T189 - Digital Photography Course (4)
Week Four - Focus and depth of field
o How lenses help gather and focus light
o What do we mean by focus
o Field of view and magnification
o Effective focal length and camera shake
o Optical and digital zoom
o Focal length and perspective
o Controlling depth of field
o Rescuing a poorly focussed image
o Putting it all together - some tips for better images
Session 4 brings in some calculator work for you. Well it’s simple arithmetic so you don’t have to go out and get a calculator. It brings together the relationship between the lens, type of lens and the camera sensor (or film) and types of sensors. A description of what ‘focus’ means is presented in terms of what the lens is doing to light entering in at one end before reaching the other.
One thing it does not tell you is how a camera automatically determines that focus is needed and then adjusts accordingly, which I think is quite important, but that could be another module.
Some rules emerged out of this learning and they are true from all photography and have some value in remembering when creating your images.
Rule 1: Depth of field decreases as focal length increases, and increases as focal length decreases (assuming object distance and aperture are constant)
Rule 2: Depth of field increases as object distance increases, and decreases as object distance decreases (assuming that focal length and aperture remain unchanged)
Rule 3: Depth of field decreases as aperture increases, and increases as aperture decreases (assuming that focal length and object distance remain the same)
- Use a wide or ultra wide angle lens (Rule 1).
- Focus efficiently (an extension of Rule 2 – see below).
- Use smaller apertures (Rule 3).
- Use the longest lens you have available (Rule 1).
- Get as close to your subject as possible (Rule 2).
- Use wide apertures (Rule 3)
...DOF left and near...
...DOF right and far...