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CHOOSING LENSES
Jim Doty, Jr.

Lenses are a photographer's most important tool. They are the photographic "eyes" that project the image onto the film or digital imaging sensor. It has been an axiom for years that smart photographers spend their best money on lenses. Given a choice between a top of the line camera and so-so lens, or a middle grade camera and better lenses, it is almost always better to put the money into better lenses.

A commonly asked question in online forums is "What SLR camera body should I get?" The most helpful answers usually begin with "What kind of lenses or lens system do you want to use?"

Several factors go into the decisions involved in choosing lenses. They include image quality, focal length, maximum aperture, specialty features, cost, size, and weight

IMAGE QUALITY

A quick survey of online forums and you will discover that this is a major concern for many photographers.  I get more emails about lens choices and lens quality than almost any other topic.  The good news is that overall lens quality has improved significantly over the last 20-30 years.  There aren't many dogs being produced. Most lenses made today have good quality optics and many have excellent optics. 

Several years of teaching photography have taught me that most photographers are limited more by their technique than their equipment.  There are exceptions where good technique yields poor results due to a poor quality lens - usually an older lens from an off brand manufacturer.

Before you spend money on an excellent to premium quality lens, answer the following questions. Do I want optimum quality? Do I want prints that are 11x14 inches and larger? Do I use a tripod, cable release (when needed), and mirror lockup (with long lenses or when doing high magnification work)? Do I want to be published? If you answered yes to several of these questions, then you want to get better to top quality lenses.

Their is good good news. A better lens doesn't always cost an arm and a leg. Assume you are looking for a lens in the 70-300mm range in three price ranges from the same manufacturer. On a scale of 1 to 10, a $150 buys a 6.0 quality lens. Spending $350 buys an 8.5 quality lens. Jumping to $1200 buys a 9.0 quality lens with a one stop faster maximum aperture. Going from $150 to $350 dollars buys a big jump in quality. Jumping to $1200 doesn't buy that much more quality and may not be worth the money unless you really need a faster maximum aperture.

QUALITY FACTORS

Several factors determine the quality of a lens, including resolution, contrast, accutance, and aberrations. Designing a lens is partly science and partly the art of balancing these factors.  It isn't that hard to produce a lens with a lot of resolution (the ability to resolve fine details), but it is often at the cost of low contrast (the difference between light and dark tones).  On the other hand, a high contrast lens may be lacking in resolution. A good lens will balance resolution and contrast. Accutance (the ability to resolve small differences in tonality) also weighs into the picture. All lenses distort the light passing through them to a greater or lesser extent.  Chromatic aberration is the inability of a lens to focus different wave lengths of light (different colors) at the same plane. Coma means an off axis point of light is distorted into a comet shape. Minimizing aberrations while balancing resolution, contrast, and accutance is the goal of a lens designer. New forms of optical glass have made this easier in recent years so high quality optics often cost less to produce than they once did.

DETERMINING IMAGE QUALITY

Determining image quality for ourselves is beyond the reach of most photographers. The best solution is to go to reliable sources. My article on trusted sources of photo equipment information (including lenses) is here.  You may want to test a lens once you have purchased it. More information on do it yourself testing is here.

FOCAL LENGTH

Lenses come in different focal lengths. "Prime" lenses have a single focal length, like 50mm. Zoom lenses cover a range of focal lengths, like 28-135mm.  20 years ago, prime lenses were preferred by the most fussy photographers since they were higher in quality, lighter in weight, and had faster maximum apertures than zooms. Now, zooms are much more popular.  The best zoom lenses are close in quality to the very good prime lenses. They are more convenient than carrying a bunch of prime lenses, and buying a zoom lens is less expensive than buying several prime lenses that cover the same focal lengths. Prime lenses usually have faster maximum apertures. Prime lens advocates would say you don't need to duplicate every focal length, just move your feet. The debate goes on.

I have both prime and zoom lenses, but when I am traveling light, I mostly use good to high quality zooms.

Short focal length, "wide angle" lenses have a wider field of view than long focal length "telephoto" lenses. Most photographers, whether using zooms or primes, like to have something in the wide angle, normal, and telephoto focal lengths.

There are do-it-all lenses that stretch from wide angle to moderate telephoto. They are small and convenient but lacking in image quality at the longer focal lengths. Quality wise, it is better to have 2 or 3 lenses to cover your desired focal lengths. If one of your lenses dies (and electronic lenses do die), you will be glad you still have one or two more lenses that still work until you get your dead one repaired. If minimal weight and maximum convenience are the primary concern, a 28-300mm zoom lens may be just what you want. Don't expect to make high quality 12x18 inch prints from photos made at the 300mm end.

Go here for more information about focal lengths, along with illustrations for digital SLRs as well as 35mm film cameras.

MAXIMUM APERTURES

The exposure article explains aperture settings. Every lens has a maximum aperture. Zoom lenses, especially the less expensive ones, often have a variable maximum aperture that gets smaller as the lens gets longer.

Faster maximum apertures mean it is easier to see, compose, and focus in low light. Faster maximum apertures mean you camera may be able to autofocus faster and more accurately. Faster maximum apertures mean faster shutter speeds for stopping action. Faster maximum apertures mean less depth of field when you want to blur the background behind your subject. Faster maximum apertures mean a higher priced lenses. Faster maximum apertures mean bigger and heavier lenses.

Apertures of f/1.4 to f/2 are very fast. Apertures of f/2.8 to f/4 are fast to moderately slow. Apertures of f/5.6 to f/8 are slow to very slow. Apertures of f/11 and slower are way to slow to be practical in all but the brightest of lighting conditions, the sole exception being some guided astrophotography through telescopes.

With better quality films at higher ISOs, and digital SLR bodies with much less noise at higher ISOs, the shutter speed factor is not as important as it once was. I use apertures of f/4 and f/5.6 without flash in average indoor conditions. Why? Because of high digital ISOs plus size and weight considerations. Still, faster maximum apertures are a plus if cost, size and weight are not significant concerns.

SPECIALTY FEATURES

Some lenses provide specialty features that are very valuable to photographers.

Image Stabilization (Vibration Reduction, Anti-Shake) technology is a huge boon and I don't know how I would live without it for a lot of my indoor candid situation photography.  It's a real plus for street shooting and some kinds of documentary photography where a tripod is not practical. It is great for doing nature photography from boats and other moving platforms. My IS lenses paid for themselves on a cruise in Alaska.

Tilt-shift lenses are a big plus to nature, architectural, and other photographers. They can be used to maximize depth of field by tilting the lens to change the plain of focus. They can be shifted to keep buildings, trees, and other objects from leaning or looking like they are falling over. Tilt and shift can even be used together. They can even be used to exaggerate rather than minimize perspective changes.

Closeup lenses are a boon to many kinds of photography when optimum quality is needed when photographing little things. The flat field performance is a big plus when shooting flat objects like stamps or money.

Soft focus lenses are used by photographers for portraits, flowers, and other kinds of photography.

COST, SIZE, AND WEIGHT

Like it or not, the cost of lenses is a big factor. Carrying a big heavy piece of glass is another. There are times I need and want my big, heavy, and exquisite 78-200 f/2.8 lens. When I need it, I take it.  Other times, a smaller, lighter 70-300 f/4.5-5.6 lens will do just fine.

 LINKS

Focal Length in Lenses with illustrations for 35mm film and digital cameras with a 1.6x field of view crop

Do It Yourself Lens Testing

Trusted Sources for photo equipment information

July 16, 2005

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