Lenses (Part 3) – Prime or Zoom?

This next post builds on where Part 2 left off. We have seen that a lens can be categorised, broadly, as being either within the wide-angle, standard/normal or telephoto category, depending on its focal length. However, there are lenses out there that change their effective focal length. These are called “zoom” lenses, since, you guessed it, they can zoom in/out. Where such zooming capabilities are not available, the lens is referred to as being a “prime” lens.

A first point of clarification needs to be made: what a “zoom” lens implies is that the lens can vary its focal length in between two fixed extremes – its widest and its most zoomed in. It is important that one does not assume that a zoom lens is the same as a telephoto lens. In fact, one can have zoom lenses with a range of focal lengths that only cover the “wide-angle” category, for instance a lens that goes from 10mm to 18mm, like the Canon EF-S 10mm-18mm. Also, you can similarly have “standard zoom” lenses, like the typical “kit lenses” provided with a new camera. Here, the zoom range would cover focal lengths falling within the standard/normal category of lenses. Also, you can have some that are telephoto zoom lenses, such as lenses that go from 70mm to 200mm. Finally, you can have zoom lenses that span across different focal length categories, starting out as a wide-angle and zooming in to telephoto focal lengths. Examples include lenses that go from say 18mm to 200mm.

The major advantage presented by zoom lenses is in the form of versatility: with a zoom lens attached to your camera, you can be shooting a landscape one moment (e.g. at 24mm), a portrait the next (e.g. at 50mm) and then a zoomed-in shot of a bird on a perch some distance away (e.g. 100mm). Whilst this of course depends on the focal lengths available to you, it is of course much easier to accomplish this with a zoom as opposed to having three separate prime lenses. For starters, you save time in the form of not having to change lenses. This also lets in less dust onto your camera sensor, an added advantage.

Given such versatility, why would anyone need “prime” lenses? To recap – prime lenses are all those lenses with a fixed focal length. Common examples of prime lenses are virtually all camera lenses in smartphones these days, albeit with a few exceptions that incorporate a lens that can zoom optically. Some of you might say, “hold on, I can zoom on my smartphones”… and that would call for another clarification. There are two methods whereby one can zoom, namely optical zoom and digital zoom. Optical zoom is a method whereby the lenses are physically moved to increase or decrease magnification – it is thus a mechanical type of zoom. On the other hand, digital zoom is a software-dependent zoom. In digital zoom, what the software does is to “crop” in on the image, enlarging the centre portion to simulate zooming in. By doing so, you are presented by a more zoomed in image, but this comes at the expense of image quality as some areas of the photo are “discarded”. Note: I will be expanding on this in future posts dealing with megapixels, cropping and other image-quality related aspects.

With that out of the way, we can go back to the role played by “prime” lenses. What advantages do prime lenses offer over zoom lenses, if a zoom lens can cover any given focal length, plus vary this according to the situation? A number of advantages, in fact…

First off, image quality is generally superior when it comes to prime lenses. The reason is simple – prime lens are constructed to be good at doing one thing, that is, taking a picture at a single, predetermined focal length. On the other hand, zoom lenses have to be good at doing this at various focal lengths. Needless to say, it is easier to construct a high-quality lens with a fixed focal length than it is to construct one with a variable focal length. This simplicity often translates in optical qualities that are superior when compared to zoom lenses set at the same focal length. For instance, comparing the quality of photos taken by a 50mm prime lens, to an image taken with a standard zoom set to the same focal length will more than likely show better results in favour of the prime lens.

Secondly, prime lenses generally have larger maximum apertures, and this also links to the construction argument above. In fact, the simpler construction parameters behind prime lenses permits designers to include wider maximum apertures in prime lenses as opposed to that technically permissible in zooms. This is not to say that there are no zooms with wide maximum apertures, but it is a known fact that primes, on the whole, are equipped with wider maximum apertures. For instance, it is common to find prime lenses with maximum apertures that vary between f/1.2 and f/2.0, whilst commonly zooms top up at f/2.8. There are, of course, exceptions, such as the recently launched Sigma’s 18 – 35mm f/1.8 lens. However, such zooms are few and far in between, whilst it is almost the norm for primes to have such wide maximum apertures.

There are other advantages too, such as that generally, prime lenses tend to be cheaper and lighter, although these do not hold true across the board. When this is the case, it generally stems from the fact that the construction is less complex, requiring less elements and components, thereby making it cheaper to produce and lighter to carry around.

As you can see, there are advantages of using both kinds of lenses, and choosing amongst the two is generally down to what you value the most. Is versatility more important to you than image quality or wide apertures? Then you probably would be better off with a good quality zoom lens. Are you constantly finding yourself wanting enhanced image quality and better low-light performance? Then you probably need to invest in some good prime lenses. Depending on what you’re after, there are lenses out there ready to play their part…provided, of course, your budget allows for it!



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