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).
• Reflex - Refers to the mirror and its movement that makes the use of a single lens possible.

When looking into a SLR camera you see through the lens via a mirror that blocks the shutter mechanism, which in tern 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 change the aperture to your intended exposure setting before releasing the shutter. In modern SLR's the aperture setting is automatically taken care for you.

When you press the shutter release the first thing that happens with a modern SLR is that the lens aperture stops down from it's wide open setting to the taking aperture. This 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 SLR's swings back into 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 would stay in its taking position and so the view finder would remain black until the photographer primed the mechanism again.

Pros

• 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 range finder or TLR camera. The only thing to be aware of is that, hidden somewhere in the frineprint of your SLR's specifications will be a viewfinder percentage value. This tells you what percentage of the taking image you will see in the viewfinder. In most SLR's 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 that do show you 100% of the taking image but these are the exceptions rather than the rule.

• Because you look directly through the taking lens you can use more extreme lenses on an SLR. Moderate telephotos and wideangle lenses are available for range finder and TLR cameras but nothing like the range available to SLR users. Also, in is considerable more difficult to change and use alternate lenses on range finder and TLR cameras compared to an SLR.

• Because you look directly through the taking lens more accurate focussing is possible. This is especially true in close-up and macro work.

• Ability to see the depth of field (DOF prior to taking the shot. As you can stop down the lens to its taken aperture prior to taking the shot with many SLR's you can see what the resulting DOF will be.

Cons

• The extra mechansims 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 quite location and/or you are trying to be discrete.

• The mirror mechanism adds extra problems with camera shake. The rapid movement of the mirror swinging out of the way can cause vibrations that add to camera shake when you are taking a picture. With normal shutter speeds this 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 SLR's add a mirror lockup function to compensate for this effect. This function allows the photogrpaher to manually select when to move the mirror out of the may in advance of taking the exposure. This facility is prized by astro, night and some micro 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.