Reciprocity
Reciprocity is the rule that states that if, beginning with the correct shutter/aperture combination to yield a perfect exposure, the shutter speed is adjusted in one direction (faster, for example) and the aperture is adjusted correspondingly in the opposite direction (opened up, for this example), the exposure will be the same.
Let’s say a correct exposure for an image is 1/125 at f8. Charting all equivalent shutter speed/f-stop combinations would look like this:
1/15 1/30 1/60 1/125 1/250 1/500 1/1000 f22 f16 f11 f8 f5.6 f4 f2.8
Each combination would yield the same correct exposure. What would change would be the amount of blur in a moving subject (or moving photographer) as the shutter speeds got longer, or the amount of depth of fi eld in the image as the aperture became progressively smaller.
Note that this is a constant rule only when the light source itself is constant. Sunlight and most available light is “constant” in that it doesn’t change over the course of the exposure. Fluorescent lights are not considered “constant” because they fl icker on and off 60 times per second. Studio strobes and on camera fl ash units are not constant sources of light because they fi re and expire somewhere between the time the shutter actually opens and closes, so the amount of light they produce may be fi gured into the equation (like fi ll fl ash) and used to advantage to either supplement existing light or overpower it.
Back in the days of fi lm, most of us were trained to think in terms of whole stops and half-stops. In other words, I might have told my assistant to “get me 11 and a half” if I wanted a little more light. Canon’s EF Series of lenses, designed for the EOS camera family, easily work in thirds of stops, much more accurate for the touchy digital environment. Similarly, digital cameras have added additional shutter speeds to refl ect the additional aperture settings. An expanded version of the reciprocity scale looks like this:
Shutter speed 1/15 1/20 1/25 1/30 1/40 1/50 1/60 1/80 1/100 Aperture f22 f20 f18 f16 f14 f13 f11 f10 f9
Shutter speed 1/125 1/160 1/200 1/250 1/320 1/400 1/500 1/640 1/800 1/1000 Aperture f8 f7.1 f6.3 f5.6 f5 f4.5 f4 f3.5 f3.2 f2.8
Should you decide to set your camera to work in half-stop increments, here’s how the reciprocity scale would work for the same 1/125 at f8 exposure:
Shutter speed 1/15 1/20 1/30 1/45 1/60 1/90 1/125 1/180 1/250 1/350 1/500 1/750 1/1000 Aperture f22 f19 f16 f13 f11 f9.5 f8 f6.7 f5.6 f4.5 f4 f3.5 f2.8
These are not complete scales, of course. There are lenses with a maximum aperture greater than f2.8 and there are lenses that stop down below f22, but the principle remains the same and can easily be charted for whatever lenses you may own.
