Part 2 – Measurements and Adjustments
Every camera has a different range of ISO values (speeds) and a different ‘base’ ISO (native or optimised).
The base ISO is the setting from which the camera is optimised to provide the best quality image.
For example a Full Frame camera (35mm) may use a base ISO of 100, whereas the Micro Four-Third systems use mostly ISO200 as their native ISO.
Like the shutter speed, the ISO values use stops that are proportionate to the amount of light required by the sensor.
1/1000sec @f/2.8 & ISO200 is the same as 1/2000sec @f/2.8 & ISO400
You may also hear about ‘Extended ISO’s’.
A Standard ISO range will depend upon the camera and sensor, but I will use Olympus OM-D’s as an example for this.
The standard ISO range on an Olympus OM-D is ISO200 to ISO6400.
The available ISO in the camera runs from low (ISO50-ISO100) all the way through to ISO25600
The ISO values above 6400 and less than 200 are the extended values and not really used in every-day photography. They are therefore normally excluded from any ‘Auto ISO’ settings.
Part 3 – ISO and Exposure
As stated previously, ISO plays a vital role in how we set up the camera for a shot.
You can see in the image above that raising the ISO doesn’t just make the image brighter, it also starts to introduce ‘Noise’ (or grain).
Because Noise plays a large role in the clarity and quality of an image, we (the photographers) must balance the options carefully before taking a shot.
If the shutter speed is too slow, we risk movement or shake, if the ISO is too high, we risk losing detail and creating a noisy image. An aperture too wide can lose sharpness at the edges and create an unnecessarily shallow depth of field.
For these reasons, ISO is normally the first setting on the exposure triangle we will adjust, either because we want a high quality image, or because it’s set to auto making exposure slightly easier.
Why do we get Noise (or grain) with higher ISO?
The noise is caused by amplification of the signal from the sensor or fluctuations in the electronic signal provided by the camera. A picture is just light and if there isn’t enough light to build a picture, the camera must ‘fill-in the gaps’. It does this in many ways from duplicating pixels to averaging the available light across the image. If there isn’t enough light to use, it simply fills it with whatever is available (not always correctly) and this provides the various forms of noise.
Here we can see a very simple explanation of what happens when there is not enough light to fill the pixels with light.
You see the camera amplifies or duplicates what it can, filling in the rest with what it knows.
Not necessarily the same as what is correct.
Part 4 – Types of ISO
Reading through this and other people talking, you’ll hear a lot of different terminology for ISO most of which comes from the film days when ISO referred to the sensitivity of the film (the size of the silver halide in the emulsion).
You may be familiar with ‘Speed’, ‘ASA’, ‘DIN’, ‘Sensitivity’ or ‘High’ and ‘Low’; all of which relate to the ISO Values.
Now with digital photography, we have two types of ISO; ‘Native’ or ‘Base’ and ‘Extended’.
A Native ISO scale is a range of ISO values used and tested for acceptable use in most (if not all) camera functions. This scale will be dependent on the manufacturer and the technology use in camera.
Most camera makers will limit any ‘Auto ISO’ settings to this ‘Native’ range.
This is a range of ISO used in more extreme conditions and/or in specific circumstances.
Extended ISO shots will almost always lose detail, effect the true colour representation and add a vintage style gritty look to your image.