In general, the viewfinder system of dSLR cameras operates very much like their film SLR counterparts. One of the primo advantages of digital cameras, the ability to see the sensor image prior to exposure, doesn’t apply to digital SLRs because they operate too much like their film ancestors.

• No live preview. You can’t preview the image you’re going to take, exactly as the sensor sees it because the sensor can’t create an image until the mirror flips up and out of the way.
• No live histograms. Histogram displays are a great way to optimize exposures. If the histogram “chart” is skewed too much in one direction or another, you can add or reduce shutter speed, aperture, or EV value to get the best possible exposure. Non-SLR digital cameras have what are called “live” histograms that show up on the LCD and/or EVF while a shot is being composed, so you can make corrections in real time. That’s not possible with a dSLR for the same reason you can’t get a live preview: The sensor is blind until the moment of exposure.
• No sneaky surveillance-type shooting. You can connect many digital SLRs to a USB cable and use a special program to control the camera from a distance. Your control program probably can download your remote photos to your computer after they are shot, but you can’t monitor what the camera sees before taking the picture.
• Shooting blind in dim light or with infrared filters. Because you’re limited to viewing through the dSLR’s optical viewfinder, if light conditions are very dim or you’re using an infrared filter, you have to shoot blind. LCD and EVF viewfinders often have circuitry that boosts the gain under dim conditions, providing you with a fuzzy, sometimes black-and-white image, but one that can still be viewed, even if you’re shooting with an infrared filter that blocks visible light.