Friday, 19 September 2008

The revolution of viewfinders

Maybe a new war has been started by the announcement of the Micro FourThirds system. Some years ago it was unimaginable that a professional or an advanced-amateur digital camera existed without an optical TTL viewfinder. The evolution of displays however forces us to rethink this assumption. In order to do this, let us take a look at the types of electronic viewfinders.

The most compact digital camera has conventional colour LCD in their EVF. In these displays, every pixel consists of three dots: red, green and blue consecutively. It is obvious that the whole display has to contain as many dots as the necessary amount of pixels multiplied by three.


conventional colour LCD basics

LCDs have not got light emission in themselves so back-illumination is necessary to see the image. Cold-cathode light tube or white-LED are usable for illuminate. The mostly used LCDs have no more than 200K dots (300x225 pixels x3) so they can not substitute optical viewfinders.


concept of LCoS display

LCoS (Liquid Cristal on Silicon) is a special kind of LCD. The liquid cristal is placed between metal electrodes and a glass. These electordes reflect the light coming through the liquid crystal. Unlike the previous case, LCoS is illuminated from its front side by LEDs. Due to the polarizing beam splitter (PBS) the LEDs do not have to be placed in front of the display. In colour LCoS the display panel is monochrome, but the color of illumination alternates. If the frequency is high enough, the human eye can not sense this flickering, so all we can see is a still colour image.


schematic diagram of a LED illuminated colour, LCoS based EVF


The main advantage of LCoS display is the high resolution in small size and high pixel fill factor. The image is very sharp and there is no visible pixelizing. The first LCoS' had less contrast than conventional LCDs but modern types have not got this disadvantage.


colour EVF (LCoS) simulation - it is three times faster in reality
(you can stop it in pop-up menu of flash player - right mouse button)

After equivalent focal length and equivalent magnification we have to learn a new abstaction, the equivalent amount of dots. The camera manufacturers never talk about real pixels, only number of dots is represented in data sheets. This is misleading however, because we have to divide this figure by three to get the usable number of pixels. There is no other way for manufactures who use LCoS EVF than representing the resolution of a virtual LCD which has same amount the pixels. In other words, they have to "upscale" their actual number of pixels, because they have no control over the communication strategy of other manufacturers.

Photographers often sceptical about the usability of EVFs, but the fact that the cinematography industry has embraced them shows that their conquest is not questionable. The main problem is that demanding photographers have never met an usable one, because EVFs are built into compact cameras (and into high-end cine cameras, such as the Panavision Genesis, unknown to most photographers). The Panasonic G1 is only a first, but significant step. Its 1.44 MegaDot equivalent (800x600 pixel) LCoS EVF is unusual among digital still cameras, but much higher resolution (>1 MPixel) is also available on the market. Maybe we will meet them in the higher category of Micro FourThirds cameras.

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