There is an idea in computing of the Turing Machine. This is a general-purpose computer that can do any computational task by changing its program. I have mentioned this before in the same sense, of being able to make a digital camera emulate other sensors or effects. My premise here though, is that there is also a general-purpose type of camera, and it’s the SLR.
The SLR can do any photographic task, because it can be adapted and has few constraints.
The key feature is that the viewing lens and the taking lens are the same: you look through the actual lens in use to frame and focus the image. This means that what you see is what you get. The only other camera that does this is the large format type, where you compose and focus on a sheet of ground glass that gets replaced by film to make the exposure. The benefit of the SLR is that the ground glass and the film are in the same box at the same time: when you have composed and focused the mirror flips up and the shutter opens to send the incoming light to the film (or sensor). There are no delays while you swap the focusing screen for the film.
The other clever trick that the SLR has is the pentaprism. This reverses and inverts the image from the lens so that you see the scene the right way up and the right way round. Compare this with the large format camera above, where the image you see is upside-down. Or a TLR, where the image is the right way up but reversed left-to-right. So the SLR shows the world in the same way that it looks without the camera. There is no struggle to follow action or level a horizon, because the camera moves in the way you expect.
Because you look through the lens, so can the lightmeter. Rather than the meter having a different view of the scene, you know exactly what it is measuring. This means that the meter automatically adjusts for filters, close-up work, odd apertures and strange lenses. By strange, think of using projector lenses, or even zoom lenses that have a variable aperture. As an aside, this is why cine lenses have T markings rather than F stops. T is the actual transmission of the lens and is true for all lenses at the same setting. A marked F stop may not be the actual value of the light that gets through though, due to the realities of multiple elements etc. This is why it is useful to be able to meter through the lens to measure the actual light rather than the marketing department’s number. It also means that with a long lens, for example, you are measuring the distant scene rather than the general light you are standing in.
The benefit of rangefinder cameras, we are told, is that you can see outside the frame. This means you can see things that are about to come into the frame. This is supposed to be a benefit in street photography. The disadvantage is that the frame you see is not quite the same as what the lens sees, and this gets worse as you get closer. You also need the camera to have the necessary viewfinder frames for your lenses, or you need to use a supplementary viewfinder, introducing another source of error and turning the focusing and framing into separate actions. An SLR, on the other hand, will focus and frame any lens you can fit to it. Rangefinders also struggle with very long or very wide lenses, as the viewfinder and focus patch become less useful. Just think of the difficulties of focusing a 500mm lens (4 degrees) or a 180 degree wide angle. With the 500 the actual field of view may be smaller than the focusing patch. With the wide angle you have no idea if your feet are in shot or not. An SLR will happily handle both.
Rangefinders do have one advantage over the SLR though, in that they are often quieter to use. There is no sound or vibration from the SLR’s mirror flipping up and down. They can also be physically smaller, as the camera body doesn’t have to hold a tilting mirror.
The big development has been in mirrorless cameras, which combine the smaller size of the rangefinder with the through-the-lens utility of the SLR. I think that mirrorless cameras took off due to mobile phone cameras. We got used to the idea of holding a camera out in front of us and looking at the screen on the back, rather than holding the camera to our eye. So the mirrorless camera gains the smaller body of the rangefinder and drops the complex mechanism to raise and lower the mirror. It does rely entirely on electronics to display the viewfinder image though, so you need bigger batteries and get through them quicker. So there is a trade-off between the bigger and heavier SLR and the smaller and lighter mirrorless that may require you to carry an extra battery. There is also no equivalent to the manual SLR. I can use my Pentax MX with or without batteries and it works just fine. On the other hand, everyone seems to be swapping to mirrorless cameras, so perhaps I am wrong about SLRs? For digital, I probably am.
The epitome of the SLR is the professional system camera. These took full advantage of the adaptability of the configuration to allow you to swap viewing/ focusing screens, viewfinders, film capacity, motor drives, macro gadgets etc. You started with the basic film holder and shutter and added other bits as needed. The need for this has gone away with digital, as you can add picture capacity by using a larger storage card and do most of the other tricks in software. So what you are left with is the viewing mechanism – the facility to see through the lens and focus the image produced by that lens.
The downside of the SLR is that they are more complex than other types of camera. There is some clever orchestration to close-down the lens aperture, lift the mirror, trigger the shutter and then reverse it all. It also means that lenses for an SLR have to be designed to have a large enough distance between the back of the lens and the sensor to allow room for the mirror and shutter. This flange distance was fixed by the camera manufacturer for their cameras and lenses and is the main reason why not all lenses work on all cameras. The precise mechanical engineering of the mirror and shutter are probably why SLRs are declining: it’s a lot easier to use electronics to display what the lens is seeing than to use mirrors and prisms to do the job. It certainly makes manufacturing easier. For what is basically a niche product in a small market, this is important. The lack of a reasonably priced shutter mechanism is probably what is preventing any new SLR being developed (although we may be in luck).
So basically, there you have it: the SLR was the pinnacle of practical usability, replaced by alternatives that were cheaper to make or more flexible (mirrorless cameras and mobile phones). RIP the SLR.