Thank you Ray for bringing us back to the topic raised by the OP.
Tony,
I'm not at all sure this is the case, that the D300 would be sharper. If one considers the situation where maximum DoF is required (without softening the image too much with diffraction effects), then one is comparing, say, F16 with the D3 and F10 or F11 with the D300. Because the D300 has greater pixel density, it needs the higher MTF of a lens at F11. Therefore, the D3 should deliver approximately the same resolution at F16 as the D300 at F11, as well as the same DoF.[{POST_SNAPBACK}][/a]
Well, if you are comparing single frames then they should be about the same, but if you stitch three images together from the D300 (recall that we were comparing 14mm on FX to 14mm on DX and I'm making an allowance for necessary overlap) then you will have more resolution from the DX camera by virtue of the stitching. Now you can zoom out and stitch with the FX, but I think the images will mostly be indistinguishable from one another except at some extreme level of tonal detail that probably exceeds what is printable.
If one moves to the other end of the range of apertures, and compares the D300 at F2.8 with the D3 at F4.5, then it's likely that the lens used will be sharper at F4.5 than it is at F2.8. In this situation, the D300 needs a lens at a sharper aperture but is in fact getting the opposite, a lens which is less sharp at the aperture required for equivalent DoF.
Yes, there is no doubt that at the other end of the aperture spectrum the FX sensor has the advantage. However, at what price and are you further ahead buying the more expensive camera and using cheaper lenses or buying more expensive lenses and using the cheaper camera? My bet is that a cheaper lens on an expensive camera delivers less than an expensive lens on a cheaper camera (considering that were comparing the D300 to the D700 or D3).
However, it might be the case that, irrespective of lens quality at particular apertures, the wider aperture that the D300 can always use for an equivalent DoF, allows for use of a faster shutter speed, and as we all know, shutter speed is often critical for a sharp result.
On the other hand, it seems quite clear that the D3 has lower noise at high ISO than the D300, just as the 5D has lower noise perhaps to a lesser extent (because it's older technology), than the 40D and 450D, and not just at high ISO.
I believe this gets to the crux of the OP's question. Are you really gaining much if you end up dialing in a higher ISO? Perhaps you are and that will depend on where you start and the quality of the light -- the D3 and D700 seem optimized for artificial lighting, the D300 also seems to be but not as much so. If you want to shoot in lowlight situations, have excellent lenses, and can afford it -- get the D700 or D3; if reach is a consideration, and related to that money -- then a D300 might be better, especially if there are improvements that can be purchased by spending on lenses.
It would be interesting to compare a D300 image at F2.8 and ISO 200 with a D3 image at F4 and ISO 400, using the same lens. Shutter speed and DoF should be the same, but what about resolution? My bet is, the FX sensor would deliver better results.
Zoom or prime? Using the same focal length and moving forward or back to maintain FOV but change perspective? Using the same focal length and standing in the same spot and changing FOV to maintain perspective? You see, now you have opened up a real can of worms -- comparing primes in different formats are apples to oranges comparisons. Here's a thought though, lets compare the same lens standing in the same spot with the same perspective -- in other words, lets compare zoom lenses. Start with a very nice moderate tele-zoom: [a href=\"http://www.photozone.de/nikon--nikkor-aps-c-lens-tests/247-nikkor-af-s-70-200mm-f28-g-if-ed-vr-review--test-report?start=1]Nikkor 70-200/2.8 VR.[/url] Wide open on DX compared to f/4 on FX (with adjustments to focal length to get equal perspective and FOV), FX has an advantage with this lens, but that pretty much goes away comparing f/4 to f/5.6. Looking at the premier mid-range zoom:
Nikkor 24-70/2.8 the two formats are pretty much indistinguishable even wide open, but that lens is just ridiculously good.
Sharpness we can measure, noise too; although measuring these can become more complicated than most assume by just looking at published charts. There are intangible qualities too, and there the larger format usually excels. At the extremes there are trade-offs that make the formats comparable, with an advantage to the larger format at the wide end and an advantage for the smaller format at the longer end, and in the middle I would choose FX. Yesterday though, Thom Hogan wrote at DPR that he would choose two D300 cameras and a GX-100 for a trip to Africa, that's his choice and YMMV. There are a lot of considerations that make a blanket FX format is always better than DX format both simplistic and sometimes just wrong.