How To Find the Right DSLR Lens For Your Needs

By Loyd Case

Loyd deconstructs and demystifies DSLR lenses so you'll spend money on the right glass for your photo needs.

Photographers are a funny bunch. One day, they tell you that it’s not the gear that matters, it’s the photographer, so you should be able to get great photos out of that old Instamatic. In the next breath, they tell you to throw away the perfectly decent kit lens you got with your new DSLR because photos shot with it will be crap. Oh, what to believe!

What you really want is the right lens for your photography. That 18-55mm f/3.5-5.6 kit lens may be just right for you, if you primarily take photos of your family in brightly lit environments, or lug it along on vacations, where the relative light weight of the kit lens makes life easier on necks and backs. On the other hand, if you’re trying to shoot your kid playing basketball in crappy gym lighting, and get blurry messes for most pictures, maybe you do need a different lens.

Let’s try to deconstruct some of the key numbers, and understand why some lenses do better than others – and why they also might cost and weigh a whole lot more. Most of the discussion will focus on zoom lenses, but we’ll touch on primes – fixed focal length lenses – at the end. Since I’m a Nikon shooter, most of my examples will be Nikon, but the principles are the same whether you use Canon, Sony or some other DSLR brand.

Understand Your Needs and Biases

It’s worth spending some time picking out your favorite photos and then making lens choices based on what you shoot, not what specs might be cool.

The person shooting birds in flight against blue skies have completely different needs than the shooter who’s trying to freeze that slide tackle on the soccer pitch in nighttime games. There’s also a tradeoff between versatility and more specialized lenses. It’s clear that a 600mm prime is a specialty lens, but what’s the difference between a 24-70mm f/2.8 and a 16-85mm f/3.5-5.6?

Understanding what you like to shoot the most helps make better lens choices. It’s not always obvious, because most photographers seem to choose the gear first. That 18-55mm f/3.5-5.6 kit lens seems so limited, so I’ll pick up an 18-200mm f/3.5-5.6. So the buyer goes for reach rather than better performance in low light, which might be the more versatile choice.

It’s worth spending some time going through your photos, picking out the ones you like the best, and then making lens choices based on what you shoot, not what you feel might be cool.

With that thought in mind, let’s deconstruct a few terms.

F-Stop (Aperture)

F-stop is the ratio between the focal length of the lens and the diameter of the lens opening. You see f-stop written like so: f/2.8. The “f” actually is the numerator in the fraction, and represents the focal length. For obvious reasons, the denominator changes as you adjust a zoom length, while a 50mm prime lens is always 50mm. Because of the way it’s written, the smaller the denominator, the more light reaches the sensor.

The fact that the focal length varies in a zoom lens is a key reason why fast zoom lenses are big and heavy. It takes a lot of glass to maintain a constant f/2.8 when you change the focal length from 24mm to 70mm, (not quite 3:1). On the other hand, a fixed focal length lens, like the 85mm f/1.4D I use occasionally, isn’t all that large and heavy, because the lens designers didn’t have to worry about a range of focal lengths.

One odd bit of jargon: lenses which have low f-stops are often called “fast” for obscure reasons. But that doesn’t always mean their autofocus works fast, so watch out for that.

Wide Apertures: the Good and Bad

Since we’re talking about lenses, it’s worth noting that the bigger the aperture, the more shallow the depth of field effect. This becomes quite noticeable with macro lenses. I have a couple of f/2.8 capable macro lenses, and it’s very easy to end up with a blurry mess unless you stop way down. So having a wide aperture isn’t a panacea, because you end up with bulky lenses, and unexpected DoF effects if you shoot wide open.

On the other hand, if you must shoot in low light and capture fast action, large aperture sizes are lifesavers. The shot below was captured in poor indoor gym lighting at f/2.8 and 1/500ths of a second.

On the other hand, you can capture fast action in bright sunlight without a pricey, bulky lens. Again, it all depends on what you’re trying to achieve.

It’s not the sensor size itself that affects depth of field, but the sensor combined with the focal length and aperture.

One final note: aperture directly affects depth of field – but the degree of the depth of field effects also depends on sensor size. Note that it’s not the size of the sensor itself that affects depth of field, but the sensor combined with the focal length and aperture. Thom Hogan has a great discussion of the impact of sensor size and lens usage.

The net effect is that given the choices of lenses, it’s harder to get a shallow depth of field with small sensors than large sensors. The flip side of the coin is that if you want deep depth of field effects, a smaller sensor may fit the bill better.

Aperture: Stopping Down and Diffraction Effects

I’d like to touch briefly on diffraction effects. As sensor pixel densities increase, the likelihood that you’ll get less sharp images at extreme apertures (i.e., beyond f/16) is possible, particularly with smaller sensors (like those used on APS-sized sensors – the Canon T3i or Nikon D7000 as examples.) It’s not a big issue – most people aren’t shooting at f/22 and beyond. But it’s worth noting that stopping way down doesn’t always make the image sharper, as your instincts might suggest. Bob Atkins has way more on this, if you want to learn more about this topic.

Autofocus Speed

The problem is that it’s difficult to tell how fast the autofocus might be on a particular lens until you use it.

Some lenses focus faster than others. It’s often the case that newer lenses, with newer types of drives, will focus faster than older ones, but that’s not always true. The problem is that it’s difficult to tell how fast the autofocus might be on a particular lens until you use it. Also, focus speed is somewhat relative. Again, if you’re shooting fast action on the go, then fast autofocus might be critical. If you’re shooting landscapes when mounted on a tripod, you may prefer to turn off AF entirely and manually focus.

