However, this reminds me of the fact that the 645D, like all other K-mount Pentax DSLRs, is actually also cropped for its sensor. So, I just wonder, how is the 645D different from those 135 FF DSLRs and how "large" is the difference? And besides, what are the trade-offs?
Here we go, for the factual data:-
| Camera | Sensor (Film) Type | Sensor (Film) Size (in mm x mm) | Diagonal Length (in mm) | Crop Factor A | Crop Factor B | Crop Factor C |
|---|---|---|---|---|---|---|
| 645 Film | 120 | 56 x 41.5 | 69.7 | 1X * | 0.79X | 0.62X |
| 645D | Cropped 120 | 44 x 33 | 55 | 1.27X | 1X * | 0.79X |
| D800/E | 135 | 36 x 24 | 43.3 | 1.61X | 1.27X | 1X * |
| K-5II/s | APS-C (Cropped 135) | 23.7 x 15.7 | 28.4 | 2.45X | 1.94X | 1.52X |
Is everything much clearer now? The myth has been resolved!
In fact, the 645D is the "APS-H" of the original 120 film with a crop factor of nearly 1.3X. The same applies when the 135 FF is compared to the 645D, which has the exact crop factor in 1.27X. So, how much advantage does it give?
And, *suppose* that there is a Full Frame 645D which exists, the 135 FF will then become a 1.6X "Canon APS-C" cropped machine in comparison, then this difference would be more significant!
On the other hand, the truly APS-C sized K-5IIs for its sensor, when compared to the 645D, is really sympathetic! It is almost in 2X, just like when the micro-4/3 sensors are compared to the 135 FF. Actually, the APS-C sensor is much smaller than the 135 FF, its total area of the sensor is just only in 43%!
Well, what are also in return with that cropped sensor in the old 645 form factor for the 645D then? A big, heavy, slow but rather expensive camera, but which is actually NOT having a significantly larger sensor, and thus no big edge in IQ as a result!
Indeed, cropping an old sensor/mount format is always stupid in the digital era, unless new lenses with a smaller image circle which fits more tightly into that smaller image sensor are re-designed and made!
This is the end of case and the case is closed! The 645D now is a meaningless offer after all! If Pentax is listening, please give us Full Frame DSLR bodies soonest, no matter for the K or 645 mounts!! >:-(
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