Before going on to chat about the relationship between aperture and shutter speed, a wee note about apertures.
A lot of newbies to photography get confused about this subject as the aperture scale on a camera is maybe a bit baffling in
that the numbers we talk about don't, at first glance, make any sense.
The number sequence runs 2, 2.8, 4, 5.6, 8, 11. 16, 22, 32 and it is difficult at first to equate that sequence with the fact
that as you move from one number to the next you are either doubling or halving the size ( diameter) of the aperture. Afer
all, the numbers themselves are not doubling or halving so how come they are having that effect on the physiacl dimensions
of the aperture ?
Well, the fact is that these numbers represent not the simple diameter of the aperture but the ratio between that diameter
and the focal length of the lens.
This is why these numbers are usually described as "ƒ " numbers, ƒ being the mathematical symbol for ratio.
(TD will get tired copying and pasting ƒ all the time so will instead call it F ).
So the sequence of numbers in the aperture scale is really F2, F2.8, F4 etc.
Now those of you with a mathematical turn of mind ( and that does not include TD so please comment if I am wrong ) you
will know that if you wish to double or halve a ratio, you multiply/divide the factor not by 2 but by 1.44 ( near enough)
Using this, the sequence of aperture values makes sense........F2 multply by 1.44 gives F2.8 (near enough), F2.8 multiply by
1.44 gives F4 ( near enough ) and so on.
Using that set of values it is now apparent that when you go up and down the aperture scale, you are in fact halving or
doubling the ratio aperture to lens...effectively doubling/halving the size of the hole ( iris or aperture ) though which the light
has to pass.
As these numbers represent ratios between aperture diameter and focal length, what happens when we change the focal
length of the lens....either by zooming in and out or by swapping lenses ?
It means that F2.8 on a wide angle lens and F2.8 on a telephoto lens will give the same exposure value......the focal length
is different, the physical diameter of the aperture will be different...but the ratio invoked in each lens is constant and the
exposure value will be constant between the two lenses
The next thing that can confuse is the fact that as the numbers in the aperture scale increase from say F2 to F2.8 the
aperture value does not increase, as might be expected....it decreases ( again because the F number is a ratio.)
So as you progress through the scale from F2 to F16 say, the aperture diameter is getting smaller..it is halving in diameter
every time you move up from one number to the next ( or of course doubles in diameter if you are moving down the scale
of numbers.)
Why do we have these varying aperture sizes ? We have talked of it before...it is to adjust and control your depth of field,
the amount of back to front sharpness in an image. In rough terms, the higher the F number used, the more depth of field
you get..............earlier posts have mentioned the effect on depth of field when using different focal lengths/ camera to
subject distance and some other factors such as that.
These F numbers are very often described by photographers as "stops" and the act of going up and down the scale as
"stopping down" or "stopping up"
In the days of very early cameras, internal variable aperture devices had not been invented and the lenses themselves had
only one F number...the size of the lens throat through which the light passed.
Some bright spark, realising the need to control depth of field, created a set of brass plates, each with a different sized hole
punched in the middle with which to represent the aperture. These plates were effectively used to stop some light getting
through and were inserted in a slot between the lens and the camera body....they were called "stop" plates.........and
although we now have built in aperture adjustment..the term "stop" is still used.
Will write out note about shutter speed, then another about the relationship between the two and post in a bit.