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Author Topic: Does eliminating film backs allow larger sensors  (Read 1380 times)

BJL

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Does eliminating film backs allow larger sensors
« on: May 21, 2008, 04:24:16 pm »

I have some questions, related to speculations as to whether sensors for medium format cameras will increase beyond the height limit of about 37mm that has held constant in the last three generations of sensors (first 37x37mm, then 37x49, then the same 37x49 with more pixels.)

I have read that some 645 medium format cameras have a film gate with an opening of about 42x56mm (true 645 dimensions) that the front of the sensor must fit through. Since there is some non light sensitive stuff around the edges of a sensor, this would limit sensor active area to something smaller than 42.5x56. The height limit might be about that 37mm, and so might explain the apparent "freeze" at 37mm maximum sensor height. The limit might be a bit higher but close enough (like under 40mm) that there is not a great incentive to increase sensor sizes by the small amount allowable, especially if the cost is fitting less chips per wafer.

Some concrete numbers: the Kodak 39MP /KAF-39000 sensor
http://www.kodak.com/ezpres/business/ccd/g...000LongSpec.pdf
has active sensor area 36.7x49mm with a frame around it of dimensions 40.6x53mm on a total device size of 51x59.6mm. If that "frame" is flush with the sensor and must fit through the film gate while the rest of the chip is recessed and so can lie behind the gate, as appears to be in pictures, then it is the 40.6x53mm.


Could the future widespread adoption of MF bodies designed solely for digital usage eliminate this restriction, and allow sensors to grow to fill more of the image circle of lenses designed for formats like 645?


One guess is that the MF market is so small that to be vaguely cost effective, DMF sensors must be usable in a large proportion of current models of MF body, so that as long as a great proportion of MF bodies are designed for use with 645 film and have this film gate, sensors will stay small enough to fit the gate. But the H3DII at least is digital only, so maybe the size limit will increase as digital only designs become the norm for 645 format bodies. (For the same reason, I doubt we will see sensors not usable in the dominant 645 format cameras, like ones fitting only 6x6 or 6x7 bodies.)
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