Balancing flash and natural light
Using flash in photography is often a source of great confusion. I’m going to try and simplify it down to the bare bones, so that if you’ve never used flash or have been a bit intimidated by it in the past, hopefully you can make enough sense of it to give it a go.
The “they say” rule of thumb for flash exposure
Flash exposure is directly controlled by aperture and your ambient exposure is directly controlled by your shutter speed
Is a generalization and there are exceptions to that rule, but for now I’m going to keep it simple and start there.
I find the easiest way to understand how this works is to see it in action using different settings with the same subject so you can see how adjusting the settings changes the resulting image. If you want to follow along find yourself a subject and set up a scenario where you can try out some flash photography.
STEP #1 AMBIENT LIGHT ONLY
Before you add any flash, take one photograph using only the light that is naturally occurring the scene. That could be sunlight, window-light, or even indoor man-made lights such as fluorescent or tungsten. You’ll sometimes see natural light referred to as “ambient” which is actually more accurate, but the two terms are often interchanged. Take it simply to mean the light occurring in the scene that you cannot control, it’s already there.
NOTE: the example photographs were taken during my recent workshop in Drumheller of Bob, an ex-miner. Bob was kind enough to pose for us at the coal mine where we did part of the workshop. The room we were in is quite dark with only a few small windows behind the camera.
So first thing I did was to take a “base exposure” using just the ambient light as described above. This is the result. Note the exposure information below in the image caption.
After reviewing the image I decided the lighting from the windos wasn’t all that bad, but I thought it could be better. I find the light on Bob is a bit flat, as it’s coming from two windows on either side of me, behind camera. What that does when the light falls on the subject from the camera angle is it makes the subject look overly flat, with little dimension and texture. So I decided to add in two flashes set up off camera.
STEP #2 ADD FLASH
I use a set of remotes that do not use TTL (through the lens) technology which means they fire the flashes but do not communicate any of the exposure data back to the camera. Why you may ask? The answer is simple – I could get remotes that work with TTL but they are about $250 per unit and you need one on the camera (sender) and one on each flash (receivers) so for $750 I could do that. However the set I use is about $99 or less so went through route and just use the flashes in manual power mode. It’s really quite simple to use even though it seems harder.
- set up your camera
- set up your flash
- fire a test shot
- if the image is too bright turn down the power on the flash, more it further away from the subject, OR use a smaller aperture
- if the image is too dark turn up the power on the flash, move it closer, or use a bigger aperture.
So in this respect this is often why you will hear that the aperture controls the flash exposure. Here is a lighting diagram showing how I set up my two flashes off camera.
NOTE: please keep in mind you don’t have to break the bank to buy a couple flashes and a set of remote flash triggers! Yongnuo radio triggers and Yongnuo has some excellent affordable options for flashes. For less than about $150 you can get two flashes, and a set of triggers.
I positioned my flashes and adjusted the powere on each so I was getting a good exposure at f4.5. For this first image using flash I used the same exposure settings as the previous image using ambient light only. Note the background appears exactly the same. This is because the exposure hasn’t changed and the flashes are not lighting it up.
Notice the only thing that changed was I used a slightly wider lens and moved in a lot closer to him. I did that because I wanted to emphasize him a bit more (things closer to the lens when using a wide angle look larger) but still see enough of the room to see the ceiling. I also had him turn on the lamp on his helmut.
STEP #3 REVIEW AND ADJUST AS NECESSARY
I was pretty happy with that but taking one more look decided I wanted a bit more drama with the lighting on his face. There isn’t a lot of contrast on him or ratio (difference between the dark and light side) because the light from the windows behind the camera was acting like a big fill light and brightening up the shadows. So how do I fix that?
Go back to our rule of thumb starting point – aperture controls the flash exposure and shutter speed control ambient. Okay so what that means in this scene is that if I change my aperture it will make Bob darker or lighter because he is being lit primarily by flash. If I change the shutter speed it will affect both Bob and the background. As I want it darker, I simply need to dial in a faster shutter speed and the background will get darker.
Dial in a faster shutter speed and the background will get darker. Huh?!
How that works is our base exposure f4.5 at 1/8th is a correct exposure for the natural light in the room. If I change it f4.5 at say 1/60th (3 stops darker) it is in affect underexposing the room light. If I had taken a shot at that exposure without any flash it would just be dark overall. But remember the flash is firing and lighting up Bob at f4.5 which results in an image that looks like this.
For some reason, which I cannot remember now, I did change my aperture to f5. If I recall correctly my exposure on Bob was a little over so I wanted to correct that and going from f4.5 to f5 was enough.
Notice the difference in the lighting on Bob here
Let’s look at what’s happened:
- there is more dramatic lighting on his face
- you can actually see the second flash coming from behind him which is lighting up his ear and side of his face as well as the side of the big wood stove on the left
- there is more of a stronger ratio (difference between shadows and light areas) on his face
- the background behind him is darker
- there is a lot more texture showing in his shirt and on his face (which in this case with a guy like Bob shows more of his character)
For this last image I took it even further and went to yet a faster shutter speed. This added more drama yet, more texture and darkened the interior of the room further. We also added a steam punk play gun as a prop, which as you can see Bob had NO fun with at all!
Why stop at 1/200th of a second?
If you are wondering why I didn’t go to an even faster shutter speed than 1/200th it is because my camera has a flash sync speed of 1/200th of a second (consult your camera’s manual to check for your camera sync speed). If I shoot faster than that, the shutter will open and close so fast the flash doesn’t have a chance to fully expose the whole scene and I get an image that has a black band across half of it. Many flashes do have a “high speed sync” setting. However – when using the flash off camera and in manual mode that setting is not available. If you are using the flash on camera you can set your flash to “high speed sync” (check your flash manual for how to set that up) and use any shutter speed you want. Or if you fork out the money for the TTL triggers it will also work with them. The only time I really find it necessary to use really fast shutter speeds and flash is when I’m working outdoors and I want to overpower the natural light, which isn’t very often.
Before and After
Here is a comparison of the before and after showing the first test shot using just ambient light. Compare that to the after one with flash added. Which do you prefer? Which has more drama? Think you can do this?
Challenge and action plan
I know you can do this, and the beauty of digital photography is that you don’t have to wait to find out! Worst case scenario is you do a test and it’s not exposed right, but you have the power to review your image immediately and make adjustments. Keep these quick tips handy or write them on an index card to help you. In fact, here’s a handy PDF Cheat Sheet for you, just print it off and keep it in your camera bag!
For another article on using flash please read: Off camera flash techniques for dramatic portrait lighting
Quick rules of thumb to use as starting points
- Flash exposure is controlled by your aperture setting
- Background and ambient exposure is controlled by the shutter speed (note the subject may get brighter if you use a slower shutter speed to lighten the background)
- If your flash exposure is too bright you can: move the flash further away from the subject; OR turn down the power output on the flash; OR adjust your aperture to a smaller opening. Ex. you would go from f2.8 to f4.
- If your flash exposure is too dark (subject is dark) you can: move the flash closer to the subject; turn up the power output on the flash unit; or adjust your settings to a larger aperture. Eg. go from f5.6 to f4.
- If your background is too dark use a slower shutter speed. Note you will want to use a tripod if your shutter speed is longer than 1/your lens focal length. (eg. if you’re using a 50mm lens if your shutter speed is slower than 1/50th use a tripod)
- If your background is too bright use a faster shutter speed.
Now get out there, get a flash and be fearless!
Until next time, cheers!