When I learned to use my old mechanical Leica there were three primary controls on the camera; a ring to adjust the focus (gauged with the aid of range finder using a ghost image overlaid on the main image); dials to adjust the shutter speed; and a ring on the lens to change the size of the aperture (lens hole) as indicated by a logarithmic scale with ‘f-stops/f-numbers’.
Other than merely pointing the camera, I needed to understand how these controls worked to make successful photos.
Today most imaging making devices take care of details such as exposure and focus, allowing image makers to snap away without concern for the mechanics.
In many instances this freedom facilitates the ability to make photos quickly and with relative ease. Yet, this loss of control steals from the photographer crucial tools.
I still like to set my aperture manually. This has less to do with obtaining the correct exposure (since in camera metering can quickly suggest or set appropriate shutter-speed/aperture combinations) and more to do with adjusting the depth of field to manipulate my composition.
A detailed discussion of how the f-stop (f-number) is determined on a lens and what the numbers mean can fill a textbook.
What is important here is knowing a few basics, such as; a smaller f-number represents a larger aperture size and, but more importantly, how you can use this.
As the size of aperture is increased more light is let into the camera, however with a big hole comes a decrease in depth of field (relative focus); conversely, the smaller the hole size (larger f-number), the less light and the greater the depth of field (relative sharpness between near and far objects).
By using a larger aperture (small f-number, say f1.4) the relative focus will be narrow, with those points not in focus appearing relatively soft compared with the subject in focus.
This relationship becomes exaggerated with longer focal length lenses. Where a super wide angle lens offers great depth of field even with a large aperture opening (small f-number), a long telephoto lens will offer relatively shallow depth of field even when using a small aperture (large f-number, say f16).
While the f-number may used as a constant gauging mark, what is most useful is controlling the degree of relative focus to achieve a desired effect.
Personally, I like the effect of a long lens with relatively shallow depth of field because this allows me to draw the eye of the viewer.
As with many successful stories, it often helps to lead your audience on an unexpected path before giving them what they want. I’ll often tease a viewer by leaving some crucial element of an image just beyond the range of sharpness, while placing the focus on something else, like say a railroad signal. Or vice versa.