(Image Source Page: http://www.pentax.jp/japan/imaging/digital/medium/645d/feature_1.html)

Since the 645D was announced, people, in particular Pentaxians, have been arguing about the advantages against the shortcomings of the new 645D when it is compared against other DSLRs of different sensor sizes, pixel counts and form factors. I think it would be of no harm for me to list out the factual data of some of the sensors used by some of the current popular DSLRs that could be compared, for an overview on the basic physical and specification differences, before heat debates and more discussions are to be carried on..

Camera / Digital Back	Sensor Size (in mm)	Sensor Area (in mm^2)	Resolution	Aspect Ratio	Total Mega Pixels	Pixel Size (in um)	ISO Range
Pentax 645D	44 x 33	1452	7264 x 5440	4 : 3	40.24	6.1 x 6.1	200 - 1000 (100 - 1600 Expanded)
Mamiya DM40	44 x 33	1452	7350 x 5562	4 : 3	40.88	6.0 x 5.9	80 - 800
Mamiya DM56	56 x 36	2016	9288 x 6000	3.1 : 2	55.73	6.0 x 6.0	80 - 800
Mamiya DM33	48 x 36	1728	6726 x 5040	4 : 3	33.90	7.1 x 7.1	50 - 800
Canon EOS 1D3 / 5D2	36 x 24	864	5616 x 3744	3 : 2	21.03	6.4 x 6.4	1D3: 100 - 1600 (50 - 3200 Expanded) 5D2: 100 - 6400 (50 - 25600 Expanded)
Nikon D3X	36 x 24	864	6048 x 4032	3 : 2	24.39	6.0 x 6.0	100 - 1600
Nikon D3S / D700	36 x 24	864	4256 x 2832	3 : 2	12.05	8.5 x 8.5	D3S: 200 - 12800 (100 - 102400 Expanded) D700: 200 - 6400 (100 - 25600 Expanded)
Pentax K-x	24 x 16	384	4288 x 2848	3 : 2	12.21	5.6 x 5.6	200 - 6400 (100 - 12800 Expanded)

So, for anyone who is interested to compare, now you can compare all these parameters and calculate their ratios and differences in percentage and etc.

Do it endlessly till the end of the days, which is just another kind of measurbation! ;-D Enjoy!

자료 출처:
RiceHigh's Pentax Blog
http://ricehigh.blogspot.com/2010/03/comparison-of-sensor-specs-645d-against.html