I used to own an old Minolta x-700. The thing I loved about that camera is that aperture was controlled using a ring on the lens. I've since used other cameras where aperture and shutter speed are both controlled with a multi-function wheel around the shutter release, and I just don't like that as much.
Do any modern DSLR or Micro 4/3 camera systems support on-lens aperture controls, or has that gone the way of the dinosaur?
This is a feature of the lens, not necessarily the camera.
For Nikon/Nikkor lenses all G-type lenses are missing the aperture ring on the lens. Unfortunately for you, this is almost all lenses these days.
You can use old lenses, however. Make sure that you get a camera body with in-built focus-motor, otherwise you won't have focus on the old lenses.
D in the name instead of G.
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There are a few digital cameras that have this, though "mainstream" cameras don't seem to.
For example there is the Panasonic Lumix DMC-LX7 and Leica Digilux 2. A quick search on dpreview with the keyword 'aperture ring' will bring up the handful of cameras that have this.
Almost any camera can support on-lens aperture control if the lens supports it. With adapters, any lens that focuses on a plane at which the sensor can sit and makes a big enough image circle can be used with any camera that supports fully manual lenses, but all advantage of electronic lenses is then lost.
There isn't anything fundamentally different about the concept of what a lens does now vs 40 years ago. They just have made a lot of the manual controls able to be adjusted automatically or from the camera itself. For legacy support, most cameras still support using a manual lens but turn off the assistance the electronic control would normally provide (for example, you may have to operate the camera in full manual mode and focus will likely be manual).
As Unapiedra mentioned, older aperture control was almost always a feature of the lens, not the camera. Modern DSLR's most of the time have a dedicated control wheel to change your aperture settings.
I don't know of any lenses for today's DSLR cameras that have an aperture control ring, but if you go and look at the Leica systems and lenses, you'll find that most, if not all, have an aperture control ring.
In the world of M4/3 the only camera I can think of having this functionality is the Fujifilm X100 and X100s.
As a side-note: Modern Nikon DSLR cameras support their older range of manual lenses, these you can change a setting in the camera to allow you to control aperture via its aperture ring. I don't know how this would work in A/S/P mode, but should be fine in M mode at least.
