Lens aperture is an important criterion in any camera’s exposure adjustments. But, for low light photographers, it is equally important in determining the choice of which lens to use.
Faster lenses (i.e. those with large maximum apertures) will allow more light to reach the image sensor, thereby reducing the need to use high ISO settings and allowing faster shutter speeds to be used, which reduces the risk of blurring due to camera shake.
Fast lenses with maximum apertures of f/2.8 or wider will allow more light to reach the image sensor, making them ideal for low light photography. (Pictured: Samsung NX 16-50mm f/2-2.8 S ED OIS zoom lens)
To capture more light, the glass elements used in fast lenses must be as large as possible, which means fast lenses are always larger and heavier than lenses that are only one f-stop slower. They are also significantly more expensive to manufacture, which means they cost a lot more.
So, choosing lenses for low light work will always involve trade-offs between lens speed, cost and the amount of space required in your camera bag ““ and how much weight you’re prepared to carry. These decisions must be made by each photographer for each situation they encounter.
Low light photographers should be aware of the relationships between f-numbers and the amount of light that can pass through the lens to reach the image sensor. In most cameras, aperture settings can be adjusted in 1/3 EV (exposure value) steps. A typical one-third f-stop scale is shown below, with the one-stop intervals bolded.
f/1.4 f/1.6 f/1.8 f/2.0 f/2.2 f/2.5 f/2.8 f/3.2 f/3.5 f/4.0 f/4.5 f/5.0 f/5.6 f/6.3 f7.1 f/8.0 f/9.0 f/10 f/11 f/13 f/14 f/16 f/18 f/20 f/22
A lens with a maximum aperture of f/1.4 will allow twice as much light to enter the camera as one with a maximum aperture of f/2.0, which in turn passes twice as much light as a lens with a maximum aperture of f/2.8 (considered ‘fast’ by today’s standards). Stopping down from, say, f/4.0 to f/5.6 will halve the amount of light reaching the sensor, while stopping up to f/2.8 will double it.
Excerpt from Low Light Photography.