The Viewfinder
Which is Best for YOU?

Let's investigate the major types of viewing systems. They are: Optical Viewfinder, Single Lens Reflex, Twin-Lens Reflex, Ground Glass Back and Electronic. The types most often found on modern digital cameras are Optical, Single Lens Reflex and Electronic (the LCD screen on the back of a camera). You look at the screen, release the shutter and rely on the computer chips do their magic.

Up until recently if you had a Digital SLR you couldn't get a live view from the LCD screen on your camera. Canon has solved the problem, sort of. You can get a live view on their newer SLRs, which have a larger LCD screen I might add, but when you press the shutter release the screen goes black until the image is displayed. It comes in really handy when focus is critical. Just zoom in on a portion of the image to check the focus! Excuse the plug for Canon, back to the subject.

Figure 1 illustrates the Optical Viewfinder. This simple system is used on, but not limited to, inexpensive cameras including most point and shoot models. It is also used on some very sophisticated cameras like the German made Leica.

While this system is simple and direct, it has a major limitation. The image is always in focus. You need a way to focus the image before it reaches the sensor. This is solved by incorporating a Rangefinder.

Figure 2 illustrates the operation of the SLR Viewfinder. This is an ingenious arrangement of a mirror and a prism that allows you to look into the viewfinder and see directly through the lens! The heart of the system is a hinged mirror, mounted at a 45 degree angle in front of the sensor. The light entering the lens is directed into a groundglass.

After passing through the groundglass the light enters a five-sided prism called the pentaprism. It's function is to bounce the image around so it is viewed in the eyepiece right-side up and correct from left to right.

So why doesn't the light reach the sensor? First, the mirror reflects most of the light up into the groundglass. Second, the shutter is right in front of the sensor and remains closed while you are viewing the image.

When you press the shutter release, the mirror swings up out of the way and the shutter opens. The mirror snaps back down right after the shutter closes. This all happens very fast but causes a situation called black out. This is when the viewfinder goes dark when the shutter is open. With instant return mirrors found in today's cameras, this is not a problem.

Instant-return mirrors can be noisy. Many Professional Photographers prefer using rangefinder cameras when stealth is required. Also, when the mirror "snaps up" it could cause the camera to shudder enough to blur your image at slow shutter speeds. A function designed to solve this problem when using slower shutter speeds is Mirror Lockup. This is usually best suited to situations requiring the use of a tripod and cable release.

Another drawback of this system is when using a very small aperture like f/16 or f/22, a very small amount of light enters the lens and focusing becomes difficult! The answer was to have the aperature remain at it's largest diameter while focusing to allow the greatest amount of light to pass through the lens.

So how does the aperture go from wide open at say f/2.8, fully open, for viewing to f/16 for exposure? The instant you press the shutter release the diaphragm closes down to the pre-set aperture for the exposure and then returns to f/2.8 and is ready for you to capture the next shot! You can see what the image would look like at the per-set aperture by pressing the Depth of Field Preview button on your camera.

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