Long lenses and DOF

The problem with long lenses is that they have extremely shallow depth of field.  When shooting distant subjects, like a mountain range, that's not an issue.  But with small subjects that fill the frame, like this variegated squirrel in Costa Rica (where I used a 500mm f/4 telephoto), DOF is an issue.  Notice how out of focus the tail is.  Usually, if the eyes of an animal are sharp, the out of focus hind end can be forgiven.  We accept that, and sometimes it's even desirable.  But in this case, the tail of the squirrel is beautiful and it would have been nice to see all of that detail with tack sharp clarity.  The only way to be able to use a small aperture with a long lens, especially in low light like this forest shot, and still have a shutter speed fast enough for a sharp picture is to be able to raise the ISO very high without a lot of noise.  Cameras that deliver low noise with high ISO's are extremely important if you want the ability to have everything:  depth of field, a sharp picture, and optimum picture quality.  For your next camera purchase, make this a priority.