This article could easily start out as a kind of chicken and egg joke, like "Which came first... the camera or the lens?" I should add in 'legacy' or 'vintage' before the word "lens" to be more in keeping with the title.
Internal mirrors and pentaprisims started disappearing from cameras somewhere around 2008-2009, and makers offered lens mount adapters for their new mirrorless designs with the new mounts they used. Most of these adapters were produced by the OEMs themselves, and provided a quick path for brand loyalists to use their existing lenses on these new digital models, or even a conversion to a new body with a differently scaled sensor. Sony's full frame Alpha and SLT series and the NEX line offered this - or Nikon's D series (both FX & DX) when the Nikon 1's were introduced.
Into this new digital soup, a growing number of foreign manufacturers began offering budget options to make it cheap to expand your lens choices even further back into lens history with most of the popular mounts and several of the eccentric ones. For example... Last month I ordered two different lens mount adapters for my NEX-F3 that cost less than $10 each - shipping included - for 2 differing mounts than the native Sony E type. If you don't already have legacy lenses of your own, there are some great decades-old lenses available for considerably less than the latest and greatest OEM digital versions; with similar optical end results, and very often, with some real visual character added in.
If you're game enough to try it, you might find yourself preferring the old glass to the new... plus you'll have far more variety in what optics you can use, IF you can see any benefit to trying a decidedly different path. The entry price to experiment is low enough to find out if it's a fit for you.
The pros and cons
In my experience, choosing to use legacy lenses hasn't been a one-to-one replacement solution. You may also find that there will probably be issues that pop up with some adaptions, and they can range from very basic mechanical mismatches, up to optical and/or electrical complications. Most of the time you won't have all the advanced functionality available with modern digital lenses when you adapt. Depending on what choices you make - you'll likely have to accept some level of trade-off.
If you're on a modest photo gear budget, adapting legacy glass will usually be much cheaper when your objective is to get high optical quality along with a 'fast aperture' lens design. For example... as of today, if you want a brand new 50mm f/1.4 prime lens to fit your Sony E-Mount camera body - from ANY manufacturer - you'll be spending at least $300-400 (USD) and you'll still end up with a manual-focus lens. If you want one capable of auto focusing, that will cost you between $700-1500 or more! A great legacy 50 1.4 and a matching mount adapter could easily be bought for $100 or less.
These trade-offs usually involve functionality and how it affects the way you work with a given lens and body pairing, and not with any compromise to the optical qualities of the lens you choose. Keep in mind that a crappy lens doesn't get better optically when it's used on a digital body, so you'll still have crappy digital photos when you take them. I'm carefully avoiding the term "compromise" when referring to lens adaptions too. To me, a compromise means I've totally lost a needed function and everything it provides, but so far that really hasn't been my overall experience. There might be some aspect of my workflow that needs some attention, but that's about it.
Not all of the differences you run into will be overwhelmingly negative or complicated to deal with. Sometimes it's a matter of modifying what you do in order to accomplish the same thing in a different way. That's the case when using a 'dumb' adapter with lenses that only provide for manually focusing on your subject. When your legacy lens has no focusing motor or mechanism for your camera to link into, there is nothing short of rebuilding it into a custom lens to add in auto focusing. There can be a side benefit to this particular circumstance that may surprise you too. It can be just as fast, and with practice, just as accurate to manually focus a legacy lens - than it would be to use a digital lens with its built-in automatic focusing function.
With minimal practice in this case, you might find that there are fewer steps to complete the whole process if your intent is to use hypercritical focusing. It takes time and extra effort to stair-step that damned little focusing square around your display panel or viewfinder to match up to your intended focus point - all while you alternately half-click away on the shutter button and continue to punch your live-view's zoom buttons. While you're doing all of that just to reevaluate what your actual focus should be, you'll still need to frame your shot along with everything else you'd normally do. That doesn't mean it's better to focus this way... it's just different and requires some effort.
Be aware too, that the trickier adaptions might not always require a difficult or expensive solution. If you have some basic mechanical skills, you could always consider some custom adaptions that you engineer for yourself. It could be as simple as removing a few screws and then removing a pin before reassembly, or it could mean swapping the entire mounting plate for an optimal infinity-focusing solution. How about using a painted cardboard tube with putty and paste? There are plenty of lens and camera experimenters online that can offer tons of ideas and inspirations if you're adventurous enough go that route.
So... You want to adopt a new member into your photo family?
Next : The practical part of the equation once you decide you want to dive in. That's another article.
By the way... The answer to my starting question is : The lens came first. Copernicus and his telescope, or Columbus with his 'spy glass' were using lenses long before anyone thought about capturing what you were seeing at the outward end of the view.