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Reciprocity

Reciprocity is a general term to describe the Exposure Triangle relationship between Aperture, Shutter Speed, and ISO. To better understand this concept, if you are not familiar with these terms, you should review them before continuing.

The suggested order of reading is:

Aperture
Shutter Speed
ISO
Exposure Triangle

Exposure is based on the Inverse Square Law, which states that any doubling of distance results a 4 fold reduction of intensity. The reciprocal factors of this law are what applies here.

In a nutshell, any time you change an exposure value - whether it be changing the aperture, shutter speed, or ISO, you have to change one of the other values by the same amount to maintain the same exposure. The assumption is that the exposure is correct, but say you want to change the aperture for more creative effect; to maintain the proper exposure, you will have to change either the shutter speed or ISO in the opposite direction by the same amount to maintain the proper exposure. This is the definition or Reciprocity (or reciprocal value).

A useful unit of measurement for determining the amount of change required is the EV (Exposure Value). This was explained somewhat in the sections on Aperture, Shutter Speed, and ISO, but we will review it again here - and perhaps it will make sense.

One EV, regardless if it is a change in Aperture, Shutter Speed, or ISO, constitutes a doubling or halving of light reaching the sensor. If you increase the value by +1 EV, you are doubling the amount of light. Conversely, if you reduce the value by -1 EV, you are halving the amount of light.

 

Aperture 1 EV Scale
f/1.0
f/1.4
f/2.0
f/2.8
f/4.0
f/5.6
f/8.0
f/11
f/16
f/22

 

Shutter Speed 1 EV Scale
Bulb
30s
15s
8s
4s
2s
1s
1/2s
1/4s
1/8s
1/15s
1/30s
1/60s
1/125s
1/250s
1/500s
1/1000s
1/2000s
1/4000s

 

ISO 1 EV Scale
64
100
200
400
800
1600
3200
6400
12800

 

From the above tables, presume that you are using Manual mode in your camera, and have the following settings in your camera, as shown in red in the above tables:

  • Aperture = f/8
  • Shutter Speed = 1/125sec
  • ISO = 400

And also presume that the metering system on your camera indicates you are -2 EV underexposed. What could you do to correct the exposure? We know that you have to increase the exposure by 2 EV.

  • Changing the Aperture will increase the exposure by +1 EV for each column you move to the left.
  • Changing the Shutter Speed will increase the exposure by +1 EV for each column you move to the left. left.
  • Changing the ISO will increase the exposure by +1 EV for each column you move to the right.

You can get the correct exposure by moving any one value by +2 EV, or any two values by +1 EV. And if you understand that, you should understand that you could also move one value by -1 EV, and any combination of the other two values by a total of +3 EV. Any combination will work as long as the net result is a +2 EV increase, which is the proper exposure for this instance.

Another example; say the values above result in the correct exposure, but you want to use 1/4 sec to get a photo of a waterfall, and want a nice silky smooth effect. The total change is +5 EV, so you need to compensate somehow with -5 EV. The recommendation would be first to lower your ISO, as lower ISOs provide lower noise. So lets move the ISO to 100, which in this case is the lowest our camera will go. That is only a -2 EV change. So we need to also move the Aperture by -3, which results in a f/22 aperture.

While this would result in a good exposure, other factors, such as diffraction come into play. Say you don't want to go more than f/11. The only option is for you to use a ND8 filter, which reduces the light by -3 EV, so you can bring the aperture to f/11. I will cover this a bit more in the filter section.