Digital medium format cameras up till now have only been CCD chipped (Leaf did have an unsuccessful dab at CMOS in 2001) so this really is quite a departure from design fundamentals. Why you may ask?
Quick answer - Higher ISO's
The older generation of pros who moved from film to digital were never affected by the low ISO values of digital backs as our backgrounds were such that shooting film at sub 100 ASA's (digital ISO) was the norm. The first Leaf digital back I owned had a native ISO of 25 and a maximum of ISO100, I recall.
A whole generation of photographer exists now that have never used film at all.
That fact coupled with the way that useable ISO has been increasing as DSLR manufacturers dropped CCD (Canon in 2000) and focused on CMOS has meant a development of a new style and method of capturing images with available light. Does this mean though that finally the CCD will be replaced in high end photography? To draw your own conclusion let's get a tad technical.
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| Simon Stewart©2014 |
Both CCD and CMOS chips convert light into electric charge and process it into electronic signals, that's where the similarity ends.
A CMOS sensor outputs digital bits and a CCD outputs an analog signal in the form of voltage.
In a CCD sensor, every pixel's charge is transferred through a very limited number of output nodes (often only one or two) which causes a bottleneck when the analog signal is being sent off chip for processing.
The upside of CCD's though is that all of the pixel can be devoted to light capture and the output's uniformity (a key factor in image quality) is high. As the CCD requires fewer on-sensor electronics this means that each photosite is bigger and can hold more charge. The result is superior image quality with better dynamic range and more accurate colour rendition than CMOS. So a CCD should have better low light performance as well? It would if the processing speeds were higher.
In a CMOS sensor, each pixel has its own charge-to-voltage conversion, so that the chip actually outputs digital bits not analogue voltage like a CCD. With each pixel doing its own conversion, uniformity is lower, but it is also massively parallel, allowing sizzling speed.
The contradiction here is that the less noisy CCD with bigger photo sites should have better low light performance. Not so - the advantage is that CMOS sensors not only have their own charge-to-voltage conversion but the sensor often also includes amplifiers, noise-correction, and digitization circuits, adding gain to an image. Circuitry is so close to each pixel, the camera can boost the exposure as it is recorded. Microlenses on a CMOS chip assist photons find the photosites so they aren't "wasted" striking the other sensor components found on CMOS sensors. Tweaked CFA's (colour filter arrays) help improve ISO's albeit at the expense of perfect colour rendition.
The result is that faster on chip processing of a CMOS sensor produces better low light images than a CCD but not necessarily better IQ or CR.
Developments in the world of digital Medium Format are always slower and more deliberate than in 35mm. It really is an expense thing. At $35 000 a pop for a new digital back photographers are inclined to hold onto their equipment as long as it is delivering and meeting clients expectations.
Simon Stewart
Simon Stewart
