Light takes a bouncy journey through a DSLR.
The Nikon D3's viewfinder system, here with the 14-24mm f/2.8G AF-S Nikkor lens, is carried over to its replacement, the D3s ($5,200, street, body only).
1. Focusing Screen And Condenser Lens: The focusing screen is a flat ground-glass on which the still-reversed image is formed. The condenser lens above it guides the image into the pentaprism.
2. Autoexposure Module: The D3's AE module is a CCD sensor with 1,005 pixels that analyze data including color, contrast, and distance. Nikon's 3D Color Matrix Metering II compares the image to a database of more than 30,000 images to evaluate exposure settings.
3. Pentaprism: Full name: pentagonal roof prism. Made of optical glass encased in mirrors, the prism bounces the image around three times to send it to the viewfinder eyepiece, unreversing it in the process.
4. Secondary Mirror: A portion of the reflex mirror is semi-silvered (a beamsplitter) to allow some light to pass through it. A second mirror behind the reflex mirror directs this image down to a lens that guides it into the AF module.
5. Reflex Mirror: Set at 45 degrees to the light path, this reflects the image from the lens upward to the pentaprism assembly. The image is reversed at this point. The mirror quickly pivots up just before exposure to allow the image to reach the sensor, then drops back down.
6. Image Sensor
7. Autofocus Module: Here, the image is divided in two and the pair compared electronically, something like an optical rangefinder. Info from the two images is analyzed to determine which way the lens should be focused, and how much. The D3 can do this at any of 51 points in the frame.
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