Richard,
If it helps, there are exactly three things that can be set on a camera, and all that the various modes
do is shift those three around one way or the other.
First, there’s the aperture. Inside the lens is an iris that opens and closes by a variable
amount. The more open it is, the more light gets in. At the same time, the more open it is, the
narrower the plane of sharp focus becomes (and the blurrier things get in front of and behind
whatever the lens is focused on). Aperture is measured in f/stops, with small numbers equating with
large apertures.
Next is the shutter speed. Simply, it’s how long the curtains that keep light from hitting
the film / sensor are left open. The longer the exposure, the more light gets in, but there’s
also more time for things the image to get “smeared” by the motion of either the
subject or the camera if it’s not mounted to a sturdy tripod. Shutter speed is measured in
fractions of a second (or seconds for really long exposures).
Last is the “ISO” rating, which determines how sensitive the film / sensor is to
light. Higher ISO ratings let in more light, but also increase the amount of grain / noise in the final
image.
For a digital camera, the “ideal” exposure is one in which the histogram (press the
“Info” button while previewing the image on the camera) is centered. When taking
pictures, start with a wild guess based on what you think you want as far as depth of field (aperture)
and stopping motion (shutter speed). Take a picture. If the preview image is even vaguely close,
zoom in all the way to check if there are signs of motion blur (smears streaking in the same
direction); if so, either use a tripod, improve your shutter release technique, or speed up the shutter. Also check to see
if you have enough of the picture in focus, or if you have too much of the picture in focus; adjust the
aperture accordingly. Finally, set the ISO to the lowest number that puts the histogram in the middle
of the graph.
And that’s 90% of what you need to know about digital exposure.
For your example of a close-up shot of a candle, I’d absolutely use a tripod. I’d
set the aperture based on how much of the foreground and background I did or didn’t want in
focus. I’d set the ISO to 100, and then adjust the shutter speed until the histogram was
centered. In this particular instance, most of the histogram will be on the left (representing the dark
surroundings), with a small peak on the right (representing the flame). I’d make sure that the
right-hand side was an actual peak with an upslope and downslope, rather than just an upslope with
the right side all the way against the right edge of the graph; doing so will ensure that you get lots of
detail in the flame. It will also probably move the left side all the way against the edge of the graph,
since the camera can only capture so much of a variation between light and dark (aka
“dynamic range”). That may be the goal of the shot, to show a bright and colorful
flame against a black background; if so, you’re all set. But if you want the surroundings
included in the picture, you have two choices: take two exposures, one for the flame (with the
surroundings dark) and the other for the surroundings (with the flame pure white), and blend them
together with Photoshop (an advanced technique, but not too difficult), or to add light to the
surroundings (probably using either regular tungsten bulbs or off-camera gelled flash).
Cheers,
b&
