Lenses
Lens Speak
Lenses are defined by several parameters; focal length, aperture, and the mount which defines what camera body or bodies the lens will fit. The focal length can be a single number such as 50mm, which would be the "standard" or "normal" focal length of a lens for a 35mm film camera or a digital camera with a full frame sensor. A lens can have a set of numbers such as 18-55, or 70-200, indicating that the lens is a zoom lens that covers a range of focal lengths.
Apertures can be a single number as well, such as f/2.8 , or a range of numbers such as f/3.5-5.6. The aperture is a mathematical ratio where the f is the focal length and the number is the denominator in the expression. So, an f/2.8 lens has a maximum aperture that is the focal length of the lens divided by 2.8. A 100mm f/2.8 lens would have a physical aperture of 35.7mm. While that may be interesting, it is not information we need to use the lens. Actually, the expression itself is the important information as it defines the ability of the lens to transmit light and that is used in conjunction with shutter speed and ISO to determine exposure.
If the lens has a range of numbers it means the aperture changes depending on the focal length. The physical aperture may be f/3.5 at the widest lens setting but since the aperture cannot get bigger its effective value is f/5.6 when the lens is zoomed to the longer setting. A lens with a single aperture number is generally considered to be better as the effective exposure is more predictable. Lenses with variable apertures are generally less expensive as they are less expensive to produce. Single apertures zoom lenses are usually heavier and more expensive, but they also generally live at the top of the product line. Since the longer the focal length the bigger the aperture needs to be, fast telephoto lenses are limited to a compromise between size and weight versus speed, not to mention cost.
Faster lenses (wider apertures) gather more light which makes them able to auto-focus in lower light. Their added light gathering ability also makes them more useful in certain types of photography such as sports, and documentary photography where high shutter speeds may be needed. Any moving subject will benefit from a faster lens that allows a higher shutter speed without pushing the ISO to exceptional values.
Information on a lens front can vary from simple to complex. The manufacturer normally includes the focal length and aperture information. This is often stated as 35mm 1:4 which means f/4, not f/1.4 as the aperture is stated as a ratio to the focal length. There may be other information including the lens type such as Macro, or the lens mount type such as F (Nikon), E (Sony), or EF (Canon). A ∅ number indicates the diameter of the lens front for attaching filters.
In addition to simple information a lens may contain acronyms to describe mounts or special features. The Nikon AF-S DX NIKKOR 55–300mm f/4.5–5.6G ED VR indicates that the Nikkor lens (Nikon) is an AF-S DX lens which means silent motor (AF-S), crop sensor (DX), 55-300mm (focal length range), f/4.5-5.6 (aperture range), G (lens series), ED (extra-low dispersion glass), VR (vibration reduction). The only thing missing is the name of the designer. Every manufacturer has their own set of acronyms to describe their special features and you will see those described on the websites with the lenses and in the instructions manuals. The biggest consideration is usually the sensor size designation indicating what camera body the lens will fit.
The Eye of the Camera
Lens choices for your camera start with compatibility with the camera body. Lenses for full frame bodies are not the same as lenses for crop sensor bodies. While full frame lenses will usually fit a crop sensor body, the reverse is sometimes not true. The lens designation usually tells you whether or not the lens will work on your camera body. In addition, some older lenses for film cameras will work on current digital bodies but the image quality will be an issue. The resolving power of older lenses was lower by comparison to the requirements of digital sensors and while the lens may fit the image results may very well be compromised.
As the pixel count in current cameras keeps rising the demand for quality lenses increases as well. It is not uncommon to have the lens be the most important factor in image production. The modulation transfer function (MTF) of a lens describes the ability of the lens to maintain variations in brightness levels which translates to image contrast. The image contrast translates to perceived sharpness in the image and is often deteriorated by diffraction at smaller apertures. Without getting too far into technicalities there are reasons why some lenses are better than others for certain photo applications.
