A single-lens reflex (SLR) camera operates on the principle that the same lens used to capture the image is also used for viewing, providing the photographer with an exact preview of the scene. This "what you see is what you get" (WYSIWYG) advantage is achieved through a clever system of mirrors and prisms.
The Core Mechanism: Viewing Through the Lens
At its heart, an SLR camera is designed so that the camera lens itself serves as the finder in conjunction with a ground-glass focusing screen to which the image is reflected by a mirror. This innovative design allows the photographer to compose and focus with precision, seeing exactly what the lens sees.
The Path of Light: A Step-by-Step Journey
Understanding the internal mechanics of an SLR involves tracing the path of light from the subject to the photographer's eye and ultimately to the imaging sensor or film.
1. Pre-Capture Viewing
Before you press the shutter button, here's how you see the scene:
- Light Enters the Lens: Light from the subject passes through the camera's single, interchangeable lens.
- Reflection by the Mirror: Inside the camera body, behind the lens, a movable mirror is angled at 45 degrees. This mirror reflects the light upwards.
- Hitting the Focusing Screen: The reflected light strikes a translucent ground-glass focusing screen, where a latent image is formed.
- Through the Pentaprism: The image on the focusing screen, though correctly oriented vertically, is laterally reversed (mirrored horizontally). To correct this, the light passes through a special five-sided optical prism called a pentaprism. The image is viewed on the screen through a pentaprism that corrects the lateral reversal of the image, presenting a correctly oriented view to the photographer.
- To the Viewfinder: Finally, the corrected light reaches the eyepiece, allowing the photographer to frame, compose, and focus the shot.
2. Image Capture
When you press the shutter button, a dynamic sequence unfolds rapidly:
- Mirror Flips Up: The angled mirror instantly swings upwards and out of the light path. This causes a momentary blackout in the viewfinder, as the light no longer reaches it.
- Shutter Opens: Simultaneously, the camera's focal-plane shutter opens, exposing the digital sensor or film to the light.
- Light Reaches Sensor/Film: With the mirror out of the way, light from the lens directly hits the imaging sensor (in a digital SLR, or DSLR) or film (in a traditional film SLR). The image is recorded.
- Shutter Closes & Mirror Returns: The shutter then closes, and the mirror immediately swings back down into its 45-degree viewing position, restoring the image in the viewfinder.
Key Components of an SLR System
To better grasp the functionality, here's a breakdown of the essential components and their roles:
| Component | Function |
|---|---|
| Lens | Gathers light from the subject and focuses it onto the sensor/film or viewing system. |
| Reflex Mirror | Reflects light upwards to the viewfinder system during composition; flips up for image capture. |
| Focusing Screen | A frosted glass surface where the image is formed before being relayed to the pentaprism. |
| Pentaprism | An optical prism that corrects the horizontally reversed image from the focusing screen for the viewfinder. |
| Viewfinder | The eyepiece through which the photographer sees the corrected image. |
| Shutter | A mechanical curtain that opens and closes to control the duration of light exposure on the sensor/film. |
| Sensor/Film | The light-sensitive medium that records the image when exposed. |
Advantages of the Single-Lens Reflex System
SLR cameras revolutionized photography, offering significant benefits:
- Accurate Framing: What you see through the viewfinder is exactly what the lens sees, eliminating parallax error common in rangefinder or twin-lens cameras. This is crucial for precise composition, especially in macro photography or when using telephoto lenses.
- Interchangeable Lenses: The single lens mount allows for a vast array of specialized lenses (wide-angle, telephoto, macro, prime, zoom) to be swapped, expanding creative possibilities.
- Depth of Field Preview: Many SLRs offer a depth-of-field preview button, stopping down the lens to the selected aperture to show the actual depth of field in the viewfinder, aiding in creative control.
- Low Light Performance: The large sensor/film size often found in SLRs allows for better low-light performance and image quality compared to many compact cameras.
While mirrorless cameras have gained popularity, the fundamental principles of the SLR system continue to influence camera design and remain a cornerstone of photographic technology.
