Among the important lens characteristics you should understand are:
- Focusing and sharpness
- Largest aperture and for zoom lenses when applicable, the range of the largest aperture at the full range of the lens’ focal lengths
- Focal Length
- Special Features such as Vibration Reduction, Focus Limit Switches, Extra-low Dispersion (ED) elements, special lens coatings, and no lens extension
- Potential problems such as focus softness in corners, vignetting (light falloff), chromatic aberration, and distortion
Most of us realize some lenses have the ability to produce sharper images than others due to their design and the quality of their optics. That’s one of the reasons similar focal length lenses, with similar features, may have vastly different purchase prices. Not many photographers other than advanced amateurs and professionals realize that even the best lenses will generally have a range of sharpness over the lens’ range of apertures. It’s important for every photographer to know their lenses’ focus sweet spot; which aperture range produces the sharpest image.
Lenses are, in part, specified by their maximum aperture, and zoom lenses may be specified, in part, by a maximum aperture range. For example, Nikon’s 18-200mm DX lens is specified as f/3.5-5.6. That means, when set at 18mm the maximum aperture is f/3.5, and at 200mm it’s f/5.6. As the lens is zoomed from wide angle to telephoto, the lens gets “slower,” meaning its maximum aperture is smaller. In low light conditions, and/or at high shutter speeds, and/or low ISO sensitivity this can be a very important specification for your lenses.
When taking close-up shots, lens magnification is important. Macro lenses, used for close-up photography, have a magnification specification. For example, Nikon’s 105mm Micro-Nikkor lens has a “maximum reproduction ratio” (when you focus very close to the subject) of 1.0, sometimes expressed as 1:1. A “maximum reproduction ratio” of 1.0 means that when a photo of a half inch diameter coin is made, the coin will take up a half inch of camera sensor space; a life size image. That ratio indicates it’s a true macro lens. If you’re taking a photo of something very small, you want that 1:1 ratio capability, so you can capture the object’s details in sharp focus.
The focal length of a lens goes hand in hand with its angle of view. As the focal length doubles, the angle of view is halved. The angle of view specification, and therefore focal lengths for lenses attached to DSLR cameras are based on “full size” sensors, which are the same size as a 35mm film frame. Many DSLR cameras don’t have “full size” sensors. Nikon’s DX sized sensor is smaller, for example.
Nikon DX sensor based cameras have a “crop factor” of 1.5. As a result, the effective angle of view of a lens on a Nikon DX camera is reduced by a factor of 1.5, so an 18mm lens would have an equivalent angle of view as a 27mm on a “full size” sensor equipped DSLR.
A 35mm lens, considered a wide angle lens for a “full size” sensor equipped DSLR, would is a standard lens (52.5mm equivalent) on a DX size sensor camera.
Telephoto lenses, (long focal lengths, and therefore narrow angles of view) magnify distant subjects to pull in its details. As a result, any movement of the lens, while the photograph is being exposed, is magnified and will blur the photograph. To offset that problem, and permit telephoto lenses to be handheld under well lighted conditions, lens manufacturers developed a mechanism to compensate for small camera/lens movement. It’s called vibration reduction or image stabilization. I wouldn’t purchase a telephoto lens without this feature.
Some lenses don’t focus quickly. As a result, manufacturers have installed focus limit switches. These limit auto-focusing to only a portion of range of the lens to speed auto-focusing. You set the lens to the range your subject is at. This feature can be a big help for focusing on subjects quickly.
Sometimes using close-up or macro lenses you are just “inches” away from your subject. Having a lens which doesn’t extend forward as you focus it helps ensure you’ll not touch your subject during focusing.
Extra-low Dispersion (ED) elements, and special lens coatings minimize chromatic aberration (color fringing) and lens flaring.
There are numerous potential problems you need to consider when choosing lenses for your purchase and photo making.
Some lenses have a very sharp focus in the center, sometimes throughout the range of aperture settings, but in the corners of the photographs, may have a softer focus.
The combination of your particular camera and each lens you have may produce vignetting in your photographs. This is manifest at the edges of your photos, especially at the corners, as the intensity of the light reaching your sensor is diminished at the edges of the sensor, producing under-exposed corners.
Chromatic aberration is generally more prevalent in large-aperture telephoto and super-telephoto lenses, and does not generally improve by stopping down the lens. It can be corrected or at least minimized by low or extra low dispersion glass elements. “Pro” telephoto, and super-telephoto lenses always use this type of glass.
According to the optics of a lens, at some focal lengths, photographic images may be distorted. For example, barrel distortion is when the image is bowed outward and pin cushion distortion is when the image is bowed inward. A third type of distortion is keystoning. Image distortion is caused by a combination of the relationship of the axis of the lens relative to the subject, and the optics of the lens. An image can have a complex distortion which is a combination of distortion types.
When choosing lenses to purchase or use, you should consider the above specifications and factors. The choices can be complex and difficult.