My favorite website for DOF is DOFmaster.com. When looking at DOF it is all about the math. at 420 mm, F/8, and 30 feet the DOF is about 6 inches from the focus point the camera used at the time of the shot.
A great blue heron (GBH) is almost 6 feet tall and about 12 inches wide when its wings are folded. Look at the close up and you can see that the detail in the wing next to the dark patch has minimal detail, but the head and eye have very sharp detail and the“powder down” feathers on the chest are in detail as well because they are on the same focal plane as the eye which was where the focus point the camera was using at the time of the shots.
When taking photos of a GBH, if you are going to get close, make sure you don’t have too much lens to get the shot you are after. At 30 feet, to get the GBH wings in sharp detail tip to tip, you would have to have about a 100mm lens at F/8 with the focus point used on the middle of the bird.
To get a sharp eye or the whole bird in focus in flight, getting the focus point on the correct spot is necessary. In most cameras, the center point of focus is faster and more accurate, depending on the lens used. The more expensive the camera in most cases the more advanced and the number of cross-type f/2.8 focus points are available. These points are able to take advantage of faster lenses if they are attached.
So to take advantage of these faster focus points, the lens has to be capable of letting through that amount of light onto the sensor. I have a Canon t2i that I was using when I started getting serious about digital photography. At that time, I switched to using a single point of focus for all photography, and then purchased some faster lenses that would take advantage of the faster focus points. Using a single point of focus along with the faster lenses really helped take my sports and wildlife photography to the next level and capture moving objects a little better.
The 7d mark ii autofocus system is the best I have used and works well in challenging light situations with a fast lens. I also often take advantage of the ability to move the single point of focus to one third of the frame or the other for creative shots very quickly to help fill the frame with a bird, or be creative with a flower as done below.
See more of my work at my website www.carymcdonald.com.