Depth of field
Depth of field (DOF) can be a little baffling if you’re new to photography, but it’s easy once you understand the basic principles. Wikipedia defines depth of field as ”the distance between the nearest and farthest objects in a scene that are perceptibly sharp in an image” so, lets keep it simple.
There are three factors that will determine depth of field:
- The aperture of the lens.
- The focal length of your lens.
- Distance from your subject.
F/stops are used to measure the aperture of a lens; the bigger the number (eg F/18, F/22 ) the smaller the aperture, which then increases the depth of field ( more of the image in focus),so, if you want to take a picture of lets say, a landscape, where you want everything near and far in focus, use a high F/stop setting, along with a shorter focal length lens.
Image by macrophile
The only downside to this is that a smaller aperture means that less light is entering the cameras sensor, therefore a slower shutter speed is needed, particularly if the light is poor, and this could cause blurry images from camera shake.If the shutter speed falls below 1/60th second (which is about the slowest speed a person can handhold a camera without camera shake) then you’ll certainly need a tripod to keep your images sharp.
Image by Schlusselbein2007
© A Webber
If you want to get a shot where your subject is isolated; where everything before and after your subject is out of focus, then a wider aperture is needed, ( and a longer focal length lens). The image above of Teasel plants, was taken with a Canon EF 70-200 f/2.8L usm at F/4. I was shooting fully zoomed in at 200mm, so any tiny movement of the camera would be exaggerated, in order to keep the subject sharp and the background nicely out of focus, I used a tripod.
Short Focal Length
Another factor that will determine DOF is the focal length of the lens. A wide angle lens, for example, has a greater depth of field (more in focus); and is considered to be ‘wide angle’ when the focal length is less than 35mm, this, combined with a high F/stop (small aperture) will keep everything, near and far, in sharp focus.
Long Focal Length
A telephoto lens, has less DOF, moreso as the focal length gets longer. So,to isolate your subject with a nice blurry foreground and background; use a telephoto lens combined with a low F/stop (small aperture). When I use the term ‘telephoto’ lens, then I’m referring to a lens that has a longer focal length than 70mm.
Image by Madmack66
When you have a photograph where the background and fore ground appears blurry and out of focus; the blurry and out of focus area is called the ‘bokeh’. You may have heard photographers use the term of a particular lens as having ‘nice creamy bokeh’; it can be flattering in portrait shots where it lets the subject stand out, perhaps from a cluttered background such as a crowd of people.
Of course, you can shoot from wide angle to telephoto with different depths of field all on one lens: a zoom lens; with todays zoom’s offering a wide range of focal lengths and apertures you can get quite creative (which is why zoom lenses are so popular).
The depth of field increases as the aperture widens from F/2.8 (left) F/11 (centre) F/22 (right), all taken at 70mm on a 70-200 zoom lens