A lens used at wide apertures will suffer most from lens design. This may be one of the primary reasons some photographers tend to prefer high quality single focal length lenses as aberrations are minimized. However, some high quality zoom lenses in longer focal lengths perform very well at wide apertures. All lenses regardless of their design quality suffer pretty equally from diffraction at very small apertures. Diffraction is scattering as the light passes the aperture. Beyond f/16 all lenses will suffer image degradation regardless of the sensor size. Depending on the camera, that diffraction degradation may start at a much lower aperture. As sensor size decreases the diffraction limit of the lens does as well. This is why compact cameras often have surprisingly limited aperture choices. The manufacturer knows that the image quality will suffer greatly if they allow you to close down too far. Since effective depth of field is greater with a smaller sensor the inability to close down is not a factor.
All lenses have a "sweet spot" which represents the aperture at which the lens creates the highest quality image resolution. This is generally in the middle of aperture range, which translates to f/8 - f/11 in most lenses. If you are inclined to do so you can test your camera and lens against a resolution chart to convince yourself of the results, but this is rarely necessary for most shooting. A quick check of a new lens to discover flaws can be done by shooting an ironed flat newspaper in even light, using a tripod of course. Look for overall color quality, fringing, contrast and image distortion. Remember that focusing accuracy, camera stability and other factors that degrade image quality will contribute to the results.
One aspect of zoom lenses that most people do not consider is the effect the zoom range has on image quality. Designing a lens that has multiple focal lengths is complex, and the ability to produce high quality lens performance decreases as the range of the zoom increases. Notice the focal length range of zoom lenses in the high end of offerings. The highest quality lenses cover the smallest range. 16-35mm is barely over a 2x range. 24-70mm is just under 3x as is 70-200mm. The greater the range of the zoom the greater the possibility that they lens will not perform well, at least in part of its range. If a lens has a greater than 5x range, especially if it transitions from a wide angle to telephoto, chances are pretty good that it will be a lesser performer. The idea of a single lens to cover all circumstances sound great, but reality is disappointing, especially in sharpness.
Lenses come in grades as well. The price alone is not the best way to judge lens quality as some rather inexpensive lenses are often quite good. This is especially true of single focal length lenses, particularly "normal" lenses in the 50mm range, exceptions being very fast versions. How the lens will be used is important and considerations such as weather proofing, material construction, maximum aperture and other details may or may not help you decide. I don't shoot candids indoors or low light photography so "fast" lenses are not important to me. They would be to a wedding, sports or event photographer. The primary purpose of lenses with wide maximum apertures is to enable auto focusing in low light. Lenses with apertures wider than f/2.8 are considered "fast" lenses, and your wallet pays the price.
The top of the line lenses by any manufacturer will obviously perform best but there is a price to be paid for them. If you need them, or can simply afford them, fine. It may be that another option would save you money without sacrificing quality. One example of that is image stabilized lenses which cost more than their non "IS/VR" counterparts. I shoot on a tripod over 95% of the time, and would certainly do so with a telephoto zoom lens, so the IS version would simply cost me more money and not gain me any image quality. Your decision on which lens to buy should be based on intended use and practicality rather than advertising hype. Lens makers obviously want to sell you the most expensive thing they make, not the lens you really need.
If you need to make serious comparisons of lenses for quality the DXOMark Lens Database is the place to visit. Here you can find serious bench tests of lenses from all major manufacturers and match the lens to the body you will put it on. The results will show you test results as numerical values so you can compare lenses in a realistic fashion. Some of the results will surprise you, and some may shock you. The second highest rated lens for one of my cameras is a $249 single focal length lens which outperforms 21 other lenses and is only beat by one lens which had slightly less chromatic aberration by one point. You can even check your smartphone.
As is true with many things, the critical tests often are not the best representation of the usefulness of a piece of equipment. While one lens may outperform another in the numbers alone, its value to the photographer may be less than another choice for other reasons. It is also true that minimal differences in performance on a bench test may not translate to results in everyday shooting. There are too many other factors that come into play. Primary among them are the skills of the photographer, careful focus, hand holding vs tripod use, and careful processing after the capture. A dirty filter can reduce image quality. Some of the best photos out there are produced with far less than the best lenses.