CHAPTER 4 HOW LIGHT PLAYS TRICKS ON PERCEPTION^51
The All-Digital Viewfinder
Theelectronic viewfinder (EVF) on a compact digital camera sits right in front of the eyepiece, where in a TTL camera you’d find the
ground glass. It works like a TV screen receiving a live feed from the lens. Not to be confused with the large LCD screen on the back of
most digital cameras—still another way to frame a snapshot—the EVF is about one-half inch diagonally; has some 200,000 pixels; and
uses a small lens in front of it to magnify the image when seen through the eyepiece. Electronic and optical viewfinders have their pros
and cons. TTL images are sharper and don’t drain the battery. EVFs are quieter because there’s no mirror to flip up, and some are
mounted on a swivel so you can pull them away from the camera body to let you frame shots that otherwise might require you to lie on
the ground to get the angle you want.
Later, at the precise moment the photo is
snapped, the mirror swings up to let the light
get to the image sensor. In cameras that use
a prism instead of a mirror, the prism splits
the light, sending some to the eyepiece and
the rest to the imager.6
More expensive cameras featurethrough-the-lens (TTL) viewfinders that
are far more accurate in predicting the frame the camera will capture. Before
light from the subject reaches the image sensor, it is diverted by a mirror.
(Some cameras use a beam splitter.)4
Whether beam splitter or mirror, either method detours the image up
onto a clear piece of glass called a focussing screen. The glass is
rough on one side to catch the light from the lens, making it a rear-
projection screen so that the photographer can see what the image
sensor will see when the picture is taken. A prism mounted about the
screen bounces the light through a loop—the loop that reverses the
upside-down, mirror image the lens produces and aims it straight at
aneyepieceso the photographer sees the image. Because the
photographer sees the same light the camera uses to burn the image
to the sensor, there is no parallax problem.5