A common question asked by both inexperienced and experienced shooters is 'which lens is best', usually with a specific focal length in mind? Understanding the factors or qualities that makes one lens different to another allows you to better define the question in the first place. There generally isn't such a thing as the 'best' lens in any particular focal length simply because it's very uncommon, and even impossible, for any single lens to have a superior performance in every possible respect.

So the question should really be altered to reflect ones particular needs or desires. A better question might be 'which lens has the smoothest Bokeh', or 'which lens is sharpest wide open', or similarly specific questions depending on the intended application.
 

Lens Qualities	Description
Fixed focal length (prime) or variable focal length (zoom)	Fixed focal length lenses are often called primes. Primes lenses are generally easier to design and manufacture than zoom lenses which need more elements in more complex optical and mechanical designs. This allows primes to be made into much faster lenses such as F1.4 or faster, or into special purpose lenses such as tilt or shift lenses. Zoom lenses might only have a maximum aperture of F2.8, at best, so they are not the best choice for low light photography where maximum lens speed is required. Zoom lenses excel where maximum flexibility and speed is required as their focal length can quickly be changed to frame or compose a shot appropriately. Composing a shot with a prime lens would potentially require you to move, to get closer or further from the subject or to actually change the lens, by which time the opportunity to shoot may have passed. Most modern zooms are virtually as good as their equivalent prime lens.
Speed, largest aperture	One of the main characteristics of any lens is it's speed or maximum aperture. A lens might be available in many versions where one version might have a maximum aperture of F1.4 whilst another might be F2.0, 2.8 or slower. The lower the aperture number, such as F1.4, the larger the physical aperture or opening which therefore allows a greater amount of light to pass through the lens. This has several effects. Firstly it allows the lens to be used in darker situations than a slower lens, such as F2-F2.8, but it also allows the lens to have a shallower depth of field (less in focus) which allows the subject to better stand out from it's background. This later quality is often the main reason a fast lens is chosen over of a slower lens. Slower lenses are also simpler and cheaper to make so they are often also an economical option offered by lens makers however they are not necessarily poorly performing second rate lenses as some might see them. Their simpler design often allows them to perform better than their faster and more expensive relatives.
Manual or Auto Focus	Not all lenses have Auto Focus (AF) nor is AF always needed or even desirable. Manual focus (only) lenses are actually quite common, especially older lenses which are often adapted to modern digital bodies. Manual focus allows greater focusing precision than AF however manual focus is more difficult and slower so it is not well suited to sport, photojournalism or many fast paced subjects such as children or pets.
Image stabilization	Image Stabilization (IS or VR) is used to eliminate camera shake at slow shutter speeds so it gives a sharper image in situations that might ordinarily have resulted in blurry images due to camera shake. IS does not work well with fast moving subjects, it only reduces the effects of camera shake. For example, if you shot 2 different subjects but in the same light and with the same shutter speed and aperture then you would potentially get 2 different results. If the first subject was stationary, such as people sitting at a table at a wedding reception, then they would potentially be rendered quite sharply but if shot people dancing on the dance floor, in the same light, then the dancers would probably be blurry due to their motion. So IS does have it's uses but it also has limitations.
Sharpness, Field Curvature/Flatness	Sharpness is one of the most noticeable and important lens qualities but it is not always the simplest to judge. A lens might be very sharp in the central pert of the image but much less sharp near the corners and this may or may not matter depending on the way the lens is to be used. Some lenses also have field curvature where the sharpness is not in a flat plane but in a curve. A lens with pronounced field curvature would be unsuitable for photographing flat subjects, such as art work, where the field curvature would cause the edges/corners to be out of focus compared to the centre of the image. Some lenses, such as enlarger, projection and reproduction lenses are designed specifically with field flatness in mind.
Does the lens perform best at close range or at infinity	Most lenses are designed to perform optimally when focused at infinity so their performance can diminish noticeably when the lens is focused at medium distances or closer. One of the reasons that specific 'Macro' lenses exist is because these lenses are designed to perform optimally at very close focus distances, although most macro lenses work quite well for medium-infinity focus applications too. Some macro lenses even have adjustable or floating elements that move, either automatically or are adjusted manually, to compensate for lens aberrations at the particular distance being used. The distance at which a lens is typically used should be considered when choosing a lens. Normal use, which may constitute subjects from several meters away all the way to infinity, would suit normal, that is non macro, lenses. Macro lenses may have an edge in terms of sharpness when shooting portraits at close distances however many portrait photographers don't like biting sharpness in their portraits as this can highlight blemishes way too much.
Minimum focusing distance	This is an important consideration if you intend shooting subjects at close range, such as portraits, product shots or macro/close up images. Specialized Macro lenses not only have a very close minimum focus distance (often abbreviated to MFD) but often have a focusing helicoid (the thread inside the focus ring) which is much finer, ie with a longer throw, in the close focus range and often not as fine at the infinity side of the helicoid. This does make macro lenses harder to focus accurately at longer distances but then they are not really designed for that application.
Cost	Lenses can vary in cost from a few dollars to a few thousand dollars. At times, ie under specific conditions, the performance of the cheapest lens may be almost indistinguishable from that of the most expensive lens, however, you generally do get what you pay for so there is almost always some kind of justification for the price paid. Price is not always a reflection of the mechanical or optical quality of the lens however an expensive lens will often, but not always, be made from better quality materials and to a higher mechanical/optical standard. The price of lenses on the second hand market is a reflection of their desirability, not necessarily quality, so any lens which is in great demand will command a high price whilst a lens which has lost favor with the market will potentially be very cheap. The cost of new lenses can vary significantly but again, you do generally get what you pay for. Some major manufacturers make very similar lenses but in different price ranges. The cheaper lenses, some times termed kit lenses, are often made from plastic instead of metal and are often slower (smaller) maximum aperture compared to the more expensive offerings which may have image stabilization, better/stronger construction, better optical performance and faster (larger) maximum apertures.
