Viewfinder

Vincent Bockaert, 123di.com

The viewfinder is the "window" you look through to compose the scene. We will discuss the four types of viewfinder commonly found on digital cameras.

Optical Viewfinder on a Digital Compact Camera

The optical viewfinder on a digital compact camera consists of a simple optical system that zooms at the same time as the main lens and has an optical path that runs parallel to the camera's main lens. These viewfinders are small and their biggest problem is framing inaccuracy. Since the viewfinder is positioned above the actual lens (often there is also a horizontal offset), what you see through the optical viewfinder is different from what the lens projects onto the sensor. This "parallax error" is most obvious at relatively small subject distances. In many instances the optical viewfinder only allows you to see a percentage (80 to 90%) of what the sensor will capture. For more accurate framing, it is recommended to use the LCD instead. For those who wear corrective glasses it's worth checking to see if the viewfinder has any diopter adjustment.

Because the optical path of the viewfinder runs parallel to the camera's main lens, what you see is different from what the lens projects onto the sensor. Sometimes optical viewfinders have parallax error lines on them to indicate what the sensor will see at relatively small subject distances (e.g. below 1.5 meter or 5 feet).

LCD on a Digital Compact Camera (TTL)

The LCD on a digital compact camera shows in real time what is projected onto the sensor by the lens and therefore avoids the above parallax errors. This is also called "TTL" or "Through-The-Lens" viewing. Using the LCD for framing will shorten battery life and it may be difficult to frame accurately in very bright sunlight conditions, in which case you will have to resort to the optical or electronic viewfinder (see below). The LCDs on virtually all digital SLRs will only show the image after it is taken and give no live previews.

Example of digital compact with a twist LCD

Optical Viewfinder on a Digital SLR Camera (TTL)

The optical viewfinder of a digital SLR shows what the lens will project on the sensor via a mirror and a prism and has therefore no parallax error. When you depress the shutter button, the mirror flips up so the lens can expose the sensor. As a consequence, and due to sensor limitations, the LCD on most digital SLRs will only show the image after it is taken and give no live previews. In some models this is resolved by replacing the mirror by a prism (at the expense of incoming light). The optical viewfinder normally also features an LCD "status bar" along the bottom of the viewfinder relaying exposure and camera setting information.

The optical TTL viewfinder allows you to look "through the lens" Optical TTL viewfinder on SLR with diopter adjustment (slider on the right side)

Electronic Viewfinder (EVF) on a Digital Compact Camera (TTL)

An electronic viewfinder (EVF) functions like the LCD on a digital compact camera and shows in real time what is projected onto the sensor by the lens. It is basically a small LCD (typically measuring 0.5" diagonally and 235,000 pixels) with a lens in front of it, which allows you to frame more accurately, especially in bright sunlight. It simulates in an electronic way the effect of the (superior) optical TTL viewfinders found on digital SLRs and doesn't suffer from parallax errors. Cameras with an EVF have an LCD as well, but no true optical viewfinder.

Example of an electronic viewfinder
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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