Continuing with this series on photography for beginners we take on a significant aspect of lens technology. The digital camera lens is a piece of optical wonder. Look at a cross-section of a lens and you would immediately appreciate the level of ingenuity and precision that goes into making each one of them. Not just that every lens is built with care but they are also tested and rated to ensure that every component work flawlessly and in tandem to be capable of producing masterpieces of imagery in the right hands.
Now, when we speak of lenses one piece of detail ostensibly comes before anything else does. Whether it is the 50mm prime or the very sharp 85mm prime or the ubiquitous 18-55mm zoom, all lenses share one significant marking that is standard across all makes and all types of lenses. I am referring to the marking 'mm'. Some lenses like the above mentioned 18-55mm has two sets of numbers associated with it. Others like the 50mm prime has only one. So, what does this 'mm' stand for?
One popular misconception in digital photography, at least one that exists at the beginner level, is that the mm stands for the measurement of the length of the lens. Let me assure you at the very outset, the mm measurement has nothing to with the dimensions of the lens. Rather, it is a measurement of the focal length of the lens, which in other words gives away an indication of the possible angle of view that you can get using that lens. Shorter the focal length of the lens, wider is the angle of view and vice versa.
So what is this focal length?
Focal length is the mm measurement that constitutes the distance from the optical center of the lens to the focal plane when the lens is focused at infinity. That's a rather queer piece of technical mumbo-jumbo. I am not quite sure if you get that. So, let me rephrase it in some other way. Let's take an example to explain it.
By now you know that light reflected off of an object travels in a straight line. It passes through the lens aperture and after traveling through the lens barrel gets focused to be pinpointed on to the focal plane. In older days that focal plane used to be the film. In the case of DSLR camera lenses that focal plane is the digital sensor. Now I have used the term 'optical center' of the lens. So a little bit of explanation is warranted.
What is the optical center of the lens?
The optical center of the lens is the point where the rays of light reflected off of a scene converges, after traveling through the lens aperture, in order to produce a sharp image on the focal plane. The lens' focusing mechanism allows you to tweak the focusing distance so that certain light rays are perfectly focused on to the focal plane while others are either focused behind or in front of the sensor. When rays of light are focused behind or in front of the focal plane a blurred image of that area of the frame is produced. We refer to these as out of focus areas in the image. Certain lenses such as the Nikon 135mm f/2 D and the revamped Petzval 58 produces excellent quality out of focus effects (also known as bokeh).
The greatest impact of focal length on your images is the amount of the scene that you can capture with your DSLR camera. I mentioned above that focal length has an inverse relationship with the angle of view that is seen through a lens. With a wide angle lens a large area of the scene can be captured. With a tele-lens, however, only a small portion of the scene can be captured. Think of it as if that the lens is seeing further into a scene. The phenomenon can be compared to seeing through a pair of binoculars. With your naked eye you can see a vast extent of a scene. With a pair of binoculars, however, only a small portion of the scene is visible.
A wide angle lens, typically a lens that is 35mm or shorter, gives a wider slice of a scene. Such lenses are ideally suitable for the purpose of capturing sweeping vistas, magnificent architecture and group shots. A tele-lens is one that is typically 135mm or longer gives a more zoomed-in perspective. That means you can shoot from much further away and still be able to fill the frame with the subject of your photos. Such lenses are typically used for shooting wildlife, portraits (around 105mm to 135mm) and for shooting sports.
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