Reciprocity

Trick Photography And Special Effects

Trick Photography and Special Effects

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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 /1 25

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 field 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 flicker on and off 60 times per second. Studio strobes and on camera flash units are not constant sources of light because they fire and expire somewhere between the time the shutter actually opens and closes, so the amount of light they produce may be figured into the equation (like fill flash) and used to advantage to either supplement existing light or overpower it.

Back in the days of film, 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 reflect the additional aperture settings. An expanded version of the reciprocity scale looks like this:

Shutter speed Aperture

1 /1 5 f22

1 /20 f20

1 /25 f18

1/30 f 16

1/40 f 14

1/50 f13

1 /60 f11

1 /80 f10

1 /1 00 f9

Shutter speed Aperture

1/125 f8

1/160 f7.1

1/200 f6.3

1/250 f5.6

1/320 f5

1/400 f4.5

1/500 f4

1/640 f3.5

1/800 f3.2

1/1000 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 /1 5

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.

Notice, as you look through these images, how depth of field increases as apertures get smaller FIGS 1.11-1.17).

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