Focus speed is called out more subtly in feature lists than other specs. You’ll see terms like “silent wave motor” or “high speed motor”, which tells you a little bit about how quickly the lens might respond. But actual focus speed is dependent somewhat on the camera body as well. Try before you buy, if focus speed is an important spec for your needs.

Bokeh: Does the Blur Look Good?

Photographers often argue about the quality of the blurry parts of the image, often called “bokeh.” A smooth bokeh is considered good, one that’s got rougher elements is considered less good. Here’s a couple of examples, one with what might be called “creamy” bokeh (good) and one where the blurry background is a little rougher looking (not so good.)

"creamy" bokeh

The quality of the bokeh seems to be most dependent on how the aperture blades are designed. Aperture blades that are curved tend to produce better looking background blur. While pixel peepers often love to argue about this sort of thing, it’s a secondary issue to how well the photograph is shot and composed. If the first thing you notice is the bokeh, then the photo itself might not be so memorable.

What is this Stabilization Stuff, and do I Need It?

not as "creamy" bokeh

Optical stabilization reduces the effect of camera shake at low shutter speeds. The result is the ability to shoot at relatively low shutter speeds and still get sharp images. Different manufacturers have different names for this. Nikon calls it “VR” (vibration reduction), Sigma’s version is OIS (“optical image stabilization”) and so on.

Optical stabilization doesn’t improve the ability to capture fast action. You still need to shoot at high shutter speeds for that.

Some SLR manufacturers (eg, Sony) build stabilization into the body, while others (Nikon, Canon) build them into the lenses.

Note that lens stabilization is not one-stop solution to all your problems. Shooting with VR on my 24-120 f/4 at 24mm means I can often get a good shot at 1/20th or even 1/15th of a second shutter speed – provided the subject isn’t in motion. However, zooming out to 120mm means that even shots at 1/50th might not look good, even with VR. Longer focal lengths magnify the effect of small vibrations.

Secondly, optical stabilization doesn’t improve the ability to capture fast action. You still need to shoot at high shutter speeds for that. Just what shutter speed is ideal depends on the type of action. I can shoot marching band action at 1/160th, but volleyball requires much faster shutter speeds – 1/320th or better.

Macros vs. Zooms vs. Primes

To zoom or not to zoom? And what to macros (or, as Nikon calls them, “micros”) really do for you?

Zoom lenses are all about compromises. In theory, I’d love to have an f/1.4 18-200 zoom. But it would likely be difficult to lift and insanely expensive. We’ve already seen how a modest 3x zoom with a constant f/2.8 aperture gets pretty hefty and expensive in its own right. Anything more aggressive would be priced beyond most people’s reach. On top of that, anything like that mythical 18-200mm f/1.4 would likely have some serious optical issues.

That’s why you often see high constant aperture zooms with limited zoom ranges. I often shoot with a Nikon 24-120mm f/4 VRII – that’s a 5x zoom range, and f/4 is about as good as it gets. Most f/2.8 zooms max out at 3x, like the Nikon 24-70 f/2.8 or 70-200 f/2.8. The Nikon 70-200 f/2.8 VRII is one of the sharpest lenses I own, and I dearly love it, but it weighs 54 ounces (over 3-1/4 pounds) by itself and costs over $2,300.

That’s why lower cost zooms are compromises. If we’re talking Nikon again, the 70-300mm f/4.5-5.6 variable aperture zoom is about $600, offers a longer reach than the 70-200 f/2.8, and weighs half as much. But the catch is that it won’t do as well in low light – the minimum aperture is f/4.5 and when zoomed out, 5.6. That’s two full stops slower than f/2.8. Since the f-stop scale is logarithmic, that means it f/5.6 lets in a quarter the light relative to f/2.8. Once again, we’re back to the issue of cost and weight: more light means more glass (literally), boosting both cost and weight.

Macro lenses allow you to get really close to your subject. A really good macro gets as close as 1:1 or even closer in some rare cases. This is great for shooting tiny insects, close-ups of small electronic components or just fun, goofy shots.

What if you really crave shooting in smoky bars or concert venues? You’ll likely want a fast prime. A prime lens is just a lens with a fixed focal length. You can’t zoom, so you have to move your body instead (sneakerzoom.) But fairly fast, f/1.8 primes are pretty affordable. A Nikon 50mm 35mm f/1.8 prime costs about $200, while the 85mm f/1.8 is under $500. Of course, if you want faster, you have to pay for it. The 85mm f/1.4G is costs three times the price of the 85mm f/1.8. It’s sharper and has better bokeh, but you’d better have a real need for it. On the other hand, the smaller 50mm f/1.4 is about $500. The longer focal length primes tend to be specialty lenses, so often cost more.

In any case, a prime lens tends to be more affordable and perhaps more importantly, can handle lower light situations than most zooms. Even low cost primes tend to be pretty sharp, too, so you get a twofer: good sharpness and the ability to shoot in low light.

Lesson the First: What Do You Shoot

Remember the first lesson: understand what you like to shoot. If you really are more of a snapshot person, and understand that, the kit lens is likely just fine, and you don’t have to obsess over lens choices. But if you love a specific aspect of photography, understanding the needs for your favorite type of photo will go a long way towards helping you rationalize lens choices.