A few notes about on-camera flash

What follows here are a few notes on the use of on-camera flash taken from discussions in my workshop “The Art & Craft of Digital Photography”: a five week course I teach in Richmond, BC (Canada). Hopefully this is useful.

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Advances over the past few years have greatly the flexibility and accuracy, and therefore usefulness of small on-camera flash. While we no longer need to be as concerned about the mental gymnastics that we used to through, there are still a few pitfalls of which you need to be aware.

First: a quick review:

1. The burst of light from your flash is very short; so short in fact that the shutter speed has no effect on the amount of light from your flash that reaches the sensor in your camera.
2. The only issue with shutter speed that you need to be aware of, is that there is a maximum shutter speed at which you can use your flash. Choosing shutter speeds shorter than this will result in the shadow of the second shutter curtain falling across your image. Most cameras today synchronize with flash at either 1/200 sec or 1/250 sec. Most modern cameras will automatically sense when a dedicated flash unit is attached to the hot shoe, and set the maximum shutter speed for you. Check your camera manual for more detailed descriptions of this issue as it pertains to your camera.
3. Given a constant light output from your flash (I.e., if you put your flash in manual vs. automatic exposure mode), the only control you have over flash exposure is the aperture setting on your camera, and the flash to subject distance.
4. Given a constant light output from your flash, the intensity of the light reaching your subject falls off according the “Inverse Square Law”. Big word, simple concept: Doubling the distance from your on-camera flash to your subject results in only one quarter of the light reaching your subject: for a two stop exposure difference. In other words, the intensity of the light from your flash is highly dependent on the distance from your flash to your subject.
5. In automatic exposure mode, modern on-camera flash systems adjust the light output from your flash according readings by the in-camera exposure metering system.  The most recent offerings from Canon and Nikon take into account camera flash to subject distance and subject reflectivity, making then very accurate in majority of cases.

Ways to enhance your flash pictures.

On camera flash is a great addition to your creative arsenal, but it suffers from a few problems:

1. It’s a small specular light source, in other words, light from your flash is quite harsh.
2. The light from a small flash is typically very cold (bluish)
3. The light from an on-camera flash is essentially front lighting. It therefore does nothing to reveal form in your subjects. (Revealing form requires light to strike the subject from the side.)
4. Because the light from your flash falls off in direct relation to the distance to the subject, it is prone to over-lighting objects that are very much closer to the flash, than those that are farther away.

Solutions:

1. Indoors and outdoors, diffuse the flash to soften the specular nature of the light. Relatively inexpensive devices are available and do at least a reasonable job of this:

• The Sto-fen Omni bounce diffuser has been around for years (http://www.stofen.com). It’s small, inexpensive, and does a reasonable job of diffusing the flash.
• Perhaps a bit more effective is a product by Lumiquest, (www.lumiquest.com ) called a Pocket Bouncer. Lumiquest also makes a diffusion screen for the little pop-up flashes common on many DSLRs today.

2.  Indoors, consider bouncing the flash off a white ceiling. Practically speaking this is only possible with the relatively low ceilings found in most homes. In reception halls and similar large social gathering rooms, the distance to the ceiling is often just too great. Increasing the ISO setting on your camera may allow you to use this technique in larger rooms, and is worth a try. Although the light from this bounce technique is soft and flattering to subjects because it is essentially top light and will tend to leave dark shadows in the eye sockets of people.

When bouncing your flash indoors, consider attaching a small white card with elastic bands to the back of the flash head so that it protrudes above the flash head in its bounce position, pointing forward. This will provide a bit of fill light directed forward to mitigate the top lighting of the bounced flash.

3.  To introduce some modeling on your subjects, consider moving your flash off camera using a dedicated extension cord. With these cords you will retain all the modern automation features available on your flash.

4.  Outdoors in sunlight, use your on camera flash to put some light into the shadows in order to reduce the excessive contrast in this situation. A good starting point is to dial down the intensity of the flash, by adjusting the flash exposure compensation to -1 2/3 stops. This is starting point, experiment with flash exposure compensation settings from -2 to -1 stop. Just remember that with fill flash, “less is more”. Properly done, the use fill flash should be nearly transparent to a viewer.

Be wary of objects that may be present in the foreground (in front of the subject you are trying to fill). Because the intensity of your flash falls of in relation to the distance to the subject, these foreground objects may appear over-lit.

Some advanced tips:

It is important to make the colour temperature of your flash match the colour temperature of the ambient light falling on your subject. Cold bluish fill flash on a subject warmly lit by the setting sun will appear unnatural and unflattering.  Fortunately this is easily and inexpensively fixed:

•   Tape a piece of a warming filter from companies like Rosco or Lee. A variety of these are available in a sample sized “swatch book” free or at nominal cost from a professional light supply store that caters to professional photographers, theatrical lighting engineers and movie production companies. These sample sizes are an almost perfect size to fit over the flash head of most on camera flash units. The intensity of these filters should be chosen as a matter of taste: for general fill flash work, try a 1/8 CTO, 1/4 CTO, or a “Bastard Amber” as a starting point.

•   At sunset you may wish to try something a bit stronger… perhaps ½ CTO to full CTO.

•   Indoors, under typical residential lighting you may already have experimented by cranking up the ISO of your DSLR to say 1000 or higher and setting your camera’s WB to “Tungsten” (which essentially allows the camera to reproduce a colour balance which is more natural and pleasing to the eye under these very warm lighting conditions).  However the lighting in most homes is quite contrasty, leading to dark shadows and/or blown highlights.  You can reduce this excessive contrast by using your flash dialled down by about one full stop.  But without filtration, the light from your flash will be extremely bluish in comparison, particularly with the camera’s WB set to "tungsten".  The solution once again is to use filtration over the flash head to balance the nominal daylight balance of the flash to the tungsten lighting in your house. A "Full CTO" filter from the swatch book taped over the flash head will reduce the colour temperature to match the ambient light in your home, and will produce pleasant fill for you indoor setting. You have now more or less made the colour temperature of your flash match that of the ambient light in most residential living rooms.

• To avoid over lighting foreground objects nearer to your flash then your main subject, particularly outdoors, try a technique that Galen Rowell referred to as “Tunneling”. Even if your lens is say a normal or wide angle, set the flash head “zoom” feature to a setting in the telephoto range. This will concentrate the flash beam on your subject (you may have to adjust the aim of the head a bit for an off centre subject), which will allow you to avoid over-lighting any objects that may be in the foreground, closer to the flash than your main subject