In my last blog, we touched upon the topic "amount of light" or exposure, and how critical it is for photography. Technically, a photograph should be neither overexposed nor underexposed. A proper or correct exposure can be defined as an exposure that achieves the effect the photographer intended.
Every photographic film or a sensor has a physically limited useful exposure range. If, for any part of your photograph, the actual exposure is outside of this range, the film or sensor can't record it accurately. Out-of-range values would be recorded as "black" (underexposed) or "white" (overexposed) rather than the precisely graduated shades of colour and tone required to describe the "detail"
Shutter speed along with the aperture of the lens determines the amount of light that reaches the film or the sensor; which implies that the shutter speed (also called f-number) and aperture are two key aspects of photography. The third key aspect that completes "Exposure Triangle" is ISO as depicted below.
Let us understand all three namely ISO, Shutter Speed and Aperture one by one.
ISO is the measure of a photographic film's sensitivity to light. Higher the sensitivity, the less light is required for a good exposure, these films are commonly termed as “fast films”. Conversely, insensitive film requires more exposure to light to produce the same image density as a more sensitive film and these are commonly termed as “slow films”. The term ISO was coined during the film days, however, is also used for digital cameras with sensor instead of film. During film days, the film roll you used in the camera decided the ISO number, therefore, photographers use to put the kind of film roll suited for the type of photography assignment e.g. to cover a sporting or live events such as car race or cricket match, they will normally use “fast films”.
With the advent of digital cameras, you have the option of choosing from a range of ISO values based on your requirement. In digital cameras, ISO numbers usually start at 100 and then double in sensitivity as you double the ISO number. So 200 is twice as sensitive as 100. Cameras these days support a very high value of ISO e.g. Canon EOS 60D has ISO range of 100-6400 (expandable to 12800).
One important thing to keep in mind; higher ISO introduces electronic artefacts that appear as speckles in your image termed as “Noise”. The reason for this is the camera is trying to amplify the signal to produce visible information. The more the image needs to be amplified the greater the amount of noise will be and grainier the image will be. Therefore, try to shoot with the lowest ISO setting possible for maximum quality.
There are few thumb rules for choosing the ISO value, depending on your level of available light. For sunny days, use a low ISO such as 100 or 200. Cloudy days might require ISO of 400.For night time shots; you’ll probably need ISO of 1600.
As I told earlier, proper exposure is crucial to creating a good photograph. The exposure is measured in units of exposure value (EV), sometimes called stops. The combination of the aperture, shutter speed, and ISO is used to achieve a proper exposure value (EV) for the scene. Change to any one of these factors requires changing one or more of the other two to maintain proper EV. This is referred to as reciprocal change. Multiple combinations of the three can give the same EV. Deciding which combination to use is one of the creative aspects of photography. In cameras with interchangeable-lens, shutter speed and ISO are camera properties. Aperture is a lens property
Now let's talk about shutter speed & aperture. Let’s say you are shooting outdoors, on a sunny day, which implies that the ISO value is fixed (say 100). Now multiple combinations of the shutter speed & aperture can give same exposure. Then why not choose any value for these two. That’s where creativity comes.
“Motion & depth of field”
We will discuss this in the next chapter. Chapter 3