DSLR : Masterate Skills of Photography
1. Understand ISO
ISO is a measure of how sensitive the sensor of your camera is to light. The term originated in film photography, where film of different sensitivities could be used depending on the shooting conditions, and it is no different in digital photography. The ISO sensitivity is represented numerically from ISO 100 (low sensitivity) up to ISO 6400 (high sensitivity) and beyond, and controls the amount of light required by the sensor to achieve a given exposure
At ‘low’ sensitivities, more light is required to achieve a given exposure compared to high sensitivities where less light is required to achieve the same exposure. To understand this, let’s look at two different situations:
Low ISO numbers
If shooting outside, on a bright sunny day there is a lot of available light that will hit the sensor during an exposure, meaning that the sensor does not need to be very sensitive in order to achieve a correct exposure. Therefore, you could use a low ISO number, such as ISO 100 or 200. This will give you images of the highest quality, with very little grain (or noise).
High ISO numbers
If shooting in low light conditions, such as inside a dark cathedral or museum for example, there is not much light available for your camera sensor. A high ISO number, such as ISO 3200, will increase the sensitivity of the sensor, effectively multiplying the small amount of available light to give you a correctly exposed image. This multiplication effect comes with a side effect of increased noise on the image, which looks like a fine grain, reducing the overall image quality. The noise will be most pronounced in the darker/shadow regions.
2. Learn the ‘Exposure Triangle’
It’s important to note that aperture, shutter speed and ISO are all part of the ‘exposure triangle’. They all control either the amount of light entering the camera (aperture, shutter speed) or the amount of light required by the camera (ISO) for a given exposure.
Therefore, they are all linked, and understanding the relationship between them is crucial to being able to take control of your camera. A change in one of the settings will impact the other two. For example, considering a theoretical exposure of ISO400, f/8.0, 1/10th second.
If you wanted to reduce the depth of field, and decided to use an aperture of f/4.0, you would be increasing the size of the aperture by two whole f/stops, therefore increasing the amount of light entering the camera by a factor of 4 (i.e. increasing by a factor of 2, twice). Therefore, to balance the exposure, you could do the following:
- Situation 1: Reduce the shutter speed by a factor of 4, i.e. to 1/40th second.
- Situation 2: Reduce the ISO by a factor of 4, i.e. to ISO100
- Situation 3: A combination of the above, shutter speed by a factor of 2 (to 1/20th second) AND reduce the ISO bv a factor of 2 (to ISO200).
3. Master Metering
Through out all of the above discussion, I have said that the camera calculates the exposure depending on the amount of available light, but what is it actually doing?
When taking a photograph, using any form of automatic exposure calculation (e.g. aperture priority mode, shutter priority mode, auto-ISO etc) the camera always tries to calculate an ‘average’ exposure. It will asses the entire scene, both light and dark areas, and determine the exposure so that all of the tones within the entire image average to 18% grey – called the ‘middle’ grey.
This is known as metering, and it is the reason that if you point your camera at a bright white scene, such as after it has snowed, and take a photograph the resulting image will always appear darker than you or I see it. Similarly, if you point your camera at a really dark scene, such as a low-lit room, and take a photograph the resulting image will always be brighter than you or I see it.
The scene is always being averaged by the camera and most of the time that results in the image appearing to be correctly exposed. However, you can control what areas of the scene are being assessed by the camera in order to influence the way in which the exposure is metered.
Generally, there are three metering modes that you can choose from:
Average – The camera will assess the tones across the entire image form corner to corner, and expose the scene to 18% grey from that assessment.
Centre-weighted – The camera weights the exposure reading for the area in the centre of the viewfinder that can total up to approximately 80% of the scene, ignoring the extreme corners of the image.
Spot metering – The camera will use a very small area of the scene, typically a small circle in the centre of the viewfinder that totals approximately 5% of the viewfinder area. It will make the assessment of dark/light tones in this area and expose the entire scene to 18% grey, from that assessment.
Practically speaking: when starting out with your camera, either average or centre weighted metering are a good starting point. They will both provide a fairly consistent measure of the exposure required and, if you select one mode and stick with it, you will soon begin to understand when a scene will be under exposed (i.e. too dark) or over exposed (i.e. to light) compared to how you see it with your own eyes.
But what can you do if a scene is under/over exposed? That is where exposure compensation comes in.