December 6, 2007
What is a Single lens reflex (SLR)
A single lens reflex (SLR) is a camera in which the same lens is used for viewing and taking pictures. A mirror in the body directs the light from the lens up into a prism for viewing, then flips up out of the way just before an exposure is made. Note that this is not an exotic technology; the standard Nikon or Canon camera body (photo at right) is an SLR.
Suppose that the photographer has chosen an exposure of f/8 and 1/125th of a second. Here is how most SLRs work during exposure:
- lens is kept open to maximum aperture (e.g., f/2.8) for ease of viewing and metering
- when the photographer presses the shutter release, the lens aperture is stopped down to the taking aperture of f/8. On old-style camera/lens interfaces (e.g., Nikon, Hasselblad), this is accomplished by moving a lever. With camera/lens interfaces designed in the 1980s (e.g., Canon, Rollei), this is accomplished by sending an electrical signal to a solenoid in the lens.
- the mirror is flipped up out of the way of the light (and parked flat up against the prism)
- now that the lens is stopped down and the mirror is up, the shutter opens and light begins to strike the CCD or CMOS sensor
- as soon as the shutter is fully open, the camera signals an electronic flash, if attached to fire
- when 1/125th of a second has elapsed, the shutter is closed
- the mirror is pushed back down to viewing position
- the lens aperture is reopened to its widest setting
SLR manufacturers generally provide a range of interchangeable lenses. This works out nicely because changing the lens simultaneously changes the scene magnification on film and in the viewfinder. It is tough to mix and match brands. Camera bodies and lenses are coupled mechanically and electronically in non-standard ways. A lens for a Canon EOS body won’t fit a Nikon body and vice versa.
The best thing about an SLR is that what-you-see-is-what-you-get. If you’ve left the lens cap on, fitted a really long telephoto, attached a strange filter, you can see the effect in the viewfinder.
One obvious problem with an SLR is weight. The prism on top of the body that lets you see a properly-oriented image is heavy.
Another problem with the SLR is noise. The mirror is light but it has to be flipped up as fast as possible, which is necessarily noisy. Photographers who work during live theater or concerts often surround the camera in a "blimp" to muffle the noise.
A final problem with an SLR is exposure latency. If you wait for the decisive moment and press the shutter, the camera doesn’t take a picture until it has stopped down the lens and flipped up the mirror. This takes between 50 and 100 milliseconds for the average SLR, which can be reduced to about 40 milliseconds by using the mirror lock-up custom function. A standard digital camera uses the final 40 milliseconds to register dark current levels from the image sensor. These levels vary based on temperature and other conditions, and must therefore be updated for every picture or sequence of exposures.
[Do not confuse an electronic viewfinder (EVF) point and shoot camera with a true mirror-and-optics SLR. The EVF camera is sending light continuously to the sensor and feeding the sensor output to a little TV screen on top of the camera. Physically the format is very similar to a true SLR, but current TV screen technology isn't nearly as good as current optics.]