What is the relationship between ISO, F stop and shutter speed and how do you use them?
Again following on from the talk I gave to the camversation group there were questions regarding how this relationship works, not an uncommon question as I have found on the courses I run. This is also coupled with the quaestion - Do I have to shoot in manual?
The photography triangle consists of three items are linked to one another
ISO
F- Stop
Shutter speed
So how are they linked and what does it mean?
Step A If I set my camera to shoot on aperture priority and have the settings set to
ISO @ 400
F- Stop @f2.8
Shutter speed 1/250
If I were to change the F-Stop from F2.8 to F5.6 (two stops) on the scale of F-Stops available to me ( F-Stop Scale = F1.4, F2.8, F4, F5.6, F8)
This has opened up the aperture and so has let more light in and to compensate for this extra light and keep the expose the same the camera must change either the ISO or the shutter speed to counteract the effect.
Step B
If we consider how it could affect the shutter speed
ISO @ 400
F- Stop @f5.6
Shutter speed 1/1000
There is too much light entering the camera so the camera will increase the shutter speed by two stops thus reducing the amount of light that enters the camera and so correcting the exposure effect of the aperture change.
The shutter speed will go from 1/250 to 1/1000 of a second.
This will mean a faster shutter speed a steadier image and less blur from items in motion
Step C
If we consider how it could affect the ISO
ISO @ 200
F- Stop @f5.6
Shutter speed 1/250
There is too much light entering the camera so the camera could increase the ISO by two stops thus reducing the sensitivity to light and so correcting the exposure effect of the aperture change.
The ISO will go from 400 to 200 which will in effect make the sensor less sensitive to light.
This will result in a cleaner image as there will be less noise at ISO 200 compared to 400.
This in summary is how these three items interact to balance the light in the image.
If you want to shoot with control over you camera but in a fast way ensuring that you get you exposures the the following setup is an ideal way forward
Set the camera to aperture priority mode This means that aperture is the most important aspect to you and when you select the aperture you want the camera will balance out the ISO and the speed. By saying aperture is the most important aspect you are saying that control of depth of field is the most important aspect to you in the images you are shooting at that point in time.
Set the camera to auto ISO This means that the camera will always ensure that the ISO is within a range you are happy with. How this works varies a little from camera to camera but on a Nikon in the settings you can set an ISO range ie 100 - 6400 and a minimum shutter speed say 1/200. What this means is that if the shutter speed falls below 200 the the camera will change the ISO to get the shutter speed back to you desired minimum.
Camera controls shutter speed When set to aperture priority the camera will then adjust the shutter speed to ensure correct exposure. So as you select the aperture you want to get the DOF you want the camera is now adjusting the shutter speed and should the shutter speed fall below 1/200 then the camera will adjust the ISO to bring the shutter speed back up to 1/200
This is a nice walk about setup for you camera. It works well for street photography as it allows for fast shooting sufficient control and will manage the transition from dark to light space for you so you don't miss a shot.
If you were to shoot in speed priority then the role of the speed and aperture would simply be swapped
Give it a try !