see also:

• super wide angle lenses
• fast 50mm focal length lenses
• standard camera zoom lenses
• portrait lenses
• lenses for macrophotography
• super telephoto lenses
• 8-25x super zoom lenses
• perspective control, shift and/or tilt lenses
• travel photography
• legacy lenses for mirrorless cameras
  • lenses with radioactive glass elements
  • lens mount cross-compatibility table
  • lens adapters

Diagram courtesy of The problem with modern optics - over correction with optical elements reduce the realism and aesthetics of people photography but work well for sports and landscapes - choose your tool wisely - check his blog out to understand this diagram better such as https://yannickkhong.com/blog/2018/12/30/work-versatile-and-x-factor-lenses-new-ways-to-view-the-lens-landscape:

• A day lens would display a pleasing amount of 3D pop in day or ideal lighting, it would break down in challenging lighting under a large amount of fringing and chromatic aberration
• A night lens would be designed to correct the fringing and chromatic aberration, yet wouldn't be able to transmit as much micro-contrast and tonality as a day light in ideal condition.

• many photographers will tell you that money spent on a good lens will be a better investment than money spent on the camera
• this is historically generally true
• money spent on “kit” zoom lenses is generally not a good investment as these lenses generally have average build quality, average optics and poor apertures which severely limits their utility in low light and for creating blurred backgrounds
• users are generally advised to buy a wider aperture lens of better optical quality
• digital cameras in particular now are almost “disposable” with even the high end expensive cameras having a real life of perhaps only 5-7 years, and will depreciate rapidly in value as they will be surpassed by cameras with more features, better sensors and better AF, IS, HD video and connectivity technologies
• high quality, mechanical, manual focus lenses will generally last for decades if they are not dropped, scratched or allowed to develop fungus on the optics
• modern AF lenses will fail over time as they have rapidly moving parts and electronic components - both have failure rates and when used frequently are likely to fail over their 1st 10 years
  • this failure rate is dependent upon build quality and lens complexity
  • the failure rate tends to be highest with AF zoom lenses with optical image stabiliser, in particular, the pro 70-200mm IS lens appear to be particularly likely to fail¹⁾
• additionally the lens features may make it redundant over time - particularly as an investment:
  • the AF motors in the lenses are continually improved, particularly for the new contrast detect AF cameras which rely on newer technology AF motors
  • the image stabiliser technology continues to improve making older lenses less effective
  • optics tend to be developed over time to provide improved sharpness to better match the higher pixel density in modern sensors - old lenses often do not allow these sensors to shine
• those purchasing a system may well consider the benefits of buying a system where the image stabiliser is built into the camera such as Sony NEX E-mount camera system or Micro Four Thirds system, as this not only means EVERY lens gains IS capability without requiring optical IS and its increased failure and redundancy rates, and you potentially gain the benefit of improved IS every time you upgrade the camera to a newer camera which you will have to do anyway with any system.
• those who cannot tolerate extended outages from delays in repair, need to consider access to service and turnaround times - this is a major reason why many pros stick with Canon or Nikon as they are more accessible worldwide
• finally, demand for particular lenses changes over time
  • the current trend is for smaller, sharper, wide aperture, more silent lenses with fast AF

• this is not an easy question to answer as it entirely depends upon YOU like to photograph
• the following are focal lengths in terms of a 35mm full frame camera system
• most will be able to photograph most subjects they need with just 3 lenses:
  • compact, light 3x zoom lens covering 24-70mm region
  • compact wide aperture 35-40mm prime lens
  • wide aperture short telephoto lens
  • other lenses as per their individual needs
• use Lens-GAS free software to rapidly chart the focal lengths of the photos in a folder to see which ones are most commonly used

• this is the lens you take when you have plenty of light (eg. outdoors, daytime), and you really have no idea what you might suddenly want to photograph, so a range of focal lengths becomes very handy
• you will have to decide upon the compromises of zoom range, aperture, image quality, cost, weight and size
• for many, a small, light 3x zoom lens covering 24mm-70mm or 28-85mm will fit the bill
• some who are prepared to carry a heavier, longer lens, may opt for a 10x zoom covering 28-280mm
• enthusiasts may opt for a f/2.8 aperture 3x zoom lens for improved low light capability - one of the smallest, and lightest is the Panasonic 12-35mm f/2.8 X HD weatherproof lens which covers 24-70mm and the Panasonic 35-100mm f/2.8 OIS X which covers 70-200mm, but these are not cheap.

