Whats the relationship between focal length, subject distance, aperture, image size, and depth of field (DOF)?
A. If you want to check the math on this, I suggest you visit Andrzej Wrotniak’s site, which also has some nice tables comparing DOF for different image sizes. I’ll briefly summarize the key points you need to remember: • DOF decreases with aperture size. Remember, smaller F-stop numbers mean larger apertures. • DOF increases with distance to the subject. This is why you can focus at infinity and get everything beyond a certain distance in focus. This is also why macro shots have such shallow DOF. • DOF increases as focal length decreases. This is why wide angle lenses have deep DOF, but telephoto lenses have shallow DOF. • DOF stays the same or decreases as image sensor size decreases. This is the one that confuses a lot of people. Do the following thought experiment: Keep everything about your camera exactly the same, but replace the sensor/film with one that is smaller. The image coming off the lens is exactly the same, so the depth of field cannot change. If you assume that you’ll
