Focus and depth of field
For this one it was the 5D and 24-104 f4. The f2.8 on the 20D is just a bit too soft at its widest settings to make an exercise based on sharpness sensible, and sharpness doesn’t get much better than L series lenses.
Here are some photos of the rooftops of my street which nicely illustrate the influence of aperture on depth of field. In all these images the camera was focused on the right hand side of second nearest brick chimney stack.
First is the stopped right down shot, f22 1/60s
Other than one stray leaf at the bottom right of the image and the very closet part of the hedge about 2m from my position the entire image, with a depth of around 1.5 miles, is in focus.
Next the lens is about half open at f13 1/200s
Here the effects of the wider aperture can already be seen, the closest section of the privet hedge is slightly less sharp and the distant village on the hillside is distinctly out of focus. Sharp focus seems to extend from about 10m to around 100m at the tree at the end of the roof line, though there is nothing in the frame between about 100m and 1500m so the sharp field of view could extend further.
Finally the lens wide open at f4
.Here the area of sharp focus is much more restricted, the sandstone house is noticeably soft as is the far end of the line of roofs, sharp focus seems to extend from around 20m to around 70m.
Given the point of focus is effectively on a horizon these photos don’t quite illustrate the point made in the course notes of the sharp area being around the focus point but do illustrate depth of field extending radially from the focus.
See and Imagine 