• this is generally a lens in the range of 35-40mm focal length with an aperture f/2.0 or wider
• it is great for candid or street photography and is awesome for taking to social events because it is small, will focus in low light, potentially allow available light shots indoors if image stabiliser is available, and can be combined with a bounce flash for really nice group portraits
• examples include Panasonic Lumix 20mm f/1.7 pancake lens, Olympus mZD 17mm f/1.8 lens

• the classical portrait lens has a focal length in the range 85-120mm and has a wide aperture to give a shallow depth of field (DOF) and allow the subject to stand out from the blurred background
• such a lens will be far more useful for most enthusiasts than a slow 3-4x 80-300mm kit lens with widest apertures around f/5.6, lower optical quality, slower AF and poorer bokeh.
• it should have:
  • a nice quality to the background blurring (bokeh)
  • sharpness across the frame at widest aperture - mostly we do not want our subjects in the centre of the image
  • relatively compact so it is not intrusive upon the subject
  • relatively compact and light so we will bring it with us
  • fast AF, preferably with precise AF selectable on one or other eye of the subject (this is generally a camera functionality - such as Olympus Micro Four Thirds system)
• my personal preference for focal length is at the long end as I find it suits my style better for compressing ugly, busy backgrounds into more aesthetic, lovely blurred ones, hence I used Canon EF 135mm f/2.0L lens, the manual focus Samyang / Rokinon 85mm f/1.4 lenses, and now the much lighter and smaller, Olympus m.ZD 75mm f/1.8 lens which has 150mm eq. focal length
• full frame users often use 85mm f/1.4 lenses, or 70-200mm f/2.8 zoom lenses although these zooms are big, heavy, expensive, intrusive and prone to failure
• Canon pro users sometimes use the 85mm f/1.2 lens but this has slow AF and is not an easy lens to use well
• Canon and Nikon cropped sensor dSLR users often resort to a relatively cheap 50mm f/1.8 lens
• other options include Olympus m.ZD 45mm f/1.8 portrait lens

• this requires the lens to be small so it can be hidden from would-be thugs, and be able to image night street scenes without a tripod
• a great option for this is a wide angle lens with wide aperture combined with image stabiliser
• examples include Olympus m.ZD 12mm f/2.0 lens

• many lenses allow close up shots, but to get really close, you need a dedicated lenses for macrophotography
• longer focal lengths in the 120-200mm range generally allow for a longer working distance which means you don't have to get too close to the subject that it will fly away
• if you are not shooting on a tripod, then a light, compact lens which will not fatigue your muscles and cause you to shake, can really help - a good example is the Olympus m.ZD 60mm f/2.8 macro lens which can also be used with ring or twin flashes and has an equivalent focal length of 120mm, fast AF, and handy focus limit switches while being weatherproof and very sharp for macro work at 1:1.

• see also: https://blog.mingthein.com/2018/07/30/repost-what-makes-a-good-lens-part-i/
• resolution
  • there is no point having the latest 50mp sensor if your lens can only resolve to 20mp
  • small apertures will decrease resolution due to the physical properties of light called diffraction - this is unavoidable - all lenses when stopped down to a small aperture will lose resolution no matter how good the lens
• micro contrast
  • ability to resolve the very finest detail structures
• macro contrast and lens flare
  • poor optical coatings on the lens elements may allow contrast eating flare to ruin your images when shooting into bright light sources - hence the use of lens hoods and modern nano coatings
• aberration correction
  • spherical aberration and bokeh
  • coma
  • astigmatism
  • distortion - barrel, pincushion or complex
  • field curvature
    • a macro lens should have a flat plane of focus to allow good focus across the field when reproducing flat material
    • many lenses have a curved focus plane such that when you focus on a subject in the centre, it is no longer in focus if you re-compose and move the subject to the edges even though the lens to subject distance has not changed
    • this can be a problem for astrophotography when photographing star fields
• transmission of light
• colour
• vignetting
  • the darkening effects of vignetting towards the edges can be corrected but at a cost of increasing noise as a result of digitally increasing the exposure
  • may also change the appearance of out of focus blurs towards the edges wide open causing an annoying cat's eye appearance of these
• focus shift
  • plane of focus may change with aperture in some lenses
• focus breathing
  • field of view may change with focus distance
• bokeh
  • this is the aesthetic quality of the out of focus areas both in front of, and behind the subject
• optimum performance:
  • any given lens tends to have a best performance for each of the above issues, and it is nice to know what this is:
    • may be for a focal length if it is a zoom lens
    • focus distance
    • aperture

• diaphragm blades
  • this is only important if not shooting wide open
  • curved or rounded blades allow nicer out of focus blurs without the sharp straight edges of non-rounded blades when the aperture is stopped down
  • odd number of blades allows for better looking “sunstars”
  • clickless aperture is better for video work
• weathersealing
• closest focus and magnification
• focus range limiters
• optical image stabilisation
  • particularly important for long telephoto lenses even if using in camera image stabiliser
  • how effective is it - current ones run at 5EV or even 6.5EV in dual IS mode
• autofocus motor system
  • most dSLR lenses uses the older USM AF motors designed to work only with PDAF technology
  • lenses optimised for mirrorless cameras have linear stepping motors which may oscillate at 240Hz to optimize both PDAF and CDAF technologies
  • how silent is it?
  • how accurate is it?
  • how much “hunting” does it do
• does the front filter rotate on focusing?
  • this is really annoying if one is using a polarising filter - fortunately modern lenses do not do this
• does the lens length change on changing focus distance?
  • most internal focusing lenses have a constant lens length during focus
  • some lenses extend on focusing and / or on zooming which can make them more intimidating and distracting as well as at risk of dust and water