OPERATIONAL CONSIDERATIONSDSLRs and mirrorless cameras provide full automation when you use lenses designed for them. You get autofocusing and open-aperture, through-the-lens metering. Be aware that some adapters interfere with these functions. And, of course, if you're attaching a manual-focus lens such as a Leica or Zeiss, you won't get autofocusing.
The focusing screens in DSLRs weren't designed for manual focusing and lack the focusing aids found in manual-focus SLRs (central split-image and microprism collars on ground-glass focusing screens, for example). Some higher-end DSLRs accept interchangeable focusing screens; if you intend to use manual-focusing lenses with one of these, it's a good idea to buy and install a screen that was designed for manual focusing.
With live-view cameras (many DSLRs and all mirrorless, interchangeable-lens models), you can zoom the live image and focus right on the LCD monitor. A magnifying loupe (and a tripod) can be very helpful here.
Some adapters come with chips that activate an AF DSLR's focus-assist system, so the "in-focus" lamp in the viewfinder will glow when you've achieved focus manually.
Canon EOS DSLRs and EF lenses have all-electronic lens mounts, where all communication between camera and lens is electronic; there's no mechanical linkage aside from the bayonet mount that holds the lens on the camera. Thus, you can't adjust the aperture when using an EF lens on a non-EOS body. However, Redrock Micro offers the LiveLens MFT Active Lens Mount, which lets you attach EF lenses to Micro Four Thirds System cameras and incorporates a battery-powered onboard electronic system that allows you to adjust apertures of EF lenses in 1?3- or 1?5-stop increments (but you must do this via the Redrock unit, not via the camera body).
Some adapters from Novoflex incorporate an aperture-control ring so you can adjust apertures with lenses that lack one, such as Nikon's G-series.