A Tedious Explanation of the f/stop. By Matthew Cole Photographers set their exposure using a combination of shutter speeds and f/stops to get the correct amount of light on the sensor (or film).
The shutter speed regulates how long the sensor is exposed to light coming through the lens. The f/stop regulates how much light is allowed through the lens by varying the size of the hole the light comes through. For any given film speed (ISO) and lighting combination there is one correct amount of light to properly expose the image. This amount of light can be achieved with many different combinations of f/stops and shutter speeds. Nature photography by Kevin McNeal. Depth of Field. Night Vision Camera. Depth of Field: What Affects it and How to Control it.
By Matthew Cole In my A Tedious Explanation of the f/stop page I go into a lot of explanation of f/stops, whose numbering system is often confusing to photographers.
That page also briefly discusses shutter speeds, since both the shutter speeds and f/stops work through sequence of doubling/halving steps which are complimentary. I note that for any given level of subject illumination and ISO, there is a whole range of exposure combinations which give the correct amount of light on the film or sensor. And I make a passing reference to the other effects that your choice from among these combinations will have on your photograph.