Programme mode (P)
Most cameras have a programme mode (P mode). This mode sets a combination of aperture and shutter speed based on the overall brightness of your scene and your ISO setting.
Usually but not always, when in P mode, you can normally ‘shift’ your combination of aperture and shutter speed. This allows you to use one dial to select the aperture and shutter speed most suited to your needs and is called ‘programme shift’. So, when shooting in P mode, if you don’t like the combination of aperture and shutter speed chosen by your camera, you can shift it to one more suited to your needs.
Most photographers will have learned of the exposure triangle and will understand that correct exposure is a combination of Aperture, Shutterspeed and ISO. If your camera supports auto ISO and you set this on your camera along with the programme shift, all three elements of the exposure triangle are set for you. This leaves you free to concentrate on your picture taking!
Setting up your camera this way allows you to quickly take photos knowing that your camera will balance out the shutter speed, aperture and ISO for you. But you still have some control, in that, you can vary all three settings by using the programme shift function of your camera. Usually, just by rotating one dial. And you still have the ability to bracket or dial in exposure compensation when needed.
Why does this work
This works because for a given exposure value there are a number of shutter, aperture and iso combinations. And the programme shift mode of your camera exploits this.
Remember the exposure triangle?
Well, something is missing from it and that is ‘Exposure Value‘
For any given scene there is an exposure value, the higher the exposure value the brighter the scene.
When you meter a scene with your camera, your camera assigns an exposure value to the scene. And then, depending on your shooting mode, your camera adjusts either the aperture, shutter speed or ISO to achieve the optimum exposure. Or if your in fully auto mode, it sets all three.
For example, shooting at ISO 200 and a scene brightness of EV12, the correct exposure can be achieved with an aperture of F8 and a shutter speed of 1/125s (ISO 200 -F8 @1/125s).
However, if we consult an exposure value chart (EV Chart), we can see that at any given ISO setting and exposure value, we can use a varied combined range of aperture and shutter speed, all equating to the same Exposure value.
So at EV12, we can see that shooting at F2.8 @ 1/1000s gives us the same exposure value as shooting at F8 @ 1/125s.
Understanding expsoure values (EV’s) helps you understand the exposure triangle and the trade off between aperture, shutter speed and ISO.
In summary, understanding exposure values, the exposure triangle and exploiting your camera’s ‘P’ mode, along with programme shift, means that you can shoot quickly, select various exposure settings with one dial and adjust exposure values through exposure compensation by dialing in more or less exposure through the exposure compensation dial on your camera.
There are some more advanced ways of using Programme shift with spot metering and auto exposure lock but that’s for another time.