Size/weight	This can be a very important consideration and can be overlooked by many. It's importance will be a very personal decision and may be altogether irrelevant to some people whilst a primary consideration to others.
Bokeh	Bokeh is essentially the out of focus area of an image which can be rendered in different ways by seemingly similar lenses. Any lenses Bokeh may be determined by factors such as lens design (lens aberrations), the shape and/or number of aperture blades, the aperture used, the distance to the subject and the position of the out of focus areas relative to the focus plane ie in front of or behind the focus plane. All of these are factors in the resulting Bokehs quality so a lens might have 'good' Bokeh in certain circumstances but relatively 'bad' Bokeh in others. There isn't really any such thing as 'good' or 'bad' Bokeh however most people would consider 'smooth' or 'creamy' Bokeh to be pleasant as it tends to be least distracting so helps a subject to stand out. Conversely 'bad', 'poor' or 'harsh' Bokeh is generally quite noticeable in an image so is often distracting and undesirable, although it can also add to the interest in an image if used appropriately. Bokeh can be a primary consideration in lens choice, over sharpness and other qualities, as a lenses Bokeh will potentially determine the overall 'look' of an image shot with that lens. A lens with 'poor' Bokeh might be described as harsh, jagged, double edge or doughnut shaped. In all these cases the Bokeh would potentially be distracting from the subject being photographed. A common misconception is that 'good' Bokeh is simply the same as having very shallow depth of field, and that the more blur the better, but in fact you might have a shallow depth of field but a very harsh Bokeh which many would consider undesirable.
Lens aberrations	Distortion: There are many forms of distortion but in all cases distortion changes the shape of the subject being photographed. Any distortion is undesirable if you are photographing geometric subjects such as architecture, art work, products etc because the distortion can change their shape to a degree where it is noticeable and undesirable. On the other hand, many subjects are not adversely or noticeably affected by distortion unless the distortion is quite extreme so the subject matter should dictate the acceptable amount and type of distortion from any lens. Chromatic aberration (CA): Visible light consists of many colours and lenses do not behave equally to all colours or Wavelengths of light, such as Ultraviolet or Infra Red. Chromatic aberration is where light is focused in slightly different places depending on it's colour. Apochromatic (APO) lenses are those which focus all (visible) colours in the same place, resulting in reduced or non existent CA.   Spherical aberration: Spherical aberration is where the shape of the lens itself cause the light passing near the edge of the lens to focus in a different place to light passing closer to the centre of the lens. Spherical aberration can be corrected to some degree by using Aspherical lens elements. Vignetting: Vignetting is the effect where the corners of the image are some times darker than the centre of the image. It is usually most noticeable with fast lenses or at large apertures and is reduced or eliminated when the lens is stopped down. Vignetting in lenses is unavoidable however some lenses do have less vignetting than others so it is still an important consideration for some types of photography.
Flare, Contrast and lens coatings	Flare is caused by bright light sources reflecting off lens surfaces and bouncing around inside the lens, usually resulting in a reduction in contrast. Sometimes reflections of the light sources themselves can appear in the image and this is the more common and noticeable form of flare. Lens flare is a much more common problem than most people appreciate as it's reduction in contrast can often be subtle compared to the common idea of flare such as when the sun is in the shot and it's reflections are very obvious. If you shoot at night or in difficult or contrasty lighting conditions then it is important to use a lens which has very good flare control. Modern lens coatings and designs have resulted in lenses with excellent flare control compared to similar lenses from previous generations but with simpler and often ineffective coatings.
Focus shift	Focus shift is an effect of Spherical Aberration. It typically affects very fast lenses and results in the focus plane changing or shifting when a lens is focused at it's maximum aperture and then stopped down. The effect of stopping down moves the focus plane slightly towards infinity and can result in an out of focus image. Focus shift is quite common but it is not well understood and certainly not always recognized as even existing. This some times leads to confusion and misunderstandings about a lenses performance which may not be as sharp as expected whilst in fact the plane of sharp focus is simply not where one thought it should be. It's quite common to see the focus plane in a portrait slightly behind the eyes, where presumably the photographer intended the eyes to be in focus, and this is often how focus shift manifests itself in the real world. Focus shift is not the same as 'front/back focusing' which is a common problem with AF lenses which may not be properly calibrated to a camera so the AF always focuses slightly in front or behind the subject. A lens can conceivably have Focus Shift and front/back focusing issues as they are different and unrelated problems.
Movements such Tilt and/or Shift	Some lenses are designed specifically to allow the lens to tilt and/or shift. Tilt and shift are different in their effects and application so it is not necessary to have both hence some lenses can only tilt and some can only shift and some lenses can do both. These movements (tilt or shift) allow the image to be controlled in specific ways which would otherwise only be possible with a View or Technical camera. Tilt/Shift (T/S) lenses are usually very high quality lenses because the act of tilting or shifting a lens can result in reduced image quality unless the lens is of very hi performance to begin with. Tilt is most commonly used to control the focal plane such as to have the foreground and background in focus at the same time without stopping down the lens. Shift is usually used to reduce or eliminate converging verticals such as when looking up at a building.