For beginners all of the numbers associated with ISO ratings, shutter speeds and aperture values can be very confusing. If this is the case for you, it might be easier for you to think about it all in terms of stops. The real unit of change in a camera is called a stop. A change in shutter speed of 2x or 1/2x is one stop. For example 1/30 second to 1/60 second is minus one stop and 1/30 sec to 1/15 sec is plus one stop. Slowing the shutter speed lets in more light and is therefore plus and the reverse is also true.
The aperture works the same way except the numbers seem weird. You just have to memorise the full stops, which are traditionally f/32, f/22, f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2, f/1.4 etc…. If you look closely you will notice that it doubles or halves every two stops.
To make things tricky, newer DSLRs have 1/3 stop increments like f/5, f/5.6, f/6.3 etc. There is no need to memorise these intermediate numbers, all you need to remember is that each click of the aperture dial is 1/3 stop so 3 clicks equals one stop.
The relationship between shutter and aperture is simple. If the metering is correct according to the camera meter and you start at a speed and aperture of 1/15 second and f/8 let’s say and you want to increase the shutter speed by 2 stops you need six clicks of the shutter wheel and you will arrive at 1/60 sec. You are now letting in 2 stops less light, as the shutter speed is two stops faster.
If you want to retain the same exposure as before you need to compensate by opening the aperture by 2 stops (six clicks) from f/8 to f/4 (remember f/5.6 would only be 1 stop). That’s all there is to it.
The ISO is similar in operation luckily, in that a doubling or halving of the ISO is equal to one stop. For example ISO 200 to ISO 400 is an increase in ISO of one stop and ISO 200 to ISO 100 is a decrease of one stop.
A change in ISO only changes the sensitivity of the sensor and does not change the shutter speed or the aperture in manual mode. It is primarily used to adjust the sensitivity of the sensor to the lighting conditions.
In bright light a low sensitivity is required and ISO 100 is best. Inside or at dusk perhaps the sensitivity may need increasing and perhaps ISO 800 is required.
The trick is to decide what is most important in the shot. In a moving, action shot with the camera set to 100 ISO, shutter speed is probably most important, so a shutter speed is selected that suits your needs say 1/1000 sec.
If setting the shutter to 1/1000 puts the correct exposure beyond the maximum aperture of the lens say f/1.4, than increasing the ISO to say 800 will raise the sensitivity of the sensor by three stops allowing you to shoot 3 stops down at f/4, which is within the aperture range of your lens.
The reverse of this scenario is also true for a shot where the depth of field is most important and the first setting is the desired aperture (experience and practice once again). The correct shutter speed may now be too slow to hand hold the camera without camera shake and the solution might be to once again raise the ISO.
Please keep in mind that the sensor performs at its best at the lowest ISO available for your camera, so it is advisable to try to use the lowest ISO possible until lack of light forces you to increase the ISO.