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Photography deals with the control and use of light.   

Camera manufacturers have been trying, with varying degrees of success, to emulate the human eye and produce images from a three dimensional environment in a two dimensional format.  This is the result of their labours.

Reciprocity is the relationship between:

the  f/stop, (size of  the aperture)

the shutter speed, (the length of time the shutter is open)

the ISO, (the sensor’s sensitivity to light), in order to get a good exposure.  

All three are measured in ‘stops of light’.

(Aperture is measured in f-stops’ e.g. f/4 or f/2.8 (the length of the focal lens divided by the diameter) and refers to stops of light. The smaller the f/ number the larger the aperture.  I know, I know, this seems back to front  and the guy who thought this up needs a good slapping, however………

The maths regarding reciprocity are quite simple.

Once the perfect exposure has been achieved, opening up the aperture by one stop, (allowing more light to reach the sensor), requires the shutter speed to be reduced by one stop, (reducing the length of time for the light to reach the sensor, to maintain the correct exposure.  

In other words, twice as much light coming in for half the time equals the same exposure.

The one constant in reciprocity is that everything either doubles of halves, as you go up and down the scale and one or two elements  can balance the third to get a good exposure.

In other words, if an action shot requires a fast shutter speed, either or both the remaining elements can be adjusted for a good exposure.

So if we have a perfect exposure, why would we want to alter the settings?  A bit later on I’ll explore how reciprocity can be manipulated for varying effects.


The aperture is the diameter of the lens opening, controlled by a diaphragm.


The easy way to remember apertures is that each stop is either doubled or halved, depending on whether you are stopping up or down.

The start of the numbers are 1 & 1.4 doubling alternately.

So the sequence would be  1 1.4 2 2.8 4 5.6 8 11 16 22 32   each one being an increase by 1 stop.

Therefore f/4 is one stop higher than f/2.8… f/11 is two stops lower than f/22, and so on.

Notice how the smallest the number equates to the largest aperture.  Note also, that the wider the aperture the shallower the depth of field, also known as the focal plane.

There is an in-depth explanation later on.


The shutter speed is much easier to understand than aperture settings and is measured in seconds and fractions of seconds.

Typically for an f/8 shot, the shutter will be 1/250th sec.

The numbers from 1 sec. decrease, (get faster), as follows.

1 sec,  ½ sec, ¼ sec, 1/8 th sec, 1/15 th sec , 1/30 th sec , 1/60 th sec, 1/125 th sec, 1/250 th sec, 1/500 th sec, 1/1000 th sec, 1/2000 th sec, 1/4000 th sec.


Notice the figures decrease by double, in much the same way the f/stops do, obeying the laws of reciprocity.

The camera’s shutter will be closed by default, opening when the you press the button.  The shutter will open for the length of time you, or the camera have set.

A shutter speed of 1/250th sec will open for twice as long  as 1/500th sec, letting in twice  the light.

An underexposed image will need either to have the aperture opened to allow more light in to the sensor, or the shutter speed slowed so that there is a longer exposure to the light.

There is, however, another string to your bow - and that’s the ISO.

Shutter Speed

ISO (Industry Standard Organisation) is the level of sensitivity to light your sensor is set to.  It’s as simple as that.

Typically, a landscape shot on a summer’s day would be in the region of ISO 200.

ISO, like f/stops and shutter speeds, are measured in stops of light.


The range of ISO’s  start at 100 and double through the range.  100, 200, 400, 800.

If you are in a situation where there is insufficient light on the day and you have run out of options with the shutter speed and f/stops, then increasing the ISO will invariably get you out of trouble, allowing a different shutter speed or aperture to claw back a good exposure.

The one draw back from high ISO’s is noise in the finished product, which can be rectified in post processing.

All the information I have given here is based on full stops of light, which is a legacy from  film SLRs, so popular back in the day.

In today’s technology these full stops of light are blunt, frankly.  Modern DSLRs are able to work in half and third stops.

In other words, for every full f/stop, modern cameras will give the option of up to 3 f/stops; every full shutter speed will have up to three options; every full ISO stop, up to three options to choose from, allowing an  ISO of up to ISO  2,400.

I would urge you to use this technology, which gives you more control over your photography by a factor of nine.