My question, specifically: why is the Fujifilm HS 10 not an SLR?
SLR - Single Lens Reflex, meaning that there is only one lens through which the viewfinder and the film are exposed to the image. This is in contrast to other types of cameras, like rangefinders, where the viewfinder was a separated optical path than the film, usually in the form of a small lens on the top-side of the body. The Reflex part comes from the rotating mirror which is used to direct the image from the lens to the viewfinder. When the shutter is released, first the mirror goes up and clears the optical path to the film.
Another characteristic of this class of cameras is interchangeable lenses. I am not sure, though, if historically all SLR's had this option, and probably some of our experienced forum members can comment on that.
Nowadays, with the DSLR technology, sensors replaced the film but otherwise the basic structure and principles remained the same.
The Fuji camera in your link is not considered an SLR for the lack of the mirror, pentaprism and optical viewfinder. There, it is an electronic viewfinder, meaning that what you see is an image generated on a tiny LCD inside the viewfinder assembly.
Definition of SLR
SLR consists of:
Single lens. That is, it does not have a separate lens for a viewfinder. If it has a viewfinder, the viewfinder looks through the main lens.
Reflex. Traditionally this means it has a mirror (a "reflex mirror") that flips down allowing the image from the main lens to be bounced upwards onto a ground glass screen for viewing in a viewfinder. This allows for interchangeable lenses, because you will always be seeing what the lens sees even if the lens is changed.
Note: Many cameras now are use a digital viewfinder which allows viewing through the main lens in the same way that a camera with a reflex mirror would, but without a flip-down mirror. These are known as Mirrorless cameras, or Mirrorless Interchangeable Lens Cameras (MILC). These don't technically meet the definition of SLR due to the lack of a flip-down mirror, but nonetheless retain more similarity to SLR cameras than compact or rangefinder cameras - in particular, that the viewfinder is still achieving the key functionality of an SLR by showing the view through the main lens in the viewfinder, and having interchangeable lenses.
SLR is a term that's pretty old, and originally distinguished itself from other camera designs which used a separate lens assembly for an optical viewfinder, such as rangefinder cameras or twin lens reflex cameras.
Now, Mirrorless is a term that distinguishes newer, interchangeable lens cameras with mirrorless designs from more traditional SLR cameras.
Benefits and drawbacks
The benefit of having the optical viewfinder look through the primary lens is that you can change that lens and still see what the camera would see, without having to also change or somehow adapt the viewfinder lens.
The drawback is that the drop-down mirror assembly means there has to be a fairly long distance between the lens flange and the focal plane (film or sensor), which restricts lens designs, particularly making wide-angle lenses bulkier and more complicated. This drawback does not apply to mirrorless cameras which use a digital viewfinder.
The Fujifilm Finepix HS10
This is not an SLR. While it has an electronic viewfinder that shows the view through the main lens, it can't be called an SLR because:
The lens on this camera is not interchangeable, which is a key strength of the SLR design.
The non-interchangeable lens and tiny sensor designate this as a compact camera; though from looking at it clearly it is not very "compact" in the literal sense. These types of compacts are often called "superzooms" because of their large zoom range, and to distract you from the fact that for a compact camera they're not very compact. Manufacturers also tried to market them as bridge cameras - though this is a misleading term designed to give the impression it is somehow better quality than a normal compact camera. The only way in which these resemble an SLR is in size and weight - the sensor and optics are still those of a compact.
To-date, the moving mirror & viewfinder mechanism that @yasp described is sort of the defining characteristic of an SLR. I think we're going to see this "clean" definition continue to erode, though, with new cameras on the horizon.
Other characteristics generally attributed to the DSLR format:
- Larger sensor size than compacts or bridge cameras
- Interchangeable lenses
- Optical viewfinders
There are already Electronic Viewfinder Interchangeable Lens (EVIL) cameras such as Micro 4/3 cameras, that deliver many of the benefits of DSLR's without meeting this specific definition of DSLR, and more are on the horizon. The new Sony A55 and A33, for instance, use a translucent mirror that doesn't move, and Nikon is rumored to be introducing a "pro" mirrorless camera soon, too.
As more of these "not quite DSLR" cameras enter the market, I wouldn't be surprised to see the traditional definition of DSLR become less important, if not less clear.