Lesson 11: Exposure Time and Aperture

ApertureI’ve decided to talk about Exposure Time and Aperture in the same lesson because it is a strong relation between them, which I’ll explain later. First I will discuss about each of them to help you understand them better.

EXPOSURE TIME: The Exposure Time is the period of time in which the sensor will be exposed to light. This is done in the moment push the button to take the picture. A mechanism inside the camera will open for the precise amount of time that has been manually set or automatically calculated by the camera. The exposer time on a digital camera can vary in a wide range. For example, on my Canon 450D DSLR the exposure time interval is from 1/4000 to 30. That means that I can expose the sensor from the tiny amount of time that is 1/4000 of a second up to 30 seconds. This is set accordingly to light conditions where the picture is taken.

APERTURE: This is a little more complicated to understand than exposer time, but I’ll try to explain it as simple as I can. The aperture is a characteristic of lenses and it is usually specified on these as a value like 1:3.5-5.6 (on lenses with zoom capability) or 1:1.8 (on lenses with no zoom). To make an analogy with the human eye, aperture is similar to the pupil. It gives us the ability to set the amount  of light that enters through the lens by closing or opening a mechanism similar to the eye pupil. A smaller number means a wider opening so more light will come in. The numbers specified on lenses means the maximum opening of the aperture that can be achieved. For example a value like 1:3.5-5.6 (or f/3.5-f/5.6) on some zoom lenses means that we can get a maximum aperture opening of 1:3.5 (or f/3.5) on no zoom and a 1:5.6  (or f/5.6) one on maximum zoom.
Another very important aspect about aperture is the Depth Of Field (or DOF). Depth of field is a term which refers to the areas of the photograph both in front and behind the main focus point which remain “sharp” (in focus). Depth of field is affected by the aperture. A larger aperture (smaller f-number, e.g. f/2) has a shallow depth of field. Anything behind or in front of the main focus point (the main point that you want to be sharp) will appear blurred. A smaller aperture (larger f-number, e.g. f/11) has a greater depth of field. Objects within a certain range behind or in front of the main focus point will also appear sharp. Here is an example:


The relationship between aperture and exposure time is that a smaller aperture will result in less light that will enter through lenses, so a longer exposure time will be required and vice versa. So you can achieve the same luminosity in a picture using different aperture/exposure time combination. The main reason for playing with these two is again the DOF (Depth of Field). There are many situations when you want a shallow DOF (so a blurred background), like in portraits, macros, etc, and there are situations when you’ll prefer a greater DOF, for example in landscape photography. To master this two in combination will require time and a lot of exercises, but I assure you that it worth the effort.

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