You won't?? Even if you use say a 7D and a 5D2 and the 7D sensor is more efficient at collecting and converting photons per area of surface than the 5D2?? With the 7D you can chose to get either: more detail (unless conditions are super bad) and more noise OR slightly better detail with less de-bayer and other artifacts and slightly better noise (if you view or convert to same scale as the 5D2).
Unfortunately it doesn't work that way in crepuscular conditions. You can have all the "pixels on target" you want, but if the sensor can't handle the low lighting (7D), you're not going to get the shot. And by "shot", I mean something you can print at 16x20.
When shooting in RAW during the November white-tail rut in Montana, my 7D becomes almost useless. The sun comes up at 8:30, and light gets shaky around 4:30 thanks to consistent, thick cloud cover. It's often snowing or raining. Once I start hitting 1600 ISO on the 7D, it's time to put it away. Out comes the full frame, where I can get usable images at ISO 12,800. Not to mention that my other cameras do a much better job of focusing on low contrast targets (brown deer with a brown background) than the 7D.
In these cases, noise is the bottleneck.
Regarding birds and DR...to be honest, I have not found that dynamic range is the issue when photographing birds.
So two golden eagles swooping in to take out a bald eagle at your back, silhouetted in the sun doesn't present a DR issue? What about bighorn rams fighting each other in uneven forest light? People wait all year for those moments, heck, they wait years. A second later, it could be gone.
Dynamic range is the single biggest issue with wildlife photography, IMHO. That's why the shadow recovery in the Sony sensors is so appealing.
Anyway, when it comes to bird and wildlife photography, dynamic range is just not an issue.
I completely disagree. It's the issue. Are you going to sneak around a grizzly bear in the bushes to get the right angle? (that's a great way to get yourself killed). Or how about tramping in the willows on a mountain lake to get just the right light on a bull moose? (another good way to get killed). What about when a squirrel decides to watch sunrise over Glacier Point in Yosemite? Are you going to command the sun to rise from the west so you can get the good light? If you are shooting tame birds or zoo animals, maybe it's not much of an issue. But for actual wildlife? Top of the list.
The only one with the control in wildlife photography is the animal. They do what they want, when they want, and under the lighting conditions they see fit. It's your job to take the punches and get to the 12th round.
I decided to combine your two posts, because I think the context of the two matter here. First, you talk about crepuscular conditions. You have brought that up a couple times in the past as well, and it is a valid point. However I think it is a point at odds with the points you make in the second post.
In crepuscular light, the low light around sunrise and sunset, you are NOT going to be using ISO 100 or 200. As you say, your going to be up at ISO 12800. You need the high ISO so you can maintain a high shutter rate, so you can freeze enough motion to get an acceptable image. There are times during the day when you can capture wildlife out and about, but the best times are indeed during the crepuscular hours of the day.
Just for reference, here are the dynamic range values for four key cameras at ISO 12800:
5D III: 7.8
1D X: 8.8
As far as dynamic range for wildlife and bird photography during "activity hours" goes, there is no question the 1D X wins hands down. It's got a 1.5 stop advantage over the D800/D810, the supposed dynamic range kings. At ISO 6400, we have:
5D III: 8.5
1D X: 9.7
At ISO 3200 we have:
5D III: 9.5
1D X: 10.5.
At ISO 1600 we have:
5D III: 10.1
1D X: 10.8
And finally, at ISO 800 we have:
5D III: 10.5
1D X: 11.1
It is not until we reach ISO 800 that the D800 series cameras start to close the cap and overtake the high ISO advantage of Canon cameras high ISO DR. It is certainly possible to shoot at ISO 800, and ISO 400, in the hour leading up to sunset...I have shot at those settings myself. However shadows are long during that hours, and it is more common that I am shooting at a higher ISO. It is only the hour or two around the two-hour period of midday that I ever find myself shooting at ISO 100 or 200...and then, it is rarely with fast moving subjects like a Golden Eagle flying directly at me to fight with another bird behind me. I'd again be at a higher ISO to guarantee I have the shutter speed I need to capture that action with just the right amount of motion blur in the wing tips, but otherwise freezing the motion of the bird itself.
