workshop session 3 – aperture and composition
The third session of the Introduction to Digital Photography workshop met February 10. Session two's assignment, white balance, was easily understood and easily completed. White balance is fairly intuitive and the assignment helped instil a good grasp of the basic concepts.
The topics for session three are:
- aperture, and
When we look at aperture, there are two aspects to come to terms with:
- aperture as a component of exposure, and
- aperture as a creative element – depth of field.
Aperture and Exposure
Exposure is a measure of the amount of light hitting the sensor. For our purposes exposure value, written as Ev or simply EV, can be defined as a function dependent on aperture size N measured in ƒ-stops and shutter speed, or exposure time t measured in seconds.
Mathematically it looks like this ...
To help our understanding, we can ignore the ‘log2’ part and treat this as a simple reciprocal function. Reciprocal functions act like see-saws. To end up with the same exposure value or Ev, when one part goes up the other must come down ...
- as the aperture N increases (the iris gets larger) the exposure time t must decrease (the shutter operate faster), or
- as our exposure time t increases (shutter operates more slowly) our aperture N must decrease (the iris get smaller).
By way of example, to achieve the same value for Ev ...
- if we increase aperture size from ƒ/8 to ƒ/4 we would have to decrease exposure time say from 1/60" to 1/125", or
- if we increase exposure time from 1/500" to 1/250", aperture must decrease from ƒ/11 to ƒ/22.
That, in a nutshell, is a working definition of exposure. Yes, there are other factors that can come into play (most notably ISO), but for the purposes of an introductory course, it suits our needs well enough.
Aperture and ƒ-stop
Aperture is specified as an ƒ-stop number. Technically, it is the ratio of the focal length of the lens to diameter of the lens iris. On digital SLRs, a lens typically has a set of marked ‘ƒ-stops’ that note stop that the ƒ-number can be set to; which ƒ-stop is used is controlled through the Av (Aperture priority) mode setting.
For point-and-shoots aperture is implicitly controlled through the various automatic or scene modes; portrait usually sets a large aperture (around ƒ/2.8 or ƒ/4) while landscape will set a smaller aperture (ƒ/11, ƒ/16 or lower).
As you might have noticed from the above discussion, a lower ƒ-stop number is a larger aperture opening, and allows more light to reach the image sensor. Obversely a high ƒ-stop number is a smaller opening, reducing the amount of light hitting the sensor.
Although not to scale, it looks like this ...
Aperture and Depth of Field
Probably one of the most effective and widely used creative elements in photography is varying depth of field. By changing aperture we change depth of field.
Objects scatter light rays in all directions. When those rays pass through a lens, that scattering is reduced. The narrower the aperture the more the scattering is reduced until the light rays are nearly parallel. It is this effect that determines how much of the scene is in focus.
- A larger aperture reduces light scattering very little, resulting in a shallow depth of field, and
- by reducing the aperture we reduce the amount of light scattering, giving a deeper depth of field.
By way of example, a small ƒ-stop number (eg. ƒ/4) is a large aperture, resulting in a shallow depth of field ...
While a large ƒ-stop number (ƒ/22) is much smaller aperture and gives us a much deeper depth of field ...
Here's another analogy that helps explain depth of field. Think of a line-up of people starting close to you and running off into the distance. then ...
- a small ƒ-stop number (eg. 4) means only few people (say 4) in that line will be in focus, while
- a large ƒ-stop number (eg. 22) means lots more people (say 22) will be in focus.
These aren't, of course, real numbers but are meant to illustrate the relationship between aperture ƒ-stop and depth of field.
Depth of Field as a Creative Element
When all the elements in a photograph are in focus, the eye will naturally wander from object to object. If only one object or even just a small part of one object is in focus, the eye tends to remain on that part that is in focus. For example ...
This photograph used an aperture number of ƒ/32. All the flowers are in focus as well as significant detail from the background. The eye moves around the cluster of flowers and perhaps even stops momentarily on the droplet of water.
However, in this photograph , taken with an aperture of ƒ/5, only the central flower is in focus. Our eye tends not to wander away from the point of sharper focus and that drop of water is barely perceptible.
There are many fine examples of depth of field in Google Image Search – check it out.
One of the more common uses of this technique is to achieve one of a photographer's magic looks – bokeh.
Here Snoopy, my smoky tabby, is caught entranced by a bird flying past. By using a shallow depth-of-field (large aperture size or iris diameter, small ƒ-stop number), the background develops a circular pattern and is blurred. The viewers attention is drawn and held to his eyes; the background almost disappears.
That's it for aperture and depth of field.
After introducing composition, this sessions assignment extends on aperture and depth of field ... assignment 3