Buy the system, not the camera. Bodies come and go every few years, but a good lens lasts decades, and once you own three or four lenses for one mount, switching brands gets expensive. So before you pick a body, pick the mount you want to build around. Here's how the six systems in our catalog compare for someone buying used, with the history that explains where each one is heading.
Canon: EF for DSLRs, RF for mirrorless
Canon launched the EF mount in 1987 with its EOS autofocus cameras, and it carried Canon from film to digital. EF stands for Electro-Focus, because every EF lens has its own focus motor and the mount uses only electrical contacts. Canon made more than 100 million EF lenses by 2014, so the used supply is enormous. Its last new EF lens design came out in 2018, the year the RF mount arrived for full-frame mirrorless bodies.
The good news for used buyers is that Canon makes adapters that let EF and EF-S lenses work on RF bodies. An EF lens bought for a 5D Mark III today will still work on an EOS R6 later. The one catch runs the other way. EF-S lenses, made for Canon's APS-C DSLRs like the 80D and 90D, don't fit full-frame EF bodies, and on an RF body they put the camera into a cropped APS-C mode.
Nikon: F and Z
Nikon's F mount goes back to the Nikon F of 1959, and Nikon kept extending it through autofocus and digital instead of starting over. Wikipedia counts over 400 Nikkor lenses compatible with it, which makes it one of the largest lens systems ever. Not every old lens works fully on every body, so check compatibility for older manual-focus glass, but modern autofocus lenses are straightforward.
Nikon splits its lenses into FX for full-frame and DX for APS-C. A DX lens fits an FX body like the D750, and the camera crops to the center of the sensor to use it. In 2018 Nikon launched the Z mount for mirrorless, with the Z6 and Z7 and an adapter called the FTZ for F-mount lenses. Their second versions, the Z6 II and Z7 II, are common used buys. At 55 mm across, the Z mount's opening is the largest of any full-frame mount.
Sony: E mount
Sony's E mount started on the small APS-C NEX cameras in 2010, and in October 2013 the original A7 and A7R put a full-frame sensor behind the same mount. Canon and Nikon didn't follow with full-frame mirrorless until 2018, so its used market has the deepest range of mirrorless bodies and lenses, from Sony and from third-party makers. The mount sits just 18 mm from the sensor, short enough to adapt almost any SLR lens.
Sony uses one mount for both sensor sizes. APS-C bodies like the A6400 take full-frame lenses without fuss, and full-frame bodies like the A7 III take APS-C lenses in a cropped mode. That makes it easy to start small and move up without selling your lenses. Sony also opened the mount's specification to other lens makers in 2011, which is why Sigma, Tamron, Samyang and Viltrox all build E-mount lenses.
Fujifilm: X mount
Fujifilm announced the X mount in January 2012 with the X-Pro1, and it has stayed APS-C only. That's a choice, not a gap. The lenses are compact, and many of Fujifilm's XF lenses have a metal mount and an aperture ring, which suits the dial-driven bodies like the X-T3 and X-Pro3. The cheaper XC lenses are lighter and mostly plastic and have no aperture ring. People come to Fujifilm for the handling and the in-camera film looks as much as the specs. If you want full-frame later, though, you'll be starting over.
Pentax: K mount
The K mount dates from 1975 and was first designed with Zeiss for a planned alliance. Pentax has used it on every 35mm and digital SLR since, so old manual lenses fit modern bodies like the K-1 without an adapter. The Pentax bodies we cover stabilize the sensor, so even a 1970s lens gets stabilized. The catch is the future. In September 2026 Ricoh said it would end production of its current K-series DSLRs while it develops a new series for the mount. Used K-mount bodies and lenses will be around for years, but the choice of new bodies is thin.
Olympus and OM System: Micro Four Thirds
Olympus and Panasonic launched Micro Four Thirds in 2008, and bodies and lenses from Olympus, OM System and Panasonic all fit each other. The sensor is smaller than APS-C, with a 2× crop factor, so telephoto lenses are tiny for their reach. The E-M1 Mark II with a long zoom weighs a fraction of a full-frame kit with the same reach, which is why birders and hikers love it. The tradeoff is more noise in low light and deeper depth of field than larger sensors give you. Olympus handed its camera business to OM Digital Solutions on January 1, 2021, and newer bodies carry the OM System name, but the mount is the same, so the lenses keep working.
How to choose
- You already own lenses. Stay with that mount, or with its mirrorless successor through the maker's adapter. That's the cheapest upgrade there is.
- You want the biggest used choice. Canon EF and Nikon F have decades of lenses at every level, and both adapt to the makers' mirrorless bodies.
- You want mirrorless now, with room to grow. Sony E has the widest range of used full-frame mirrorless bodies and lenses, and APS-C and full-frame share the mount.
- You want small and light. Fujifilm X or Micro Four Thirds, and decide whether you care more about handling or reach.
- You love old lenses. Pentax K takes them natively with stabilization, and any mirrorless body adapts them.
Once you've picked a mount, the lens mount pages list every body we cover for each one, and each camera page has the checklist for that body. If you're also weighing a DSLR against mirrorless within a system, read DSLR or mirrorless, bought used.
