Aperture and shutter speed

Aperture, the size of the opening in a lens through which light passes, determines how much light strikes the CCD/CMOS sensor as does the length of time that the shutter is open. Large apertures (small f/numbers such as f/2.8) allow more light in than small apertures (large f/numbers such as f/22). Each full stop increase/decrease in either aperture or shutter speed allows in twice/half as much light as the stop before. Therefore if we had an initial aperture setting of f/8: f/5.6 (one stop more than f/8) would allow in twice as much light and f/11 (one stop less than f/8) would allow in half as much light. Likewise, for an initial shutter speed setting of 1/250th second: 1/125th second (one stop more) would allow twice as much light to strike the sensor and 1/500th second (one stop less) would allow half as much light through.


Aperture (f/number)      2.8       5.6       8          11        16        22        32

                                                                                   Wider aperture                                                    Smaller aperture

                                                                                (less depth of field)                                              (greater depth of field)


 Shutter speed               1/60     1/125   1/250   1/500   1/1000 1/2000 1/4000

                                                                             Slower shutter speed                                                        Faster shutter speed

                                                                           (shutter open for longer)                                                  (shutter open for less time)


Aperture and shutter speed are linked together to achieve the correct exposure. Each time we increase/decrease the aperture by one stop a resulting one stop decrease/increase in shutter speed occurs and vice versa. For example, if we were shooting in aperture priority mode with settings of f/8 and 1/250th second and we wanted to change to f/5.6 then what would happen to the shutter speed? As we have increased the aperture by one stop a corresponding one stop decrease in length of time the shutter is open must occur to ensure that the correct amount of light passes through the lens. Therefore the new exposure setting would be f/5.6 and 1/500th second. Likewise changing from f/8 to f/11 equals a one stop decrease and therefore a one stop increase in the length of time the shutter is open must occur to maintain the balance. Thus the new setting would be f/11 and 1/125th second.

Aperture, as well as controlling how much light passes through the lens also determines depth of field (the amount of detail in front of and behind the main focal point that will be in focus). Large apertures (small f/numbers) produce a shallow depth of field whereas small apertures (large f/numbers) produce greater depth of field. One thing to bear in mind is that depth of field extends from 1/3 in front of and 2/3 behind the main point of focus. Therefore if you are photographing a large aircraft from 1/4 or 3/4 face on for example and you want the entire aircraft to be in focus do not focus on the nose. Instead, select a small aperture and focus on a point 1/3 of the way along the fuselage.

One final thing about depth of field. Shorter focal length lenses, such as 28mm wide angle, have a much larger depth of field compared to longer focal length lenses, such as 300mm telephoto, for any given aperture.


ISO speed

In the "olden days" film was classified based on its ISO (International Standards Organisation) number. ISO is still used by digital cameras manufacturers to denote sensitivity to light and ranges from 50-3200. Quite simply, the higher the ISO number the higher the sensitivity to light and the faster shutter speeds that are possible. Unfortunately this higher sensitivity to light comes at a price, namely noise. Therefore images shot at 800 ISO will not look as good as ones shot at 100 ISO.

ISO behaves in the same way as aperture and shutter speed. Each increase/decrease in ISO is twice/half as sensitive as the one before. For example 200 ISO is twice as sensitive to light as 100 ISO but half as sensitive as 400 ISO. Therefore if you are shooting fighters flying through the Machynlleth Loop in Wales using 100 ISO and the fastest shutter speed you can get is 1/250th second but you want 1/500th simply switch to 200 ISO or even to 400 ISO for 1/1000th. Are you seeing the pattern emerging now? Each successive increase in ISO allows a corresponding one stop increase in shutter speed and vice versa.





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