June 15, 2007
This page contains advice on choosing lenses for your digital single-lens reflex camera (DSLR). My experience is with Canon DSLRs with cropped-frame sensors, like the Canon Rebel. But if you have a full-frame or non-Canon camera, some of the general information on this page may still be helpful to you.
What should I look for in a lens?
It's easy to spend lots of time and money on choosing and buying lenses, so before we start let's think about what we really want.
As others have written about, equipment isn't the most important factor in determining whether you can make great images. Good lenses, like other good equipment, will make an incremental difference in your work if you know how to get the most out of them. No lens will make you a better photographer!
So before you get a new lens, it's good to have a specific goal for how it will improve your work. Some of the ways in which a different lens could help you are:
Focal length
Each lens has a focal length, measured in millimeters (mm). Lenses of different focal lengths are used to shoot different subjects. The smaller the focal length, the wider the angle of the lens, meaning you can capture more of what you see in one image. The greater the focal length, the closer the lens gets you to your subject, meaning you can capture something farther away.
Lenses with a fixed focal length are called prime lenses, as opposed to zoom lenses, which have adjustable focal length.
Very wide angle lenses, around 10 to 14 mm, are called fisheye lenses because they produce a distorted, nearly circular image. More common are wide angle lenses around 20 to 28 mm. These are helpful for interiors and landscapes. Lenses around 35 to 50 mm capture an image very similar to what you normally see with your own eyes. A 50 mm lens is sometimes called a normal lens. Lenses between 50 to 120 mm are commonly used for portraiture because the photographer can capture a face without standing too close to the subject. Lenses over about 100 mm are telephoto lenses. The longest telephoto lenses, 300 mm to 600 mm or even more, are used when the photographer is necessarily far from the subject, as in wildlife photography.
Focal lengths aren't linear. This means that the difference between a 20 mm lens and a 30 mm lens is far more significant than the difference between a 200 mm lens and a 210 mm lens. To really see this you'll have to play with some lenses.
For a cropped-frame DSLR such as a Canon Rebel, the focal length shown on the lens isn't the effective focal length for your camera. There is a multiplier of 1.6 (Canon) or 1.5 (Nikon). So if you buy a 50 mm lens, it is effectively an 80 mm lens on your Canon Rebel. As a result, manufacturers like Canon and Nikon have started to make lenses that are specially designed for cropped-frame DSLRs. For example, Canon makes a 17-85 mm zoom that "turns into" a 27-136 mm zoom on a Canon Rebel.
In general, the widest wide-angle lenses and the longest telephoto lenses are more expensive than the lenses in between of similar quality.
Here is a good article with more detail on using different focal lengths to get different aesthetic results from landscapes.
Maximum aperture
Every lens has a maximum and minimum aperture, for example from f/4 to f/22. The maximum aperture (the smaller number) is generally more important because it determines the most light the lens can gather (for shorter exposures in low light) and the shallowest depth of field the lens can achieve (for blurring backgrounds).
Lenses with a large maximum aperture are called fast; lenses with a smaller maximum aperture are called slow. In general the fastest lenses are prime lenses. A very fast prime might have a maximum aperture of f/1.2, and many primes are around f/2.8. A very fast zoom lens could have a maximum aperture of f/2.8, but most are around f/4.
This helps determine what a lens can be used for. For example, sports photographers need fast telephoto lenses (events are often indoors in low light and the action is often far away). To achieve this, the lens needs to be relatively large.
Many zoom lenses have a variable maximum aperture, for example, from f/4 at 17 mm to f/5.6 at 85 mm. This can mean that if you set an aperture and then zoom in, the aperture you set could change. For this reason many photographers prefer zoom lenses with a fixed maximum aperture.
For lenses of different focal lengths, the same aperture produces different depths of field. The widest angle lenses have very deep depth of field, meaning that even at f/4, the lens may be focused from one or two meters to infinity. Telephoto lenses have much less depth of field, so f/4 would create a blurred background.
In general, faster lenses are more expensive than slower lenses.
Convenience
A big benefit of your DSLR (or SLR) is that you can switch lenses. However, extra lenses add more bulk and weight to your camera bag, and switching lenses takes time and can introduce annoying dust into your camera body.
Zoom lenses are attractive because they reduce the number of lenses you need to carry around. You may be able to find one zoom lenses that seems to cover all the focal lengths you normally use. This could make you more likely to take your camera more places, and get more shots.
The tradeoff is that in certain other ways, prime lenses are superior to zoom lenses, and zoom lenses with a shorter span of focal lengths are superior to those with longer spans. Prime lenses are generally faster and sharper than a zoom lens can be. Of course, some very expensive zoom lenses are just as good as some inexpensive prime lenses.
