There is a particular pleasure in mounting a thirty year old lens on a brand new camera and watching it draw. Adapting lenses across systems opens up cheap, characterful glass and lets a single body wear decades of optics. But it is not magic, and it is not universal. Whether a given lens will work on a given body comes down to a small amount of mechanical reality that is worth understanding before you spend money on an adapter.
The one number that decides everything
Every interchangeable-lens system has a fixed distance from the lens mount to the sensor, and that single specification governs almost all of adapting. It is called the flange focal distance, and Wikipedia keeps a clear definition of what flange focal distance is and why it is fixed to fractions of a millimetre. The short version: a lens is designed to sit a precise distance from the film or sensor plane, and if it ends up closer or farther than that, focus breaks. An adapter’s entire job is to preserve that distance across two systems that disagree about it.
Why some adapters are impossible
Here is the rule that saves you from buying the wrong thing. A simple adapter can only add distance, never remove it, so you can adapt a lens onto a body whose flange distance is shorter than the lens was built for. The adapter is just a spacer filling the gap. Go the other way, and the math turns negative: the adapter would need to have a thickness below zero, which is another way of saying it cannot exist. Pulitzer-winning photographer Brian Smith lays this out with real mount figures in his guide to flange focal distance, including why a short-flange mirrorless body is the most adaptable camera you can own and a deep-flange film body is the least.
Optical versus mechanical adapters
When the flange distances do not cooperate, manufacturers sometimes add a correcting optical element so the lens can still reach infinity. Avoid these when you can. Every cheap glass element you insert between a good lens and the sensor is a chance to throw away the sharpness you adapted the lens to get. A plain mechanical adapter, just metal holding the lens at the right distance, keeps the optical path honest. Pay for precision in the metalwork instead, because slop and wobble in a loose adapter will cost you focus accuracy in exactly the moments that matter.
What you give up
Most adapters trade away electronics. Autofocus may slow down or vanish, image stabilization may not carry over, and lens metadata often stops being recorded. None of that is a problem if you plan for it. Manual focus on a static subject is no hardship, and this is where reading your screen well pays off, since you will be confirming focus by eye rather than trusting a beep. Our guide to reading a camera screen covers the magnification and focus-peaking habits that make manual adapted lenses genuinely usable. It gets harder when the subject moves, which is its own discipline covered in our notes on tracking subjects that will not hold still.
Where to go next
If you want the practical, mount-by-mount version of this with specific bodies and lenses we have actually adapted, read our field notes on adapting lenses across camera systems. And if the glass you are eyeing is genuinely old, the same flange logic shows up when you digitize film and slides, where lens choice and working distance decide how much detail you can pull back, something we get into in bringing old slides into the digital age.