Components of Exposure
The three main components of exposure are;
ISO – This is the film/sensor sensitivity to light.
Aperture – This is the “hole” in the lens through which light reaches the film/sensor.
Shutter Speed – This is the amount of time the shutter is open allowing light to fall on the
film/sensor.
ISO
Are the initials of the International Organisation for Standardisation. This is the standard
that ensures all manufacturers are talking on the same wavelength.
I know, wrong way round but it doesn’t matter to us, this is just for your information.
Most of our DSLR’s have a minimum setting of ISO 100 (Nikon 200) I’m going to make some
comparisons to film here as that’s where it all originated!
Films with a low ISO number were less sensitive to light but the grain of the film was very
fine, resulting in a much higher quality and allowing for larger prints to be produced.
As the ISO speed was increased so was the sensitivity to light and also the size of the film
grain, resulting in less sharp “grainy” images.
The same principle applies to Digital images but now the “grain” shows itself as “noise”.
Therefore we choose a low ISO speed for best quality or a higher ISO speed to increase the
sensitivity of our sensor enabling us to shoot in lower light conditions.
Convention says that we gauge things by “f stops” (see Aperture) so to relate ISO to f stops;
If you can shoot a scene at 1/125 @ f5.6 with ISO 100 then you could shoot the same scene
at 1/125 @ f8 with ISO 200, you will see a pattern emerging here with things doubling or
halving as we progress, so;
ISO 100
ISO 200
ISO 400
ISO 800
ISO 1600
= + 1 f stop
= + 1 f stop
= + 1 f stop
= + 1 f stop
= + 2 f stops
= + 3 f stops
= + 4 f stops
There are also intermediate settings which give us 1/3 or 1/2 stop increments.
Aperture
As touched on earlier, the Aperture is a “hole” inside the lens which lets light through to the
film/sensor; the aperture is often referred to as “the f stop”.
Also mentioned earlier was the point that we normally gauge things by “f stops”, that’s a
true statement, but conversely when we talk about the maximum aperture of a lens it is
often referred to as “the speed” of the lens and people often refer to lenses with wide
maximum aperture such as f2.8 or f1.8 as being “fast” lenses.
So where do these numbers, f8, f11, f16 etc. come from and why does f8 on say a 50mm
lens look smaller than f8 on say a 500mm lens?
The answer is simple mathematics (don’t go, stay with me for a moment) the f number is
simply a ratio between the diameter of ”the hole” and the length of the lens and is actually
shown on your lens identification as being a ratio e.g. Canon EF 85mm f1:1.8, you can clearly
see that the maximum aperture is expressed as a ratio.(for the non-mathematicians
amongst us the symbol : between two numbers indicates a ratio between the two numbers)
To keep the maths simple, if we say our 50mm lens has a maximum aperture of 25mm
diameter then 50 divided by 25 = 2 therefore this would be a 50mm f1:2 lens, commonly
called 50mm f2.
Different lenses, same aperture, different “hole” size;
50mm lens @ f8 = 50 divided by 8 = 6.25mm diameter
500mm lens @ f8 = 500 divided by 8 = 62.5mm diameter
So you’re thinking, bigger hole lets more light in, how can they both be f8?
Oh dear more theory! The “hole” on the 500mm lens may be letting more light in but
because the lens is longer than our 50mm lens the light has to travel further and the further
the light travels the more its intensity diminishes. This is governed by something called The
Inverse Square Law.
Don’t panic! You don’t need to know it, but believe me you all use it every time you pick up
your camera.
Similarly you don’t need to know the actual diameter of each f stop but the above, I hope,
has given you some appreciation of where all these terms originate, which can sometimes
be useful.
Earlier I said we will see a pattern emerging where things double and half, as we saw with
the ISO settings, doubling the ISO sensitivity allows us to use one more f stop for a given
scene, and so with f stops each FULL f stop increases or halves the amount of light getting
through the “hole”.
The FULL f stops are considered to be; 2.8, 4, 5.6, 8, 11, 16, 22, 32
Again, there are intermediate settings giving us 1/3 or 1/2 stop increments.
The choice of f stop also governs the depth of field or sharpness within the scene, but for
now we will limit this to exposure.
Shutter Speed
Having now gained an understanding of ISO and Aperture, Shutter Speed should be quite
easily understood.
We have seen that increasing ISO setting can make our film/sensor more or less sensitive to
light and that doubling our ISO can be equated to one full f stop.
Also we have seen that one full f stop will let twice (or half) the amount of light to reach the
film/sensor.
To capture our image we need (for a given ISO rating) to have a given amount of light
reaching the film/sensor for a given amount of time (this is the bit your light meter works
out for you).
So, as with ISO and Aperture, the Shutter Speed settings double or half the amount of time
the light is falling onto the film/sensor for. Again we are talking about the FULL increments
here, e.g. 1/30, 1/60, 1/125, 1/250, 1/500 etc.
These settings originate in the film days when shutters were basically controlled by
clockwork and no intermediate settings were available, since the introduction of electronics
into our cameras we have been able to utilise intermediate settings.
Some examples using all three elements, we are photographing the same scene here under
the same lighting conditions;
ISO 100, 1/125 @ f8 = ISO 100, 1/60 @ f11 you can see here that because we have halved
the amount of light by changing from f8 to f11 we need to let that smaller amount of light
fall onto the sensor for twice as long by selecting a slower shutter speed of 1/60 instead of
1/125
Can you follow the logic in the following examples?
ISO 100, 1/125 @ f8 = ISO 200, 1/250 @ f8 (double the sensitivity = half the time)
ISO 200, 1/60 @ f5.6 = ISO 200, 1/30 @ f8 (double the time = half the light)
ISO 100, 1/30 @ f8 = ISO 400 1/125 @ f8 (quadruple sensitivity = quarter the time)
OK, tricky one!
ISO 100, 1/30 @ f5.6 = ISO 1600, 1/125 @ f11 = Answers on a post card 
In this one we have increased the sensitivity by 4 f stops (remember I said we usually equate
everything to stops) so we need 4 stops less light;
We have reduced the light in this example by using both the shutter speed and the
aperture, so by increasing the shutter speed from 1/30 to 1/125 we have taken care of 2
stops
The other 2 stops are taken care of by reducing the aperture from f5.6 to f11.
So by now hopefully you can see that to maintain a correct exposure for any given scene
you must alter at least two of the three elements, which two I will leave up to you, your
choice will be determined by what it is you are trying to capture in your image and will vary
tremendously between for example capturing a landscape to capturing a bird in flight.
I hope you have found this of some help, Steve Lindsay-Blagg LNPS