Yesterday I was telling you a little about how I have my yard setup for photographing birds. I mentioned that I have my blind setup right at the minimum focusing distance for my lens. The lens I use most often for these types of shots is the Tamron 150-600mm. When using the lens at 600mm at the minimum focusing distance the depth of field (DOF) is pretty small.
Hover over the image to view the camera settings used. Click the image to view it larger.
If you look at the above image, you can see how shallow the depth of field is. You can see the eye is tack sharp, but 3 or 4 inches away the bird's tail is out of focus. It is important to remember this when using a telephoto lens at the longer end close to your subject. Since the DOF is so shallow your focus is critical. If I had missed the eye by even a tiny bit, it would have been soft. Images of wildlife or birds rely on the eye being in sharp focus!
When I hear people complain about getting soft images out of any telephoto lens, it is usually related to distance. Either they are so close that the DOF is so shallow that they simply miss the focus point, or they are too far away and end up cropping the image heavily and expect the image to be sharp. Keep this in mind when using those longer lenses.
This image was made using my Sony A77 II and Tamron 150-600mm Lens. Both mounted on my Sirui Tripo and PH-20 Gimbal Head.