Lenses with a larger maximum aperture tend to be bigger and heavier. For example, Canon makes a 70-200 mm f/4 lens and 70-200 mm f/2.8 lens that are very similar except for the one stop difference in maximum aperture. But the f/4 lens weighs 700 g while the f/2.8 lens is thicker and weighs 1,300 g.
Lens manufacturers are starting to take advantage of the fact that cropped-frame cameras don't need full-size lenses. For example, Canon makes a line of EF-S lenses that are smaller and lighter than the equivalent full-size lenses. The tradeoff is that they only work on the cropped-frame DSLRs, whereas most other Canon lenses will work on nearly any Canon camera you might own in the future.
In general, lenses that are higher quality and more expensive are bigger and heavier (and usually more durable) than cheaper lenses that cover the same focal range.
Image quality
Most photographers prefer to use lenses that produce images with sharp details, good contrast, accurate color, and little distortion. Overall "image quality" can be highly subjective, but most lenses excel in some areas and not others.
Sharpness is a measure of how sharp the lens can make the in-focus parts of an image. It is related to resolution, which is the smallest amount of detail the lens can capture. Apparent sharpness can be increased in Photoshop, but resolution can't. Many lenses are sharper toward the center and less sharp toward the edges. A new lens should be tested to make sure the falloff in sharpness is acceptable and consistent.
Contrast is a measure of how much adjacent areas of an image differ in brightness. Contrasty images tend to be more pleasing. In the past, photographers looked for contrasty lenses because it was tedious to add contrast in the darkroom. Today, it's easy to work with contrast in Photoshop, and digital cameras are more sensitive to blown highlights than film. (A blown highlight is when the camera overexposes the bright areas of a high contrast scene so they no longer contain any detail.) As a result some photographers actually set their cameras to reduce contrast when taking a picture, then adjust it later.
Color accuracy is a measure of how faithfully the lens reproduces real-life colors. Other factors such as the camera sensor and white balance have a big potential effect on color accuracy. But some lenses do produce more accurate, vivid, pleasing colors than others.
Chromatic aberration is when a lens (or the lens together with the sensor) adds unwanted color artefacts to the image. This is most commonly seen as purple or red "blooming" in light or dark areas that are next to much darker or lighter areas (not necessarily just white next to black). It is more common along the edges of the frame than toward the center. Some chromatic aberration can be removed in digital post-processing.
Bokeh is the way a lens blurs areas of the image that are out of focus. In general, the most pleasing bokeh is smooth, without distracting shapes. It's more often found in prime lenses, and it's not easy to produce in Photoshop. Many lenses have distracting bokeh (lots of circular or angular shapes) and are therefore less suitable for portraiture or macro work.
As you would expect by now, in general, lenses with good sharpness, contrast, color accuracy, low chromatic aberration, and good bokeh are larger, heavier, and more expensive :)
Stabilization
More and more new lenses are being made with stabilization technology to reduce the effect of hand shake. Canon calls it image stabilization (IS); Nikon calls it vibration reduction (VR). Sensors in the lens detect movement and move the lens elements to compensate. Similarly, some camera bodies detect movement and move the sensor to compensate. Stabilization is useful for all kinds of shooting where a tripod isn't practical, but it's most useful for telephoto lenses, which magnify the effect of hand shake.
In low light, stabilization adds two or three stops to longest exposure you can hand-hold with the same aperture. If you would normally use a shutter speed of no more than 1/50 s with a 50 mm lens (the general rule of thumb), now you could shoot at 1/15 s. Of course, moving subjects will be blurred, and depth of field and your ability to blur the background are still determined by the lens maximum aperture (which hasn't changed). A non-stabilized lens with a larger true maximum aperture would get better results, but it would typically be much bigger.
Sometimes seen as a consumer feature, stabilization is starting to make its way into professional lenses because it makes any lens more flexible, as long as the limitations are understood.
In general, lenses with stabilization cost more than non-stabilized lenses, but significantly less than a non-stabilized lens with the same low-light capability (assuming one exists).
Special purpose lenses
Some lenses are designed specially for certain types of photography. They usually don't make good general-purpose lenses, so they represent an investment in a particular goal.
Macro lenses are designed to let you get closer to a small subject. Most lenses can't focus within a meter or more of a subject. A good macro lens can get close enough to show the subject 1:1 or magnified.
Tilt-shift lenses allow you to shift the centerline and focal plane of an image. This means you can straighten converging lines to make them parallel and bring near and far subjects into the same focus. This is done by physically moving the lens elements within a frame. It's useful for architectural and landscape photography, but it takes another skill set to do properly. There are only a few tilt-shift lenses, and they are prime and manual focus.
Soft blur lenses intentionally add a particular blurring effect that may be desirable for some kinds of especially tacky portraiture (ok, just my opinion).
Pinhole lenses are literally just a tiny hole in a flat surface. They have a fixed aperture that is very small. The result is that while all of the image will appear out of focus, everything is equally out of focus, no matter how near or far from the lens. Typically a relatively long exposure is needed. The earliest cameras were pinhole cameras, since the simplest camera is literally just a box with a tiny whole in it. Some photographers enjoy the special effect created by pinhole lenses.
