"So basically they have a higher concentration of pixels in a smaller area allowing for greater magnification in printing and post production. "

Yes.

What they should say is that their D300 with a 600mm lens has the same field of view as a D700 would have with a 900mm lens. Since both cameras have 12MP the D300 images appear to have more magnification, because they are enlarged more, than the D700 image at the same focal length.

"Albeit, possible with greater noise due to the AP-C Sensor."

The smaller sensor will have more noise at high ISO settings. The difference is minimal up to about ISO-800 above that is where the FX sensor shines.

Dave
Quad Cities, USA
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