I’ve spent the better part of last week’s leisure time (and some work time) researching, toying with the camera and mulling about – which type of ‘prime’ lens I should buy. A prime lens is a lens which has a fixed focal length (as opposed to zoom lens which has a range of focal lengths). The optical zoom in point-and-shoot cameras is due to the fact that the mechanical assembly of the lens element makes it possible for the effective focal length to be varied.
So why am I going for a prime lens, when people in this age and time, go for 15x optical zoom? [for a SLR lens, such ginormous zoom to be available in one lens, is almost impossible]
Because I want to take photographs with a lens with a big aperture and consequently, a very fast shutter speed.
And why can’t I have a very fast zoom lens? I can, but they are freaking expensive (over a thousand dollars) and beyond a certain point, in today’s world, is impossible to manufacture.
Most importantly, why do I need a fast lens, with big aperture? That is the main question.
So that I can have a shallow depth of field, and hence take ‘portrait’ photographs.
I’ll try my hand in explaining a little about these.
Keeping the distance between the subject and the camera constant (that is, eliminating any zoom), there are 3 key aspects to the exposure of the image that the camera captures.
- ISO speed
- Lens aperture (f-number or f-stops) and
- Shutter speed
The ISO speed is a remnant from the time when film was used in photography, but is still important. The ISO speed says how sensitive a particular film (or sensor for digital SLR) is. Higher the ISO, more sensitive a film (or sensor). The more sensitive a sensor is, the lesser the light it requires to take the photograph. For the sake of this particular post, we’ll assume the ISO is constant too.
That leaves us with the lens aperture and the shutter speed. Shutter speed is easy to understand. It is the time for which the camera sensor is exposed to light. It is measured in seconds and, usually in, fractions of seconds like 1/125 (125th of a second) or 1/250 (250th of a second). The aperture is the ‘hole’ through which the light enters the camera; so the aperture controls the amount of light. Aperture and f-number enjoys an inverse relationship. A low f-number denotes a bigger aperture. For example, a f-number of f/1.4 allows double the amount of light into the camera as compared to a f-number of f/2.
Why double, you may ask?
Mathematically, f-number is represented by =>
f-number = (focal length)/(diameter of the aperture)
Now area of a circle (as the aperture is a ‘hole’ and circular in nature) is =>
area of circle = pi * (radius)squared
Keeping the focal length constant, to have a true comparison of 2 f-numbers (and consequently 2 different diameter or radius [half of diameter]) you have to square them both and then get the ratio.
So, square(1.4)/square(2) = 2/4 = 1/2. QED.
That still doesn’t explain why portraits are better with a faster lens. A fast lens captures the subject in focus clearly, while leaving the background and other out of focused area, well, out of focus or not sharp. Which means a shallower depth of field. The artistic use of this phenomenon is known as bokeh.