I am not interested in a D800 or any other FX camera for nature photography. I like the ability to know that what I see framed is what I will get. I may crop but I don't want to see a smaller image and then later have to crop to size.
But, I do want better dynamic range and ISO performance at higher ISOs. I am limited to what I can handhold in a kayak. Currently that is the Sigma 50-500 lens. That and my D7000 weight in at 7lbs. 11ozs.
I have looked at better zoom lenses. None of them which may have better performance are similar in weight and size to the Sigma.
This morning was a good example why I would buy a D400, regardless of its price. I went out to my favorite birding lake. There is a wonderful rookery there. However, it is closed to boats starting Feb. 15. So, this is my last chance to get up close and personal to Egrets, Herons,and others during the mating season.
Last night the weather prediction was for partly cloudy and getting sunny later on in the morning. I was on the lake at 7:00. It was foggy and total overcast. I stayed till 10:00. It was still overcast as I loaded up the kayak by into my pickup.
I just looked at the pictures. I see ONE (a Killdeer closeup) maybe two keepers. The rest will go to the digital grave. There was just too little light. I was shooting at ISO 1600 and never saw over 1/400 of a second. Most of the time the shutter was around 1/200.
Now, I know there is no camera which will give me 1/1000 sec. at a crystal clear ISO 1600. Still, I will take all that Nikon can give. As I see it, a better performing camera body is at this time the more viable route for me.
If someone had a 200-500 F/4 zoom similar in size and weight to the Sigma, I would buy it in a heartbeat.