One of the questions that crops up often on photography forums, sites, and even in photography conversations over a pint is "which lens should I buy next?" It is said with such sincerity and met with so many recommendations that are, in the end, mostly meaningless. It even rears its ugly head in the form of "What is the best lens for 'X' photography?", as though somehow, another person's answer will guide the asker to greatness.
In order to truly ask the right questions about the lenses we need or desire, we must understand what makes them different from each other. There are optical differences between lenses that cause different focal lengths to render scenes differently. For the sake of simplicity, all focal lengths in this article will be talked about in 35mm "full frame" terms.
Field of View
Let's first consider two lenses, one 24mm and one 85mm. These two lenses differ in their field of view quite significantly. The 24mm "sees" approximately 84 degrees, whereas the 85mm sees around 29 degrees. In practical terms, this means a 24mm will "include" more of a scene or background than an 85mm. When photographing a person, for example, and keeping them the same size in the frame, a 24mm lens will see a lot more of the background.
Again, looking at the two lenses of 24mm and 85mm, we can see a significant difference in perspective distortion. With the 24mm lens, objects, or parts of objects closer to the lens appear larger than those farther away. The distance between objects can also be greatly exaggerated with wider lenses. This might be useful for showing how large and powerful a boxer's fists are in a portrait, or for creating the "converging lines" effect with railway tracks. Tilting wide lenses up or down will also have a much greater effect on the vertical lines in your image than with a longer lens.
With the 85mm, almost none of this effect will be seen. Longer lenses will do the opposite, drawing farther objects "closer". This is great for rendering far away backgrounds effectively closer to your subject in a portrait, for example.
Depth of Field
Although depth of field is a function of many variables, including aperture (larger aperture means less depth of field) and focus distance (closer focus distance means less depth of field), it is also greatly influenced by focal length. An 85mm lens, for example, has an inherently narrower depth of field than a 24mm lens. If you're looking to send your backgrounds into buttery oblivion, a longer lens may be the ticket. An 85mm lens at f/2.8 will have a shallower depth of field than a 24mm lens at f/2.8, given the same focus distance.
My Lens May Not Work for You
The initial question is not a bad one in and of itself, but it's the wrong question to ask most of the time, and asked of the wrong people. The way I use a 24mm lens may differ from the way you do. You might be asking for a recommendation for a portrait lens and I would certainly recommend my 24mm for portraits, whereas a beauty photographer will most likely tell you to stay away from a 24mm lens and go for something closer to 85mm. This lies in our differing intentions when photographing portraits. I like to be physically close to my subjects and am first and foremost looking for a genuine moment, not necessarily a flattering portrait. Thus, a more beneficial question would be: "what lens would achieve the aesthetic I want?"
Your Way of Seeing Is What's Important
So, when looking for a new piece of equipment, or even simply taking a lens out of your bag to shoot with, consider the aesthetic before anything else. Consider what you want to achieve. After all, we are visual artists. We need to understand which tools to use so that we can create the image we want. This is what will make great images, not the lens that another photographer thinks we should have.