With the imminent launch of two brand-new flagship sport/wildlife cameras from Canon and Nikon, I find it interesting to think a little about what we might expect from the two new cameras.
I believe that for those of us who shoot wildlife or any subject where getting closer is important, that the camera sensor is of great importance. The size of the individual pixels on that sensor will have an effect on how large ones subject finally is. Smaller individual pixel sizes usually translate into bigger subjects which will become apparent if the images are cropped to the same number of pixels. On the other hand, bigger pixels, given the same or similar state of technology as the smaller pixels, usually end up providing images that appear to be lower in noise, and of better quality. Of course, there is more to image quality than just pixel size and all sizes of dslr pixels have been getting gradually better every few years as computer tech and hardware like circuitry gets better and smaller. In a very broad sense, one should consider whether a bigger subject size is more important than low-light image quality. There are of course many other considerations including final output size - big prints or computer screen? Cameras with many megapixels are better for large prints, those with fewer megapixels usually sufficient for all else.
Given my thoughts above, when one looks at specifications of the Canon 1DX and the Nikon D4, one will notice that the Canon has individual pixels of 6.95 microns in size, whilst those of the Nikon are 7.3 microns in size. So I am going to guess that given how close their pixel sizes are, that image quality is going to be similar between the two. Perhaps the D4 may be slightly cleaner of noise at really high iso, and the Canon will definitely be able to produce either a slightly bigger subject if cropped, or a slightly bigger print, but in my opinion these two cameras are going to be very close in terms of image quality. I would even guess that image quality/subject size will no longer be deal-breaking or brand-switching differences between the two cameras as they may have been in the past. Differences may be apparent in AF, buffer rates, video, battery life etc, and only time will tell.
However, it becomes equally if not more interesting to think about what differences users of the current pro sport/wildlife bodies might expect, the Canon 1Dmk4 and the Nikon D3s.
Users of the Canon 1Dmk4 will be replacing their 16mp sensor (which has a 1.3x field of view crop) and smallish pixels of 5.7 microns in size, with the full-frame 1DX sensor and its pixels of 6.95 microns. Almost certainly there is going to be a reduction in noise at similar iso settings with the new camera, perhaps as much as one full stop or even a bit more. In other words, the 1DX should show similar noise levels at iso1600 or even lower perhaps, than the 1Dmk4 does at iso800. This will be due to the larger individual pixels (and larger sensor surface area) and two and bit years plus of sensor tech advances. On the opposite side, the subject will be smaller in the frame, given the same lens and shooting distance. With the 1DX, one will need to either mover closer to the subject, and if this isn't possible, then a bigger lens will be needed to match the 1Dmk4 subject size. You also have a little more cropping ability with the 1DX 18mp resolution but not enough to make up the difference in subject size that will result from the significantly smaller individual 1Dmk4 pixels.
Current Nikon D3S users, make use of a full-frame camera with 12 mp and an individual pixel size of 8.45 microns. As the D3S has sensor technology just one generation, (say roughly three years) old so the D4 will benefit from the latest advances in sensor technology. The D4 has smaller pixels than the D3s, reduced from 8.45 microns to 7.3 microns. Usually bigger pixels outperform smaller pixels in low light, but Nikon will have made use of tech advances to improve the efficiency of the new, smaller pixels. Given how steady and incremental such improvements have been though, I would guess that the D4 will perhaps match the D3S when it comes to low light/high iso performance. I do not expect it to produce RAW files that are better in terms of noise and image quality than the D3S at the same iso levels. If it does indeed match the D3S then I would take that as a very favourable result, given that the D3S is held in high regard for its low light performance. When it comes to subject size the D4 is superior to the D3S, as its smaller pixels mean that one could crop a 16mp file from the D4 to 12mp (the same as the D3S) and end up with a bigger subject. On the other hand, those users who need bigger prints would benefit from the extra 4mp. And D4 users could be a little further from their subject and crop it closer than the D3S.
When I write of image quality here, I prefer to really make use of RAW images for reference. I find that the high-iso jpg images posted on the internet that come from the in-camera jpg processing engines to have had so much noise reduction applied globally that fine detail is smeared right out.
What I have written here is of course my own opinion, and much of it is also easily open to misinterpretation. I have tried to share my thoughts as clearly as possible to avoid this. And I am not saying Canon or Nikon is better. I am saying that understanding some of the differences between dslrs cameras, especially with regard to their sensors characteristics, can be important in selecting a camera for particular applications. Being a nature photographer just gets better and better!