A step-by-step procedure for using off-camera flahses with manual exposure control.
Using Off-camera Flashes in Manual Mode
Controlling exposure using an off-camera flash unit often relies on proprietary metering systems like Canon’s ETTL. These automatic metering systems work well in some occasions, but I often find that for full control of subject and background exposure, I end-up turning to manual exposure mode. While I don’t use flash very often in my work, it’s a technique that, when used discretely, can be very useful on the right occasion. Therefore, it's worth spending some time mastering flash exposure. In this short tutorial, I’ll describe the steps I use when exposing a photograph using an external flash unit set for manual mode.
The External Flash Unit
Figure 1 shows a typical flash unit. One other reason for using manual mode is when you are presented with a flash accessory that cannot communicate with your camera’s metering system, but that can be triggered by the camera nonetheless. This was the predicament I encountered with the Sigma EF-500 DG Super shown below.
Figure 1 - Sigma EF-500 DG Super Flash Unit
This flash was designed specifically for Sigma cameras, so it doesn’t "understand" the communications protocol from the Canon T1i body where I mounted it. However, this Sigma flash model is actually very versatile, offering a manual “M” mode where one can adjust the output power manually. See Figure 2. After setting the flash to manual mode (press the "MODE" button), the output power is simply adjusted in fractions by pressing "SEL" and then using the "+" and "-" buttons (1/1 for full power, ½ for half-power, ¼ for quarter power, etc). In Figure 2, the flash is set for 1/4 (one quarter) of full power. With the flash set this way and mounted in the camera we are in business.
Figure 2 - Manual Power Settings
How flash exposure works
Let's look first at the case of daylight, outdoors photography. Most people think of flash as something to use indoors in dim light. However, I find flash to be most useful as a "light-filling" element when one is confronted with harsh overhead sunlight. In this situation, the fill-flash eliminates harsh, unflattering shadows, often in the subject's face.
Generally speaking, I like to think about daylight flash exposure as exposing two components separately: the ‘background’ and the ‘subject’. The key considerations to understand in this scenario are the following:
a) Your choice of ISO and Aperture influence the exposure of both background and subject
b) Shutter speed (exposure time) affects primarily the background exposure (for daylight photography)
c) The flash output (flash power) affects primarily the subject’s exposure
Once you understand these guiding principles, the rest is easy. I’ll use an analogy that may help some photographers understand this. I like to think about flash exposure as equivalent to ‘light painting’. Light painting is the exercise where you shoot a (usually dark) background for several seconds or even minutes and paint with light over a main subject, say an old car or the insides of a cabin using a flash or a lantern. The result is that the background scene’s exposure depends mostly on how long your exposure was (shutter speed) and the subjects exposure depends mostly on how much light you ‘painted’ on it with the lantern or flash. Similarly, shooting a subject against a background using flash involves the same tradeoff. It’s just that in the later scenario, the time-frames are compressed. However, the relative subject/background times are similar because the flash burst lasts only about 1/1000 s, whereas your total exposure time is typically an order of magnitude larger at 1/100s or 1/50s (for human subjects typically).
With these principles in mind, let’s see how this all works with an example.
Manual Exposure Procedure, Step-by-Step
These are the steps I follow when lighting a subject against a background in daylight. These steps assume you set both your camera and your flash to manual “M” mode. The steps are:
1) Choose an Aperture and ISO speed
2) Choose a Shutter Speed (exposure time) to Adjust Your Background Exposure
3) Adjust the flash power output to control the subject’s exposure
Let’s look at each step in more detail.
Figure 3 - Canon DSLR with Sigma Flash
1) Choose Aperture and ISO Speed
These two are the global parameters that affect exposure for both subject and background.
The aperture choice is often an aesthetic decision (within the aperture limitations of your lens). For a shallow depth of field and blurry background, you may choose a wide aperture of say f/2 or f/4. On the other hand, if your goal is to have most of the scene in focus and maximum sharpness, you may choose to shoot at f/8 of f/10.
The ISO choice is going to depend of how dark your scene is and how much noise you can tolerate (the higher the ISO, the more noise you get). For daylight scenes, you may want to stick to ISO 100 to 200. Dark indoor scenes may require a higher ISO. As a rule-of-thumb, an ISO setting of 400 to 800 should be adequate for most indoors situations. Generally speaking, it’s a good idea to use the lowest ISO that allows you to expose your photo properly and not any higher.
