Maniputlating Depth of Field

Camera Terms used in this blog:

1) large aperture
2) small aperture

Aperture: The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor. (txt and images from Wikipedia)
Remember: Smaller F-Stop= Larger Aperture. Bigger F-Stop= Smaller Aperture




Focal Length: The focal length of a lens is the distance between the optical center of the lens and the film plane (for Film Cameras). The longer the focal length, the more it "magnifies" the subject.


The relationship between f-stop, focal length and the diameter of the lens opening is as follows:


f-stop = focal length / diameter of lens opening


ND Filters: In photography and optics, a neutral density filter or ND filter can be a colorless (clear) or grey filter. An ideal neutral density filter reduces and/or modifies intensity of all wavelengths or colors of light equally, giving no changes in hue of color rendition. (from Wikipedia) 
Situations where ND Filters are particularly useful include:

• Smoothing water movement in waterfalls, rivers, oceans, etc. If your aperture is already at a minimum, an ND filter will let you cut out some light so you can achieve this effect.
• Achieving a shallower depth of field in very bright light. If you have too much light in a scene, the camera will struggle to not blow out the image at wider apertures, so an ND filter will help you out immensely.
• Making moving objects less apparent or not visible (such as people or cars)
• introducing blur to convey motion with moving subjects

Left: No ND Filter               Right: ND Filter
Photo from Fotohacker

Some video cameras have built-in ND Filters that you can access on the side of the video camera or by going into the menu settings.


















Manipulating Depth of Field
Aperture and focal distance are the two main factors that determine how big the circle of confusion will be on your camera's sensor. Larger apertures (smaller F-stop numbers) and closer focusing distances produce a shallower depth of field. The following test maintains the same focus distance, but changes the aperture setting:

f/8.0

f/5.6

f/2.8f

Here is another example:

Shallow Depth of Field

Wider Depth of Field