Regarding pixels on subject...I'm confused why you would say that doesn't matter. If you increase the size of your subject relative to the size of your pixels, that means that the frequency of the noise becomes higher and higher relative to the subject. It really doesn't matter if you are at a higher ISO or not...the frequency of noise is still based on the pixel pitch. If I take two shots of a deer, at ISO 12800, and in one the deer fills 33% of the frame and in the other the deer fills 66% of the frame. Which image is going to have better IQ? The one where the deer fills 33%? No, of course not. The image where the deer is larger in the frame...the image where there are "more pixels on the deer", is going to have the better IQ. The deer is much larger, so all that ISO 12800 noise is going to be less intrusive, as in terms of relative frequency, the noise is much smaller.
Whether the 7D is useful or useless depends on exactly that. Pixels on subject. I've been able to make very good use of my 7D under very difficult lighting conditions by taking the time to get the subject framed right, and getting it frames large enough. Even at very high ISO:
This night heron was shot at ISO 3200, but underexposed by about a stop (I'd had my 7D configured to limit which ISO settings would be automatically selected at the time, and I was shooting Auto ISO). The shutter speed was 1/6th second. This was with the 600mm f/4 L II, well after sunset...blue hour was on, and there were dark clouds blocking a lot of the remaining post-sunset light. The only reason I was able to make anything of the shot and still have this much detail was because I managed to get enough pixels on the bird that it could withstand the editing.
So, first off, I do not believe that the argument that the enhanced DR of the Sony sensors is useful when it comes to wildlife or bird photography. On the contrary, given the measurements, it seems like the Canon cameras do indeed have the DR edge, especially the 1D X, and especially at the higher ISO settings that are critical during the hours wildlife is most active.
To address a couple points more specifically. The silhouette scene...a bird silhouetted is a bird silhouetted. Either it is a dark subject against a very bright background, or it's not. If we are talking about something where a bird is silhouetted against bright sunlit water, even a D800 is going to struggle with that....assuming you even had the option of shooting at ISO 100. The likelihood is that your using a much higher ISO (pretty much GUARANTEED in the "two golden eagles flying in with the sun at their backs" case) in which case there is no advantage to using a D800 or having more DR. In that situation, you do something like this:
You make the images REAL silhouettes...pure shadow superimposed over a brighter background.
When it comes to other animals, like bull moose or elk, bears, or the squirrel that wants to watch sunset. Well, starting with the squirrel...I would certainly do what I could to get on the western side of it. I mean, we are talking about good lighting here, lighting that does your subject justice. Aside from silhouetting your subject, shooting from the back side directly into sunlight is not the most flattering light for a wildlife subject. It might make for one interesting photo one time, but in general, I look for the scenes and angles where my subjects are better lit. The sun does not necessarily need to be directly at your back. Actually, having the sun DIRECTLY at your back is not good either, as it flattens your subject. There needs to be a certain angle, and sometimes you can suffer a little bit of loss on the DR front (i.e. end up with slightly too much DR in the scene than you can really handle) in order to get a shot that might otherwise not be possible.
Regarding moose, elk, deer, etc. I absolutely do what I can to get a better angle on them. But you pick your battles, for sure. Deer, elk, etc. are quite docile during the earlier parts of the year. It is only really during the rut and their mating season that they take on a hostile stance. That is where having a big long lens, and some TC's and, maybe even, a cropped sensor, become really handy. They give you a much more comfortable working distance when photographing rutting wildlife.
Here is an example where I did what I could to get the right angle of light, during crepuscular hours. Light still coming from more of an angle than I wanted, the elk's body is decently lit but the beard is more shadowed than I wanted, but shot still came out quite nicely. In this particular situation, for the given exposure, a D800 would not have offered me much in the way of advantages over the 7D...I was reach limited, I was at very high ISO. The D800 might have had slightly more DR, but less resolution, and similar per-pixel noise...but a considerably slower frame rate.
So, at least in my experience, given that I rarely have the opportunity to shoot wildlife of any kind at ISO 100, and were not talking about zoo animals here, but real WILD life...I do not believe DR is a critical issue with wildlife photography. You simply don't have 12 or 14 stops at ISO settings from 400 and up. At the really high ISO settings we use during crepuscular hours, ISO 3200, 6400, even 12800, we might not even have EIGHT stops of DR! I also do not believe it is impossible to situate yourself, the photographer, at the right angle to your subject such that it is properly lit, therefor minimizing or eliminating DR issues in the first place. Even in the cases where DR might become an issue, such as backlit subjects...if you look at the photography of well-respected wildlife pros, like Andy Rouse...he doesn't try to lift the shadows of a shaded backlit subject like an African wildcat by many stops. He leaves them shaded, he leaves them contrasty. Sometimes it just isn't about dynamic range...sometimes, dynamic range really, truly, doesn't matter...