When buying a DSLR lens, the first decision to make is what focal length you want. The answer depends entirely on what you'd like to photograph.
Before we get too far in a discussion of the importance of focal length, let's get the technical definition out of the way.
Focal length is measured in millimeters (mm) and it represents the distance from the optical center of a lens to the digital camera sensor when the subject of the photo is in focus.
This is the standard textbook definition, but it's still not entirely obvious WHY you need to know about it before your purchase a new lens.
Instead of the textbook approach, let's try the plain English explanation:
How do you decide which focal length lens you need? It all comes down to what you want to photograph.
Let's say that you have a short 50mm lens. In order to get a close-up of your friend you stand 5 feet (1.52 meters) away.
You switch to a long 200mm lens. Now you must stand farther away from your friend (say 15 feet/4.57 meters) to capture the same image.
Let's simplify further: you're taking a photo of a kitten. You use a lens with a short focal length because you can sit right next to the kitten.
Now you're taking a photo of a crocodile in the wild. Do you really want to sit as close as you did to the kitten?
Unless you are a thrill seeker, the answer is probably no.
For those of us who don't like to live on the edge, a lens with a long focal length will get us up close and personal with that croc even if we keep our distance.
Lenses specialize in specific focal lengths.
Let me put it this way: you won't find a do-it-all lens that covers the entire focal range from 11mm to 600mm. They don't exist.
Instead, lenses are grouped into four primary categories based on their focal lengths:
|Lens Type||Focal Length|
|Wide Angle||28mm or lower|
|Standard||Anything from 35mm to 85mm|
|Telephoto||Anything from 100mm to 300mm|
|Super-Telephoto||300mm or higher|
This is why the very first decision you should make when purchasing a new lens is what focal length (or focal range in the case of zoom lenses) you'd like it to cover.
In the section above, I mentioned that you want a lens with a long focal length if you want to stand far away from your subject but still get a close-up.
Lenses with different focal lengths have different uses in the world of photography. Let's add some additional information to the focal length table:
|Lens Type||Focal Length||Best Use|
|Wide Angle||28mm or lower||Landscapes and Interiors|
|Standard||Anything from 35mm to 85mm||Portraits|
|Telephoto||Anything from 100mm to 300mm||Portraits and Sports|
|Super-Telephoto||300mm or higher||Wildlife and Sports|
That's the quick summary, but let's dig a bit deeper.
Wide angle lenses are ideal for photographers who want to capture all of the scenery in front of them.
These lenses can capture virtually everything that your eyes see in front of you - from the ground at your feet to that mountain in the distance.
This is why they work well for landscape photographers who really want to draw the viewer into the photograph.
They are also used to photograph indoors, since you don't have a lot of working room. To photograph an room full of people you have to back into a corner and use the widest angle lens you can get.
Wide angle lenses are not good for portraits because they distort facial features: the most noticeable problem is that they make noses look HUGE which is not terribly flattering for your subject.
Standard lenses are versatile and can be used for a wide range of photographic subjects.
They aren't at the extreme edges of the focal length range, but instead fall right into the middle.
You can use standard lenses to capture landscapes and closeups, but their main strength is portraits.
Lenses with focal lengths between 28 and 70mm capture faces that are free of distortion so that they look "natural".
Telephoto lenses also work well for portraits but cause some distortion: the longer the focal length of a lens, the more it compresses the visual space.
This means that it's harder to tell the distance between objects when you use a telephoto lens. Telephoto photos just have less depth and three-dimensionality than lenses with wider focal lengths.
What are they really good at? Getting you up close and personal with subjects that are far away.
This is why they are ideal for non-pro sporting events - you can stand on the sidelines, but still get a close-up of the game winning goal.
These lenses are almost exclusively the domain of professional photographers, and are extremely expensive.
They are used by wildlife photographers, where getting up close is not an option.
They are also the lenses you see at any professional sporting event: those massive ones that require daily strength training to tote around.
A super-telephoto lens really compresses visual space, and requires a lot of skill to use effectively.
I'm a visual learner, so here are some examples that should help you really understand focal length.
Focal Length: 28mm
Focal Length: 75mm
Focal Length: 200mm
I don't own a super-telephoto lens, so I don't have any good examples.
Don't worry, I've got you covered.
Scotch Macaskill runs a web site called Wildlife Pictures Online with some exceptional examples of wildlife photos taken with super-telephoto lenses.
I recommend that you take a look at his site, not just for the great photos, but also for the detailed information about taking wildlife photos on African safaris.
Before you move on to the next section, have in mind the type of focal length you'd like in your lens.
Even if you don't have an exact range, rely on the knowledge of what you want to photograph. Here are some examples to get you thinking:
Why is this important?
If you can decide right here and now that you really want to take great landscape shots, then you won't have to evaluate every single lens out there.
Just focus on the Wide Angle lenses, which will make the following steps significantly easier.
In order to demonstrate my 6-step process to finding the best digital SLR lens, I'll use a concrete example to help you out.
Chris has just purchased his first digital SLR: a Nikon D5100 with an 18-55mm f/3.5-5.6 kit lens.
The kit lens works well for landscapes and portraits, but Chris really wants to take photos of his son's little league games.
Since he wants to get close to the action, Chris decides to look for a Telephoto lens with a Nikon mount.
Chris starts his search for a digital SLR lens by adding these two items to his wishlist:
In the next section he'll add even more.