Field of View

When using the telephoto lens for the bird photography such as my Nikon 80-400mm VR lens, should I use it with the D700 which is FX format or my D300 which is DX format?  Both of them have almost exactly the same pixel count, around 12 megapixel.  D700 has higher ISO sensitivity, which also means better shadow detail.  It is very important for the bird feather.  On the other hand, D300 with the 1.5X factor, gives me a better reach for the subject.  The only other lens, other than the super heavy, super expansive fixed focus lens, is the Sigma 150-500mm.  It sells for about half of the price of my 80-400mm and weights about 50% more.

The Piping Plovers shots using the 80-400mm lens and D700 were done under the bright sun light on the beach.  I shot the pictures at ISO200, f/8, 1/2000 offhand.  Thus the ISO and the widest aperture of the lens do not matter.  It make me wonder if I were using D300 and the 150-500mm lens, what the image would look like in the viewfinder.

The Angle of View of lens at different focal length for FX and DX cameras are as following

                    FX     DX
200mm     12.2      8.0
300mm       8.1      5.2
400mm       6.1      4.0
500mm       5.0      3.1
600mm       4.1      2.4

We can use them to calculate the size of the image at the different focal length for different camera format.

Here are what the different combinations would look like in the viewfinder:

400mm with D700 (FX)

500mm with D700 (FX)

400mm with D300 (DX)

500mm with D300 (DX)

The bird at 500mm with D300 (DX) is twice the size of the bird at 400mm with D700 (FX).    

As show in the first image, using 400mm with D700 (FX), it was quit challenging just to pint the focusing point steadily on the bird, even with the VR.   The autofocus of the camera also had as much difficulty to hunt for the precise focus.    

If I were using the 150-500mm, I would probably have to use it on a tripod.  Then I will no longer be just a tourist wondering around with a big camera.