For the photographer it means when you look through the viewfinder you are actually looking through the lens you have mounted to your camera body. Who hasn't looked into the viewfinder and discovered pure black....because the lens cap was still on. The main advantage of a SLR camera is you see pretty much what the image sensor will see when you trip the shutter.
Here's how the image gets from the lens up to the viewfinder:
- Light from the lens and strikes the 45 degree angle mirror being turned up by 90 degrees.
- The light passes through a focusing screen which is where the focus points, gradation lines and so forth the you see in the viewfinder really are.
- The light then enters the bottom of a pentaprism (or pentamirror), reflects twice, being turned through 270 degrees to present an image right side up in the viewfinder for you to look at.
Cross-section view of SLR system.
1 - 4-element lens
2 - Reflex mirror
3 - Focal-plane shutter
4 - Sensor
5 - Matte focusing screen
6 - Condenser lens
7 - Pentaprism
8 - Eyepiece
So far so good, we can see almost exactly the field of view the image sensor will see
but that 45 degree mirror is blocking the light path to the image sensor. For now that's ok because we aren't yet ready to capture the image.
If we assume the camera is setup in Auto mode and we have the image framed in the viewfinder the way we want; it's time to take the next step and press the shutter release.
When we do the camera will:
- Focus, if we have an auto focusing lens mounted, and will select all the other settings necessary to capture the image on the image sensor.
- Lift that 45 degree mirror up out of the way (blocking your view in the viewfinder)
- Drop the front (first) curtain on the shutter (which is right behind the mirror), letting light hit the image sensor
- Drop the rear (second) curtain on the shutter to block light from reaching the image sensor
- Raise the front curtain to it's original position
- Raise the rear curtain to it's original position
- Reset the trigger for the shutter so it's ready for the next image
- Lower the mirror to it's regular 45 degree position (restoring your view in the viewfinder)
- Write the image data to the electronic data storage card.
Wash, rinse, repeat.
The image sensor is made of millions of individual light sensors. Each one records how much light hits it during the exposure. Additionally, most image sensor pixel locations have a covering called a Bayer color filtering mosaic that adjusts the wavelength of the light reaching the sensor (kind of like multi-colored sunglasses) to the same as the light reflected from that part of the real world in the image field of view.
That unprocessed image data is either written directly to the memory card as a Raw data file or it is run through a rendering engine and converted to a JPEG (Joint Photographic Experts Group) or TIFF (Tagged Image File Format) image file.
So, you can see there is a lot going on when you trip the shutter on your camera.
In the next installment we'll look at the other main part of a DSLR, the lens.
See you then,
All images courtesy of Wikimedia Commons.