Swapping out lenses is one of the greatest advantages of modern photography, allowing for photographers to take vastly different kinds of pictures with the same camera. But what should you know before you  buy an expensive new lens?

It’s important to ask yourself the right questions before making a major purchase, as even cheaper lenses tend to be pretty expensive. We’ll run down some of those important questions and help readers understand what they’re looking for before they commit to a new lens.

Do I Have the Right Kind of Camera?

Many cameras aren’t equipped with what we call “Interchangeable Lenses.” Those professional looking, big SLR cameras, digital SLRs, and the MILC cameras we discussed last week are the ones we’ll be looking at today. Point-and-shoot cameras have fixed lenses, and are engineered to have the ability to take good snapshots with a so-called “normal” lens. Chances are, if your camera came with a lens, it is a normal lens, made to replicate an image similar to the one produced by the human eye. But cameras aren’t like human eyes—they can be made to do things that our eyes aren’t very good at. Interchangeable lenses (metaphorically) add more  colors to the palette of clever photographers, allowing to tailor a lens to a certain kind of shot they hope to reproduce. This is the advantage of DSLR, SLR, and MILC cameras over point-and-shoot, and not any amazing difference in image quality caused by larger camera bodies or more megapixels. Let’s take a quick look at the kind of info interchangeable lens camera owners need before shelling out lots of cash for new lenses.

What Type Of Lens Do I Want?

There are three major types of lenses, but today we’ll be talking about two others, just for the sake of being thorough. The three main types are normal lenses, telephoto lenses, and wide angle lenses. The other two types are specific types of wide angle and telephoto lenses, made for special kinds of photography—macro lenses, and fish eye lenses. This is the first important question you should ask yourself—what do I want to buy a new lens for? Let’s briefly talk about how each lens is used, and why you might want to buy one.

Normal Lens: All lenses are differentiated by their focal length, or the distance it takes for light to criss-cross and focus on the photosensitive material inside, be it film or a sensor. Like we’ve said, normal lenses are made to create pictures similar to the images you see with your eyes, and have a focal length of around 50mm for the so-called “standard” format. Cropped format DSLR cameras will require a smaller focal length, but that’s a complex subject we don’t have time for today. Any lens you buy should tell you if it is a normal lens for the format you’re shooting or not—more on that later.

Wide Angle Lens: Lenses with shorter focal length (around 35mm and less) allows for light to strike at the photosensitive material more easily, allowing for a greater angle of view in your image. Wide angle shots will capture more image from a wider field of view when shot from the same spot. They also have greater depth of field, allowing you to keep clean, tighter focus on more parts of the image, even in image data near and far. The shorter the focal length, the tighter the focus. Very wide angle lenses also create image distortion, which is a good lead in to our next topic.

Fish Eye Lenses: When lenses have incredibly short focal length, they fall into a sub-category of wide angle called “fish eye” lenses. These lenses squeeze so much information into the same image format that they greatly distort the images, and create a bizarre, otherworldly effect we’ve probably all seen in movies. Fish Eye Lenses are fun, but are not widely used in photography, except as a novelty.

Long Focus or Telephoto Lenses: These lenses are the ones that really seem to impress people—huge barrels of glass, metal and plastic mounted on a professional looking camera body will make an impression on just about anybody. Turns out telephoto lenses are some of the most useful to take certain kinds of photos. It seems a given that these kinds of lenses are good for long range shots, but you may be surprised that many of these lenses are perfect for intimate portrait photography. Lenses of around 85-100mm are sometimes called “portrait lenses” because they can effectively eliminate the distortion of shorter focal length lenses, and keep faces looking natural. They also allow photographers to keep a good standard distance of 10-15 feet from a subject and still get a tight, intimate shot. And photographers interested in bokeh shots will be happy to know that the smaller depth of field is perfect for bokeh.

Macro Lenses: Some telephoto lenses are specially designed to focus in on smaller objects, and we call these macro lenses. There’s not a lot to be known about this, except that telephoto lenses are good for extreme closeups and photographing small objects as well as taking shots of distant images.

How “Fast” Does My Lens Need to Be?

