Photo: Rick Clemson
From N.K. Guy, Photonotes
Flash photography takes on two very basic forms. In regular flash photography, the flash is the primary light source for the photo. Flash metering is done for the foreground subject, and the background is metered by the camera’s regular exposure metering system. This can lead to the background being underexposed and dark if ambient light conditions are low. This is how most people think of flash - as a way of taking photos in dark places.
However, flash can also be used in bright locations or in daylight to lighten shadows, reduce the harsh contrast of full sunlight or brighten up dull images without being the primary light source for the photo. This is called “fill flash” or “fill-in flash,” though the latter term’s more commonly used in the UK. And it’s often a source of surprise for non-photographers, who don’t expect to see photographers using flash units outdoors on sunny days in brightly lit settings. In such situations the fill flash is being used as a sort of portable reflector - shining a little extra light in certain areas.
A typical example might be a person who’s wearing a hat outdoors on a sunny day. Hat brims often cast dark shadows over the subject’s face, and a little flash can lighten up this shadow nicely. A backlit subject is another common use for fill flash - you can’t simply crank up the exposure compensation to expose the subject correctly as then the background lighting would be too strong. Or perhaps you want a little sparkle of light reflecting back from a person’s eyes - the “catchlight.” Sometimes wildlife photographers use flash units at great distances from their quarry for the same reason - they aren’t using the flash to illuminate the animal but to provide a lively catchlight to the eyes.
In all these cases you are, from the point of view of the camera, using two light sources at the same time. There’s ambient lighting, which is all the available light around you - reflected light from the sun or artificial light sources. And there’s the light from the flash unit, which is supplementing this existing light. As always, ambient light levels hitting the film or sensor are governed by the lens aperture and shutter speed and flash levels are governed by flash metering. By adjusting the output of the flash unit you’re essentially adjusting the ratio between flash-illuminated and ambient light-illuminated scene.
In fact you could argue that the two cases present above - flash as primary light source and ambient light as primary light source - are an artificial distinction and that all flash photography is fill photography in a sense; just that in the first case the ambient lighting is so low as to be insignificant, whereas in the second case it’s the reverse. This is true enough, but the distinction is useful to make, particularly in terms of the way full auto and P modes work versus Tv, Av and M modes.
Unlike certain other camera systems (particularly Nikon), Canon EOS cameras always default to fill flash mode when the camera is in Tv, Av and M modes. They also perform fill flash in P mode if ambient light levels are high enough. There’s no separate switch or pushbutton to engage fill flash. For details have a look at the section on EOS flash photography confusion below.
Fill flash Ratios
The “fill flash ratio” is commonly described in terms of the ratio of ambient light plus fill flash combined, compared to the fill flash alone. Canon EOS gear, however, usually lets you adjust the fill flash in terms of stops of flash output, in either 1/2 or 1/3 stop increments. What’s the relationship between the two ways of describing fill flash?
* A ratio of 1:1 would mean that the flash unit is the sole source of light (0 ambient + 1 flash) and therefore you wouldn’t have a fill flash situation.
* A 2:1 ratio would mean that the ambient light and flash are at the same level (1 ambient + 1 flash). That basically means 0 stops of compensation given a fairly flat-lit scene, and usually results in rather unnatural looking fill flash.
* A ratio of 3:1 means that there is twice as much light from the ambient source as the flash (2x ambient plus 1 of flash). Such a ratio requires a -1 stop fill flash setting on the flash unit, since each stop means a doubling or halving of the amount of light.
* A ratio of 5:1 means that there is four times as much light from the ambient source as the flash (4x ambient plus 1 of flash). This is a -2 stop difference. Typically photographers use between 1 and 2 stops of fill flash to lighten shadows without creating a phoney flash-illuminated look.
However, the term “ratio” is confusing and seems to mean different things to different people. Sometimes people talk about a 1:1 ratio when the ambient and fill lights are of equal intensity. So a 2:1 ratio might mean -1 stop fill flash and 4:1 would mean -2 stop fill flash. In this case they’re talking more about the light output than they are about the reflected light.
The concept of ratios works well in studio situations where you have total control over the lighting. You can turn off the main light and measure the fill lighting with a meter, you can move lights around to vary their strength, etc. But if you’re taking a candid photo outdoors you have no such control. You can hardly turn off the sun, and automated TTL flash is going to have its own ideas as to what constitutes correct lighting.
