no big black areas. This legibility is important in the final image. Also, notice that since we closed down the aperture we now have more depth of field and the wall in back is now more in focus.

We have created a "safety net" of darkened ambient exposure. When we add flash, no part of this image will get any darker. So we'll end up with drama PLUS legibility.

Now, let's bring in our flash. (Flashes, actually.) We work with manual flash—for predictability and repeatability. One less variable to screw up. And because of this, adding the right amount of flash exposure to a photo is simple and straightforward.

I'll bring in a flash, on manual power, in an umbrella positioned out of the frame and from camera right. Take a test shot. If the flash is too dim, I'll turn up the power. Say it was at quarter power (on manual, as nearly always) when I made my first frame. If too dark I might turn it up to half power. Or vice versa if it was originally too light.

Also, I am going to do the same thing with a second flash back on the wall. Just to make the wall pop a little bit. And here is the result:

Wow, right? Same exact spot as the first shot above, which was properly exposed open shade. Then we dropped down that exposure to get the sort of "safety-net" ambient-only exposure. Then we lit Jessie (and the back wall.)

This, in a nutshell, is balancing ambient and flash. Everything going forward with your lighting kinda hinges on you getting this. So if you don't understand it, re-read it and stay with it. But be aware that it may not really make sense until you get out there and actually do it.

When working this way, I like to think of my flash as a main (or "key") light and the ambient as my supporting (or "fill") light.

The Process, in steps:

1. Get a full ambient exposure.

2. Drop the exposure down to create a little "drama." How much, is up to you.

3. Bring your subject back up to full exposure by adding flash.

It's Almost Not Fair

How often have you heard this, usually with a tone of superiority:

"I am a purist, I only shoot available light."

(Translation: I am scared shitless of flash.)

As an ambient light photographer, you only have one "correct" exposure. Maybe a little wiggle room if you are being interpretive.

But as a lighting photographer, we control everything in the frame, independently of the other areas, by how and where we expose and add light.

I have been doing this for almost three decades, and I still think that is the coolest thing ever. If you want more detail on the Jessie shoot, it is laid out in more detail (but also assuming a little more knowledge) in the On Assignment section, here.

Otherwise, let's flip the process and use flash to control the harsh shadows created by directional ambient light. Same process, just backwards.

Next: Balancing Flash Intensity With Ambient, Part 2


New to Strobist? Start here | Or jump right to Lighting 101
Got a question? Hit me on Twitter: @Strobist
My current project: The Traveling Photograher's Manifesto