High quality lenses can make a large difference in the appearance of your photos, but they can also be unnecessarily expensive. When you upgrade your lens, make sure you have a valid reason to do it. Here are some reasons you may want to venture beyond the ‘kit’ lens included with many consumer-oriented camera bodies:
It is quite purposeful that I’ve left the optical quality as the last item on this list. It’s simply not as important as too many people make it out to be. The sharpness of your photos is often times hindered more by your technique than by any modern lens, and the sharpness of a photo is often not really a major factor in its effectiveness as a photo.
First and foremost, make sure your lens fits the physical mount of your camera. Pretty well any DSLR lens can be adapted to fit on a mirrorless camera, even if the brands differ, but the opposite is not true (Google “flange distance” or take the advanced course for more info there). Common mounts include:
Lens choice is affected by sensor size which you should be starting to understand. Make sure your lens covers the sensor of your camera. Often, a cropped sensor lens can be used on a larger sensor camera, but not always. Nikon cameras will use this smaller image circle and simply “crop” the larger sensor (from “FX” to “DX” size in Nikon speak). Canon, however, will not allow you to put a cropped sensor lens (“EF-S” in Canon speak) on a full frame (“EF”) camera body. Though it can be done with a hack saw.
At this point you also need to determine how important auto focus is to you. Many fantastic lenses can be had quite affordably if you’re able to manually focus for your style of photography. Things like product photography, astro photography, studio portraits of adults; many of these don’t require that your camera automatically focus the lens.
If you’re mounting older DSLR lenses on mirrorless cameras, there are many third party adapters available. Some of these will even relay focus and metadata information via electronic contacts on the lens and camera body. To cite a currently popular specific example: mounting Sigma “Art” lenses on Sony mirrorless bodies (e.g. A7III) using a Metabones adapter.
Prime lenses are a single fixed focal length as opposed to zoom lenses where focal length changes. There was a day when prime lenses were the indisputable choice if image quality was to be prioritized. Not so much anymore. There are some excellent zoom lenses that so closely rival the optical quality and speed (maximum aperture) of their prime counterparts that the convenience advantage makes them superior for me.