Kodak DCS620x Review, Phil Askey, June 2000

Say hello to Kodak's latest Professional Digital Camera, the DCS 620x. You'd be forgiven for doing a double take as this is (purely from a body, photographic and controls point of view) identical to the DCS 620 (which I reviewed last year).

So the big question is, what's new about the DCS 620x? Well essentially Kodak are targeting their largest audience for the camera, and that's news photo and sports journalists, they've enhanced the camera to perform at amazingly high ISO levels (400 - 4000, at a push 6400). This is incredibly important to working in medium / low "available light" situations where you still need high shutter speeds to capture "that moment" (think floodlit football, basketball, ice hockey, a dusk shoot-out in a war torn country).

Kodak have done several things to produce such high sensitivity, to understand this totally we have to go a little deeper into the technical details of how digital cameras capture images.

Full Frame vs. Interline Transfer CCD's

Interline Transfer CCD

Your typical consumer digital camera uses what is called an Interline Transfer CCD, put simply the CCD can itself control the start / stop of when it measures light falling on it, otherwise known as an electronic shutter, it does so by shifting values out of the photodiodes into "shift registers" then pushing all of that data out as a final image. The advantages to Interline Transfer CCD's are simply that, they can be controlled by software and don't require a mechanical shutter (though are often used in conjunction with one) and can produce a video feed output (a requirement for a live preview LCD feed).

Because of the extra electronics required around each pixel the "fill factor" (size of the photodiode) tends to be quite small (about 30% of the pixel area). To get around this Interline Transfer CCD manufacturers place a layer of "microlenses" (click here for electron microscope image of microlenses) over the CCD to capture more light and focus it onto the smaller photodiode area which then gives them an better effective fill factor of about 70%.

Full Frame Transfer CCD

Kodak's professional CCD's are Full Frame Transfer, they don't have a shift register, this means that a mechanical shutter is absolutely required to control the start / stop measurement of light. The shutter is opened and then closed again (say 1/60s later), the whole CCD shifts data off itself into the serial register where it's processed as the "RAW" image. As Full Frame CCD's are simpler (don't have shift registers and associated electronics around each photodiode) they have a much better Fill Factor (around 70%) and don't require or use microlenses.

The disadvantage is that you can't get a video feed out of them which is the main reason we don't see more manufacturers using Full Frame CCD's (we're all too used to our LCD preview).

Pro's & Con's associated with each CCD type:

Colour Filter Arrays (CFA's)

Photodiodes, the tiny light sensitive materials used to measure the amount of light for each pixel on a CCD are essentially monochrome devices, that is they can't themselves tell the difference between different wavelengths of light. To produce a colour image a CFA (Colour Filter Array) is placed over the monochrome sensor pixels, in reality the CFA is made up of very thin layers of coloured dye. This CFA filters out all but the chosen colour for that pixel. Software interpolation later produces a full colour for a pixel based on the value of surrounding pixels.

The GRGB Bayer Pattern is the most common CFA used, it was used by Kodak on the DCS series cameras and is used in most consumer digital cameras (except for Canon up until the S100 who used a CYGM CFA). A primary colour (GRGB) Bayer Pattern is produced by placing two layers of coloured dyes over each other as such:

	Red = Yellow + Magenta
	Green = Yellow + Cyan
	Blue = Magenta + Cyan

Kodak's new CMY CFA

One way to increase the sensitivty of a CCD would be to do away with the CFA, unfortunately if we did that we'd have a monochrome sensor, but there is another answer. Kodak Professional decided they would produce a CCD with a CFA which uses only one layer of dye using the three primary additive colours, Cyan Magenta and Yellow, this would of course results in a more sensitive imager (less light sapping dye over each pixel). The new CCD offers a wide ISO range (400-4000), 8-9 stops of dynamic range and with the addition of new lower noise electronics and a 12-bit ADC the overall result is the most sensitive CCD yet available.

Kodak's new CMY CFA CCD

In addition to the updated CCD and electronics Kodak have also introduced a noise reduction algorithm into their "Acquire" software (TWAIN driver) which is required to open the RAW TIFF files the camera produces. This (optional) filter removes noise and effectively cleans up very high ISO or noisy images.

As I have previously reviewed the DCS620 I'll be covering the camera itself in less detail and the image quality / noise reduction in slightly more detail, that said I'll still cover the major features and controls of the camera.

If you're new to digital photography you may wish to read the Digital Photography Glossary before diving into this review (it may help you understand some of the terms used).

This review is Copyright 2000 Phil Askey and the review in part or in whole may NOT be reproduced in any electronic or printed medium without prior permission from the author. For information on reproducing any part of this review (or any images) please contact: Phil Askey.