2) Choose a Shutter Speed (exposure time) to Adjust Your Background Exposure
As noted above, in daylight situations, the shutter speed influences primarily the background's exposure. Experiment with different settings to make the background look as dark or as bright as you wish. Usually you also want to set a minimum shutter speed. A minimum of say 1/50s or 1/100s for shooting people is a good choice as people have this annoying tendency to move! Also, if you are shooting hand-held (i.e. without a tripod), consider that, as a rule of thumb, your minimum shutter speed should be about 1/focal-length. So if you are using a 135mm lens hand-held, a 1/135s minimum speed is a good target. Image stabilized lenses may give you some extra margin here, but often you run into the 1/50s minimum owing to people's movement. Remember that stabilized lenses only stabilize the optics, but not the people:)
When you can't get enough exposure for the aperture and shutter speed needed, then adjust the ISO. The key is to experiment, and experiment... Experimentation is 'free' with digital cameras.
Figure 4 shows a photo of the Nordic super-model "Russel" I purposefully hired for this shoot. Both images were taken without flash. Aperture and ISO were fixed at f5.6 and ISO 200 respectively (see Step 1) which gave me the blurry background I needed (this scene was shot using a Canon 100mm f2 lens). As you can see, I experimented with the shutter speed to get the background as bright as I wanted it to be. In this case, I chose to go with 1/100s (image on the left). Notice also that, although Russel is well exposed, one might want to brighten the shadows under his chin a little. We will use the fill-flash on the next step just for that purpose.
Figure 4 - Background Exposure, Flash OFF
Technical detail: there's also a maximum shutter speed that you should use and that's a limit imposed by the camera. This is the so-called flash sync-speed and is something like 1/200s or the T1i I'm using for these examples. Some cameras offer a much faster limit, so refer to your own camera manual for details. Using a shutter speed faster than the maximum flash sync-speed (say 1/500s) can translate into a visible black bar across the frame. You may also find that you can no longer increase the subject's exposure by adjusting the flash power under these circumstances. For these reasons, it's advisable to shoot below the maximum flash sync-speed (below 1/200s in my case).
From the discussion above, it should be clear that, in practice, your useable shutter speed range is quite limited: 1/50s on the low end (to avoid motion blur) to 1/200s on the high-end (to avoid maximum sync speed limits). If this range is not enough to properly expose the background, play with the aperture and ISO speed instead to control your background exposure.
3) Adjust the Flash Power Output to Control the Subject's Exposure
Russel is by all accounts a gorgeous looking model, but the chin shadows in Figure 4 don't do him any favors. Clearly, this photo can benefit from some fill flash to 'fill-in' these shadows.
I prefer my flash to be a subtle player in lighting my model. With full-power 1/1 setting (see Figure 5 below) I've clearly gone too far, and Russel has turned into an albino bear. Experimenting a little, I've found the 1/64 setting in Figure 5 to work best. It's just the right balance of natural and fill-flash in my opinion.
Figure 5 - Flash Fill. Left: Overexposed. Righ: well.. Just Right
Notes on Indoors Usage
The examples above involved outdoors shooting and using the flash as a "fill-light". Indoors shooting is somewhat different because the background light is normally much weaker and the space is smaller. Therefore, chances are the flash will contribute not only to your subject's exposure, but also to the background's exposure. The procedure is not so different from what was explained above, but under these circumstances, I would set a shutter speed fast enough to freeze human motion (say 1/50 to 1/100s), a relatively high ISO (400 to 800) and then play with aperture and manual flash power to get the right exposure. Indoors light varies quite a bit, so you will have to experiment. Let's look at an example.
For the indoors shoot, I contracted with super-model Naomi (See Figure 6). ISO was set to 400 as this was a dimly lit room. I chose a relatively fast shutter speed of 1/100s (to avoid camera shake and subject motion blur). For the aperture, I chose f/2.8 which gives me a nice blurry background. The result is the image on the left in Figure 6 (flash was OFF at this stage). This first attempt is clearly underexposed and too dark. This is where the flash comes-in. The image on the right of Figure 6 uses the same shutter-speed/aperture and ISO combination, but the flash was set to its lowest power (1/128). The result is not too bad, with the model properly exposed. That said, I don't like the contrast between the dark background and the very bright subject.
Figure 6 - Flash Fill. Left: Overexposed. Right: well.. Right
The solution? When shooting flash indoors, try to bounce the light from the ceiling rather than using "frontal/direct" flash (as was used in the example above). Figure 7 shows the final result with bounced flash and some adjustments to the shutter-speed and aperture. Notice that I had to increase the flash power substantially (from 1/128 to 1/2). This is normal as bounced light from the ceiling is dispersed and diffused, so you need to launch more power to compensate for that dispersion. The result however is much more pelasing as the light becomes softer and more even. Because the light was dispersed all over the room, the background brightness is now much closer to the subject's brightness, and the photo is much more pleasing to the eye.
Figure 7 - The final result
This is basically it. Manual flash exposure may sound intimidating at first, but once you understand the tradeoffs involved, it can be easily mastered.
Comments, questions, suggestions? You can reach me at: contact (at sign) paulorenato (dot) com