Eyes in the dark
Darkness is a common failure of LCDs and EVFs. This seems obvious because night-photography requires exposures of several seconds or even minutes. Since a camera's sensor is read every fraction of a second to produce the view on the LCD, it does not have time to accumulate sufficient light to produce a comfortable preview. Sadly this is the case of most LCDs and EVFs on cameras available today.
To allow the use of LCDs and EVFs in the dark, a few companies have implemented a system which amplifies the sensor's output to produce a brighter preview. The one company that has managed this far better than all others is Konica-Minolta. Several of its prosumer digital cameras amplify the brightness of their displays and, in very low light, turn the previewed image into a monochrome image (black and white images have higher contrast than color images). This corresponds well to the behavior of the human eye in low-light because low-light sensors in our eyes are achromatic (they don't see colors).
As for optical viewfinders, they are ideal for low-light photography because our eyes adapt well to low levels of lighting. This is done in part by using sensors which are very sensitive to light but not sensitive to colors.
Of all the types of viewfinders, the LCD is the only one which is positioned differently. While it may not seem important, those who have frequently used an LCD for composing a picture will disagree. Precisely framing a picture using an LCD is more difficult than with any other type of viewfinder. This occurs because when using other viewfinders the photographer is looking from (or close to) the point of view of the camera. It is easy to frame this way because we are almost seeing the picture. When using an LCD, what we see is an LCD which is showing a picture. Most people who frame using an LCD complain about having difficulty keeping the horizon horizontal. Some digital cameras optionally overlay a grid to help keeping the camera level. Most LCDs exhibit another problem: difficulty to see in bright light. Since LCDs are open to ambient light, they can reflect light which makes the image almost impossible to make out. An anti-reflective coating on an LCD can diminish that problem. Some LCDs reflect ambient light from behind the display which greatly diminishes the problem. Such LCDs are called transreflective displays. Only a few camera models have trasreflective LCDs.
The position and size of LCD displays provide a unique advantage among viewfinders: an LCD can be seen without having the camera right against our eye. Although it was mentioned earlier that this makes framing more difficult, some shots may not be possible to frame using any other type of viewfinder. Indeed poor framing is better than no framing. After a picture is taken its framing can be possibly improved by cropping. Rotational corrections however damage the quality of digital pictures, so it is most important to keep shots level. Many cameras are equipped with LCDs which can be moved and rotated in various ways which augment framing possibilities: overhead shots, ground level shots, self portraits, etc.
The length alone of this article demonstrates that issues surrounding viewfinders are numerous. Each type of viewfinder has its advantages and each type has its disadvantages. For those who have been keeping score, it seems that EVFs and SLR viewfinders have the most advantages. The principal trade-off between the two is exposure preview versus precision. Most photographers will prefer SLR viewfinders because that is what they are used to. Photographers from the film days have all gotten used to not being able to preview exposure.
Choosing a viewfinder can't be done without choosing a camera. This is where choice becomes compromise. Digital SLR cameras all have SLR viewfinders, by definition. They also have an LCD but not for preview. Long zoom (6X or more) fixed lens digital cameras all have EVFs, so do very wide angle fixed lens cameras. There was a fixed lens digital SLR by HP, but it has been long discontinued. In the end, the needs of a photographer will dictate the type of camera he uses. This choice will then dictate the types of viewfinders available. Only then will knowing how each type of viewfinder works help a photographer choose a specific camera. For a photographer the experience given by a viewfinder is important, unfortunately the choice is not completely up to him.
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