The promise is irresistible. A drawer of old lenses, glass with character that no modern formula quite copies, and a new mirrorless body that can, in theory, wear all of it. Adapting lenses across systems is one of the genuine joys of digital photography, but the gap between in theory and in practice is where money gets wasted and focus gets missed. This is the hands-on version: what actually works, what quietly does not, and how to tell the difference before you buy.
The flange distance rule, in practice
Every system fixes the distance from its lens mount to the sensor, and that single number decides almost everything about adapting. A lens mount is a precise mechanical interface, and Camera-wiki keeps a useful overview of how lens mounts and their registers differ across systems. The rule that follows is blunt and worth memorizing: a plain adapter can only add distance, never subtract it. So you can put a deep-flange lens, an old SLR optic, onto a short-flange mirrorless body, because the adapter just fills the gap. Try the reverse and the adapter would need negative thickness, which is a polite way of saying it cannot exist. This is why mirrorless bodies are adapting champions and why some pairings you wish for are simply impossible.
Active versus passive adapters
A passive adapter is metal and nothing else. It holds the lens at the right distance and lets you focus and set aperture by hand. For old manual lenses, that is all you need and all you want, because there is no electronic translation to go wrong. An active adapter adds electrical contacts and circuitry to carry autofocus, aperture control, and lens data between two systems that were never designed to talk. When active adapters work, they feel like magic. When they do not, you get hunting autofocus, wrong aperture behavior, or missing metadata. Treat active adapter performance as something to verify on your exact body and lens, not something to assume from a compatibility chart.
When the adapter is also optics
Some adapters do more than space the lens. A focal reducer, often sold under the speed booster name, adds a glass element that squeezes a full-frame lens’s image circle onto a smaller sensor, buying back field of view and roughly a stop of light. It is clever and genuinely useful on crop bodies, but it is also more glass in the path, so quality varies and the good ones are not cheap. B&H has a clear introduction to the range of lens adapters and what each type does, from plain spacers to focal reducers to specialty options, which is a good map of the territory before you spend. The rule of thumb stays the same: every extra element is a chance to lose the sharpness you adapted the lens to get, so add glass only when the tradeoff clearly pays.
What you give up
Most adapted setups cost you some automation. Autofocus may slow or vanish, in-body stabilization may not know your lens’s focal length, and EXIF data may stop recording. None of that is fatal if you plan for it. Manual focus on a calm subject is no hardship, especially with focus peaking and magnification to help, which is exactly the screen skill we cover in our guide to reading a camera screen. The difficulty climbs when the subject moves and you are focusing by hand against the clock, a challenge we get into in our notes on tracking subjects that will not hold still.
Buying an adapter that does not wobble
The cheapest adapters fail in boring, expensive ways: play in the mount, light leaks, and a flange depth that is slightly off so infinity focus never quite lands. Precision is what you are paying for. A good adapter mounts with no wobble, holds the lens dead square to the sensor, and respects that fraction-of-a-millimetre tolerance. If you are working with genuinely old glass, the same care applies to handling and to the macro work of copying film, which we cover alongside scanning in bringing old slides into the digital age, where lens choice and working distance decide how much detail you can pull from a tiny frame.
Aperture without electronics
One detail trips up newcomers more than any other: aperture control on a passive adapter. A vintage lens with its own aperture ring is easy, since you set the opening by hand on the lens itself and the adapter never needs to know. But many later autofocus lenses, especially Canon EF and Nikon G designs, have no aperture ring at all and expect the camera body to command the iris electronically. Mount one of those on a plain mechanical adapter and it can default to wide open or stopped fully down with nothing in between. The fix is either an adapter with its own aperture control built in or simply choosing lenses that still have a physical ring. Check this before you buy, because nothing is more deflating than a beautiful lens locked at a single aperture you cannot change.
Adapting is not a hack so much as a discipline. Respect the flange math, keep the optical path as clean as the job allows, plan for the automation you lose, and buy metal that does not flex. Do that, and decades of overlooked glass open up on a single modern body.