Digital camera makers emphasize megapixels and zoom length (and attempt to blur the difference between optical and digital zoom by combining both zoom factors into one). The emphasis is on the digital. With 8-megapixel sensors and 12x optical zooms, it's now time to return the emphasis to the camera.
What spec is the most important? Aperture size. The size of the lens opening has three effects on your picutres:
1. Speed. The larger the aperture, the more light gets in, the faster the shutter speed can be, the sharper the image. Also, the ISO (ASA) can be lower, also resulting in a clearer image.
2. Sharpness. The larger the aperture, the sharper the image. Smaller apertures, like f16, result in fuzzier images.
3. Depth of field. This is the trade-off: the larger the aperture, the shorter the depth-of-field. (In some photographic situations, of course you want shallow depth of field.) Indeed, aptertures became very small when the pocket-size focus-free film cameras ruled the marketplace in the 1980s. Camera makers could dispense with focussing by using a small aperture that allowed nearly infinite depth-of-field.
The drawbacks to large aperture are:
- more glass means higher expense
- larger size negates compactness
- increased zoom means decreased aperture
The lens on the Minolta XE-7 I bought in 1976 had an aperture of f1.4; cheaper cameras had f1.8, wjile more while expensive ones had f1.2. I recall reading a photography magazine that reviewed an f1.0 lens: very expensive, very big, and able to take pictures at very low light levels.
These thoughts came about when I got my Canon S1is a year ago, and wondered why pictures I took inside a banquet hall turned out shakey. It was months later that I realized its maximum aperture was f2.8, compared with f2.0 on my former G1 camera.
The relationship between aperture and shutter speed is not linear. This chart can be helpful:
f1.4 - f2.0 - f2.8 - f4 - f5.6 - f8 - f11 - f16 - f 22
Each successive aperture setting lets in half as much light, because aperatures deal with circular areas. f2.0 lets in twice as much light as f2.8 and four times as much as f4.0. Conversely, the shutter speed at f2.8 must be twice as slow as at f2.0, leading to the problem I experienced with the S1is.
Shutter speed is linear, and this table might be helpful. Assuming a scene where f1.4 requires a shutter speed of 1/1000, changing the aperture results in these changes in shutter speed:
f1.4 - 1/1000
f2.0 - 1/500 <- G1 maximum aperture
f2.8 - 1/250 <- S1 max aperture
f4 - 1/125 <- slowest handheld speed for National Geographic photographers
f5.6 - 1/60 <- recommended slowest speed for handheld shots
f8 - 1/30
f11 - 1/15
f16 - 1/8
f 22- 1/4
Aperture size doubles, shutter speed quadruples