This tutorial is specially designed for first timers struggling to understand the concept of aperture, depth of field and bokeh. We came up with a non scientific but photographic and very easy to understand explanation of the relation between the aperture of the lens, the light and the image.
Lets start with the lens. The lens aperture controls the amount of light reaching the sensor of the camera. A lens will have aperture values marked from f/2.8, f/3.5, f/5.6, f/8, f/11, f/16, f/22 etc on the lens mount of older lenses. Newer lenses show this data on the LCD. Refer to your camera manual for this.
Now, ‘Closing’ the aperture of a lens ie. from f/16 to f/22 will reduce the amount of light falling on the sensor by 1stop (it has the same effect on exposure of the image like when we reduce the flash by 1 stop).
Simmilarly, ‘Opening’ the aperture from f/16 to f/11 will double the amount of light falling on the sensor, and will have the same effect as doubling the flash power used.
The Depth of Field(DOF) is a term, which is determined by the subject distance to the camera and the lens aperture, and the setting of the aperture(f-stop).
Formats(sensor sizes) of the camera also affects the depth of field. The larger the sensor, the shallower the depth of field. Thats why 645 medium format digital backs(eg. phase one) produce more DOF than 35mm sensors(eg. Canon 1Ds3, Nikon D3x), which in turn can produce more DOF than cropped sensor cameras(all entry level SLR's as of 2010).
For a given aperture(f-stop), decreasing the magnification, either by moving away from the subject (or using a wide angle lens)increases DOF. In the image below, we reduced the magnification by moving away from the model using a 50mm lens at f2.8.
In the same way, increasing the magnification, either by moving closer to the subject (or using a tele lens), decreases the DOF. Low DOF in real world, means out of focus background! In the image below, we increased the magnification by going close to the model. Also, notice that the model is far away from the background now.
For a given subject distance, increasing the aperture (ie. From f/16 to f/2.8) decreases the DOF and viceversa, decreasing aperture(ie. From f/2.8 to f/16) increases DOF. The image below is simulated to show you the difference the aperture does when every other setting is the same....EXCEPT for the shutter speed, which needs to change to compensate the light falling on the sensor, to keep the exposure the same.
Lets take a look at this japanese term 'Bokeh' now. Bokeh is the aesthetic quality of the way the lens renders out-of-focus points of light (blur areas of an image). Differences in lens design and aperture shape cause some lens designs to blur the image in a way that is pleasing to the eye, while others produce blurring that is unpleasant or distracting— "good" or "bad" bokeh, respectively. Bokeh occurs for parts of the scene that lie outside the depth of field. Lenses with aperture going to f1.8 produce more prominent bokeh than lenses with f3.5-5.6 apertures and are generally more expensive. Photographers sometimes deliberately use a shallow focus technique to create images with prominent out-of-focus regions. See the circles of light in the image below....that is bokeh!
In the image above, we have used the Octabox to throw partial 'skimmed light' on the model. The angle in which we place it was to light the model and the background at the same time. There were some metal objects on the table in the background on a table. With shallow DOF they would all be visible and this picture would look like an ugly snapshot. So, we use DOF for our benefit and shoot at f2.8 to blur the background. The light reflected from the metal surfaces on the background creates circles of light aka bokeh.