To have a list of SLR cameras, see the Category: SLR.
SLR is an acronym for Single Lens Reflex. What does that stand for?
- Single Lens — The camera uses one lens for both taking and viewing. In other words you look through the same lens as the camera uses to expose the film or sensor (as opposed to twin-lens reflex cameras).
- Reflex — Refers to the mirror and its movement that makes the use of a single lens possible.
When looking into the viewfinder of an SLR camera, you are looking through the lens via a mirror that blocks the shutter mechanism, which in turn blocks the film. In most modern SLRs, the aperture of the lens is stopped wide open when you are focusing and composing the image. This allows the brightest possible viewfinder image. In older cameras with 'manual stop down lenses' you would have to set the lens aperture to its widest setting while focusing and composing, and then manually change the aperture to your intended exposure setting before releasing the shutter. In modern SLRs, the aperture setting can be set for you automatically.
When you press the shutter release, the first thing that happens with a modern SLR is that the lens aperture stops down from its wide open setting to the taking aperture. This is why the view through the camera often goes darker than it was immediately before the shutter is tripped.
Simultaneously to the aperture stopping down, or following it, depending upon the exact mechanical specifications of the SLR used, the mirror you had been using to look through the lens with swings out of the way of the shutter and film. This is the reason that the viewfinder goes black in an SLR when you press the shutter release.
With the mirror out of the way the film is now directly in line with the lens but still shielded by the shutter mechanism. The shutter mechanism is now tripped resulting in the film frame being exposed via the lens.
Once the exposure is complete and the shutter is closed the mirror, in most modern SLRs, swings back into the taking position and you can once again see through the viewfinder. This is known as an 'auto return mirror'. In older and more primitive cameras the mirror will stay in its 'up' position and so the view finder will remain black until the photographer has primed the mechanism again.
In many vintage SLRs (and few modern ones with interchangeable finders), the image on the ground glass is directly seen from above (waist-level finder) — it's upright, but reversed (left and right). In most modern SLRs, the ground-glass image is seen through a prism that resides on top of the ground glass screen. The view through the prism gives you an upright, unreversed viewfinder image (eye-level finder).
- What you see is what you get, or almost. Looking through the same lens as will be used to expose the film means that you do not experience parallax errors like you might in a rangefinder or TLR camera. The only thing to be aware of is that, hidden somewhere in the fineprint of your SLRs specifications, will be a viewfinder percentage value. This tells you what percentage of the taking image you will see in the viewfinder. In most SLRs this value is less than 100% indicating that slightly more of the scene than you see through the viewfinder will be exposed on the film. There are rare exceptions, like the original Nikon F and the current Olympus E-3, that do show 100% of the taking image -- but these are exceptions, rather than the rule.
- You always see the image of the taking lens and do not need any additional viewfinders for different focal lengths. This makes the use very comfortable. Wide-angle lenses and moderate telephoto lenses are available for rangefinder cameras, but the longer telephoto lenses can only be used on an SLR, because focusing them with a rangefinder would not be possible.
- Because you look directly through the lens, more accurate focusing is possible. This is especially valuable in close-up and macro work.
- On many SLRs, you can preview the depth of field prior to taking the shot, by stopping down the lens to its taking aperture. The viewfinder image becomes darker but you can see exactly what parts of the picture will be in focus and what will not.
- You can see the effect of any filters or auxiliary lenses fitted, and a TTL exposure meter will automatically take any loss of light through these additions into account.
- The extra mechanisms for the mirror and prism add bulk and weight to a camera design. Camera designers are skilled at hiding this extra bulk but compare a Nikon F with a Leica of similar vintage to see the difference in size and weight.
- The mirror mechanism adds extra noise when you trip the shutter. Again, compare a Nikon F with a Leica of similar vintage to hear the difference. This extra noise can be a problem is you are in a quiet location and/or you are trying to be discreet. Modern DSLR cameras can be programmed to operate on silent mode with the mirror retracted and use the sensor to focus.
- The mirror requires space to pivot, therefore lenses with focal length less than about 45mm require retrofocus design to bring the lens elements away from the mirror travel. This additional feature makes wide angle lenses more expensive, darker and with worse image quality if compared to rangefinder lenses.
- The mirror mechanism adds extra problems with camera shake. The rapid movement of the mirror swinging out of the way can cause vibrations (an effect known as mirror slap) that add to camera shake when you are taking a picture. Camera designers try to remove this effect by buffering the mirror's movement. With normal shutter speeds this effect is rarely a problem but with longer exposures it can cause an image to slightly less sharp than it might otherwise be. Some fully featured SLRs add a mirror lockup function to compensate for this effect. This function allows the photographer to manually select when to move the mirror out of the way in advance of taking the exposure. This facility is prized by astro-, night- and some macro-photographers.
- The mirror motion before the shutter can open adds a slight shutter latency to SLR cameras i.e. when you trip the shutter in an SLR it takes a split second longer to expose the film with and SLR than it does with a range finder and TLR camera.
- Since the mirror flips out of the way to allow the film to be exposed, the viewfinder blacks out for the period of the exposure (in fact from a little while before to a little while after). Although modern mirror mechanisms are very quick it does mean that you can never see the subject at the actual moment of exposure.