When it comes to focal length and focus, is there really every distance to focus for?
Firstly, lets clear one thing up. “Infinity focus” doesn’t mean as far as the eye can see. Infinity in terms of focus is the point in the distance where everything beyond that point is equally in focus. Let’s say you are stood under a line of power-lines that have 100 pylons stretching into the distance. Now you can adjust your focus so the first second or third pylon is in focus. There will be a pylon, maybe 10 or 20 ahead of you. If you focus on this pylon, all the pylons after it will be in focus.
Focal length refers to the angle of view of a lens. For example; a “17mm focal length” is wide angle, a “400mm focal length” is telephoto.
Focal length determines the point of infinity focus.
Different focal lengths will have a different infinity points. For example a 24mm lens will have infinity at around 20 meters whereas a 200mm lens will have infinity at 500 meters.
This then relates to aperture. A photographer may use aperture to get everything sharp from the closest object to infinity.
Depending on how close the closest object is, the photographer would choose a small aperture (big number such as f/16) and focus on a point one third beyond the closest object and one third before infinity. This is known as Hyperfocal distance. This become very challenging with a telephoto focal length such as 200mm unless the objects are far away.
Bokeh is the blurred background effect. This effect is created with a large aperture (small number such as f/2.8) with the focus on the subject. This is challenging with a wide angle focal length such as 17mm, unless the subject is very close. This is a shallow depth of field technique.
These 2 concepts are only applicable in the space leading up to the point of infinity focus.
Depth of Field (DOF)
Depth of Field refers to the range of sharpness in a photograph. There is only one point of focus (focal plane) but the range of sharpness is determined by the Aperture, Focal length and distance to first object.