Focusing
Most modern autofocus lenses have very similar, fast autofocus mechanisms. But it's still the case that some lenses focus faster than others. If you have a special need for very fast focus, you know who you are.
The actual autofocusing process is carried out by your camera, not the lens, but the faster the focus motor, the faster the camera can do its thing. In general, cameras need some light to be able to autofocus, so lenses with a larger maximum aperture will be easier to focus in low light.
A side effect is that some slow, low quality focus motors are also much noisier than better focus motors. This might matter to you if you're taking candid pictures of people, who might be distracted by the noise.
Brand
When shopping for a lens, in many cases you can choose between a lens made by your camera manufacturer and one made by a company that makes lenses for different brands of cameras, such as Tamron, Tokina, and Sigma. Often these look very similar on paper, and they are certainly compatible with your camera (as long as you pick the right lens mount).
Most reviewers would agree that in general, "off-brand" lenses are intended to provide consumers with a lower-cost alternative to the lenses from your camera manufacturer. Some of these lenses may be just as good, others may be significantly worse. No matter who makes the lens you're considering, you should read specs and reviews and determine what's best for your needs and budget.
That's a lot of information. Can we summarize it?
Depending on the kind of photography you do, You probably want some or all of the following:
- A variety of focal lengths, suitable to your work.
- A large maximum aperture (f/2.8 or better), for shallow depth of field.
- Good overall image quality, within the limits of what you can afford.
- A lens with especially strong image quality in a certain area that you care about, like great sharpness or bokeh.
- A lens with image stabilization, if you use telephotos or shoot in low light.
- No single lens can do everything. In general, the more varied your photography, the more different lenses you'll use.
- Most prime lenses have larger maximum apertures and better image quality than most zoom lenses. But one zoom lens could replace three or more prime lenses, reducing bulk, weight, and cost.
- Money can buy you lenses with better image quality, including zooms that are almost as good as the best primes.
But to achieve that, the "best" lenses are usually bigger and heavier.
Ok, but what lens(es) should I actually get?
Obviously there isn't a single best answer to that question. But here are some steps to help you decide.
Define the range of what you need
It helps to know what you want before looking at the available lenses. For example
- What's the widest angle lens you'll use regularly? For many photographers it's around 24 to 28 mm.
- What's the longest telephoto you'll use regularly? For many photographers it's around 135 to 200 mm.
- Do you need a lens with a wide maximum aperture? Having one lens that goes to f/2.8 or f/1.8 adds flexibility in low light and for shallow depth of field.
- Are there any special purpose features you need? These will generally lead you to an additional, particular lens.
Here is a good article that takes the step-by-step approach into more detail. Here is another article with more scenarios on lenses for different uses.
Look at what's available
Only a few new lenses come on the market each year, so your choices are actually fairly constrained. Looking at what lenses are available will force some choices and tradeoffs.
I can't maintain a good list of lenses here, but other sites do that very well. Here are some popular lens review sites:
- The-Digital-Picture.com for reviews of Canon lenses and gear.
- FredMiranda.com user reviews for Canon, Nikon, and More.
- BobAtkins.com for reviews of Canon lenses.
- "Canon" means it's manufactured by Canon (though some parts may be made by someone else).
- "EF-S" is the Canon code for a lens designed for Canon cropped-frame cameras such as the Rebel or 30D.
- "17-85mm" describes a zoom lens. As you zoom, the focal length will change from 17mm to 85mm. On a Rebel, the effective focal length will be 27-136 mm (multiply by 1.6).
- "f/4-5.6" is the maximum aperture. As the focal length changes from the wide to the telephoto end of this particular lens, the maximum aperture will change from f/4 to f/5.6.
- "IS" is the Canon code for a lens with stabilization.
- "USM" is the Canon code for a lens with a fast, quiet ("ultra-sonic") focusing motor.
- "L" is the Canon code for a professional-quality lens, i.e. it probably has better image quality and is more expensive.
- "VR" is the Nikon code for stabilization.
- "T/S" refers to a tilt/shift lens.
Think in terms of an upgrade path
You don't always have to buy the best possible lens today. For many focal lengths, you can move on to "better" lenses over time, bearing in mind that your development as a photographer is more important than your lenses anyway. If you aren't making money from your photography, you probably have little excuse to spend thousands of dollars on lenses. But in the future, when the expense is justified, you might take the next step up.
It helps that compared to consumer electronics, lenses hold their value surprisingly well. If you take good care of your lenses, you can recover half of what you spent even after a few years.
Finally, before investing a lot of money in a particular lens, see if you can rent it from a good camera store such as Penn Camera (DC metro area). You might find out it's not worth it to you, or you might find out a rental was all you really needed.
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