Anyone that opts for a Canon EOS DSLR today has 30 years’ worth of native optics to choose from, and many more when you factor in compatible third-party options. Nikon and Pentax users are arguably catered for even better, with their respective systems happily accepting lenses from even older models without adapters (albeit with some limitations).
This is one of the main issues that continues to prevent people from adopting mirrorless cameras: there less chance of the lens you want being available for your shiny new system than there is for a more established DSLR line. Furthermore, as DSLR manufacturers have had a chance to update these lenses over time, and with further assistance from third-parties, there’s potentially less chance of a particular lens being offered for a mirrorless camera at a variety of price points, which makes things more difficult if you're on a tighter budget.
Trying to develop a lens range that can rival these requires time and a lot of investment, and this is something every mirrorless manufacturer has had to deal with. As Panasonic was first to market with its Micro Four Thirds Lumix G system, and with the support of Olympus and third parties, the entire lens range for these cameras now contains pretty much every option you could reasonably expect. The same is not true for every other mirrorless manufacturer.
As size is such a priority for mirrorless systems, manufacturers have been keen to make the lenses for these systems as small as possible too. And some lenses are indeed very small, particularly those with small maximum apertures, and collapsible constructions that allow the inner barrel of the lens to retract into the lens when not in use. Pancake-type lenses have also become popular for mirrorless cameras.
DSLR lenses on mirrorless bodies
Some people are very happy with their DSLR lenses and are able to use them on mirrorless bodies via adapters. The specific combinations that work depend largely on the flange-back distance of the camera (the distance between the lens mount and the sensor), but this has persuaded many people to get the best of both worlds by retaining their favourite lenses while benefiting from the latest in mirrorless technology.
Combined with the fact that the rear of the lens on a mirrorless system will typically sit further towards the sensor – something that can’t be changed on a DSLR as they are typically based around a physical design that was defined in the days of film – you really do get a more efficient design with mirrorless cameras.
As manufacturers have sought to develop the more professional end of their lens spectrums, those using these kinds of lenses may see less of an advantage over DSLR systems. Wide-aperture lenses designed for mirrorless systems, for example, particularly those with telephoto focal lengths, can be comparable in size and weight to equivalent lenses designed for DSLRs.
So what's the takeaway from all of this? If you are keen on adopting a mirrorless system, think about what lenses you are likely to be using with your camera body and do some research into what's out there. For everyday shooting you may find a simple lens will give you an overall package that’s considerably smaller than an equivalent DSLR setup, but for more specialist shooting you may find this difference to be far less significant.