When you look at the info on an online listing of a lens, you might notice that it also includes the f number of the lens, or two in the case of zoom lenses. This refers to the maximum aperture of the lens, or by some terminology, the speed of the lens. The lower the f number, the wider the aperture, the more light the lens allows in. A lower f number on the lens means that you can use lower ISO and faster shutter speed settings, so a drop in f number (particularly on zoom and telephoto lenses) means a dramatic increase of quality in the lens (and probably expense!). Longer telephoto lenses allow in narrower, more intimate images, but also block more light and have smaller f numbers. All other things equal, get the smallest f number you can afford.

What Format Am I Shooting In?

Digital photography has created a problem, in that it has created lots and lots of new “formats.” Film only photographers won’t have to worry about formats, because almost every SLR that uses film will be using the 35mm format. Digital photographers have to deal with cropped sensor formats, and have to use lenses designed to create clean images on sensors smaller than the 35mm image area.

You might buy a lens for the wrong format, but you’ll most likely be returning it if you do. Most lens mounts don’t allow for cameras to use lenses for the wrong format, with one notable exception. Nikon is particularly proud of the fact that it uses a standard mount for its interchangeable lenses (it has for many years), so a photographer might be tempted to use the wrong format lens. This is never really a good idea, as the wrong format lens can affect your camera’s ability to resolve detail properly or create an improperly cropped image. (Feel free to ask questions about this—if there’s interest, we’ll probably be writing an explainer about this very confusing topic.)

You likely won’t have this problem when shopping for lenses—just google “lenses for “ and then your camera model to get an idea of what to start looking for. It’s very (very) likely not going to sell as a lens for that camera if it’s in the wrong format!

What Do Other People Think?

This is a critical step, and an obvious one, but we’ll briefly talk about it anyway. Just like any piece of software or hardware, read lots of reviews before making a purchase. It’s important to be well informed before sinking a couple of hundred bucks into a new lens. But keep in mind what the reviewers are saying. What level do they sound like they’re at? Are they describing the kinds of pictures you want to take? Are they taking pictures in the kinds of situations you’re taking pictures in? Really think about if the lens is a good fit or not, from the perspective of accomplishing what it needs to accomplish.

Does the lens resolve detail well? Does it have any anti-shake, or other technology? It’s amazing to think of the level of engineering that goes into a lens to create a quality image, so spend enough time reading both professional and customer reviews to be sure you truly understand what you’re getting. Here’s a quick example. An aftermarket telephoto lens for a Nikon camera might be excellent at resolving detail and cost three hundred dollars less than a comparable Nikon lens, but might have some strange quirks (Author’s note: I’ve seen a zoom lens that would slide forward and backward, ruining an image, unless it was held in place by hand). Reviews can educate you about these problems so you can decide if the extra few hundred is worth all the frustration, or the minor frustration is worth saving a few hundred.

How Much Use Will I Get Out of This Lens?

This is always one of the most important questions to ask before committing to that lens. Are you going to get a lot of use out of it? Do you need to take portrait or long range shots? Do you really need to take goofy fish-eye lens images? If you’ve got money to burn and photography is a passion for you, go bananas and buy all the lenses you think you can use. Keep in mind that new lenses will not make you a better photographer, but they can help you take a different kind of picture.

What do you look for in a good interchangeable lens? Tell us about your experience buying lenses and your preferences in the comments section below, and maybe add any other thoughts that go through your mind before shelling out the big bucks for a new quality set of optics.

Image Credits: Let’s Go Shopping Part II by Yueh-Hua Lee, Creative Commons. 7D DSLR Rig version 1 by Dean Terry, Creative Commons. Canon Digital Elph PowerShot SD780 IS (3) by Studioesper, Creative Commons. 50mm f/1.4 G by Rick (瑞克), Creative Commons. Longleat House Gardens (Ultra Wide Angle) by Phil Holker, Creative Commons. Fish-eye + exposure blending by Dino Quinzani, Creative Commons. Golden Portrait by Geraldine, Creative Commons. Macro by August Kelm, Creative Commons. Nikon 35mm f/1.8 DX by Isaac Hsieh, Creative Commons. Macro Herreras by Roberto, Creative Commons.