Exposure Part II : Intensity

As we have learned before in order to control exposure we have to control intensity and time. Here I will talk about controlling light intensity!
Think of the lens like a window, the more you open the window the more light will come in. The concept is the same for a lens. The larger the opening of the lens the greater the light intensity on the photo. Similar to a window you can open and close the lens (aperture).
A diaphragm is the mechanism which control the opening and closing of the lens aperture which also provides us a system to measure this change. Modern lenses use a iris diaphragm, build in replica to the iris of our eye. Thin metal plates are assembled to allow the lens to open and close as needed.
In theory the opening of the aperture is easy to measure. However, different lenses have different focal length, the long, short lenses and by using the zoom feature. For example 100mm lens is twice as far away or long as a 50mm lens when both are looking at the same object. It is like when you stand close to a window you will receive more light while when you move far away from the window the light will become distributed to the whole room and you will receive relatively less light while the opening of the window is the same.

f/number calculation
Hence, f/number system is used to measure or express the amount of receiving light regardless of the focal length! An f/number is the ratio of the lens focal length to the diameter of the lens aperture. For example an 1 inch focal length with an aperture size of 1 inch, the f/number is 1/1 hence 1. Similarly a lens of focal length 4 with an aperture size of 1 inch, the f/number is 4/1 hence 4. In order to identify when we are talking about this ratio f/numbers are written as f/1 or f/4. So when we have a camera A attached with a lens of focal length 8 inch with a aperture diameter of 1 inch it will give f/8. Similarly another camera B with focal length of 16 inch will need an aperture diameter of 2 inch to give f/8. With these settings Camera A and camera B will give the same light intensities. Always keep in mind f/number is not the actual aperture size but the light intensity when aperture size is compared to the focal length.
The standard f/number sequence is 32, 22, 16, 11, 8, 5.6, 4, 2.8 and 2. Digital cameras have in between variations. It as a real pain memorizing these number at first but it was worth it… As each number in this sequence represent a change in the intensity of light by a factor of 2. As the number increase the amount of light passing through the lens decreases to half that of the previous f/number. So if the lens is initially set to f/8 and it is changed to f/11 the light passing through has been reduced exactly half and another change from f/11 to f/16 cuts by another half. The reverse is also true, if changed from f/16 to f/11 the light passing increases by double. Just keep in mind as the f/number increases the light passing through the lens decreases as the aperture size is reducing.
Sequence Table
If you look at the above diagram of the sequence you must be able to note that every other number is twice the value of its predecessor. The key numbers are 1 and 1.4 (an approximate of square root of 2, actual number 1.4142135). If you consider this real number then every other number is either very close or exactly double its predecessor. So this must give you can hint on how to calculate the sequence (but subject to a lot of error due to the ignored decimal place). An easier way to remember is a change is always 2 to the power n. So a change of 3 f/number is 2 X 2 X 2 = 8, a factor of 8.
This f/number is called “stop” as they stop light in exact ways. In practice any change of exposure are referred to as stops. For example to change your shutter speed to 2 stops up.
Even though we have a sequence, lens settings can easily be set at any intermediate points between numbers. I am practicing using my EOS 100, an old film model but the true analogue quality is still unbeatable with the digital cameras (close!). This f/number comes in handy as I do not have a live visual image on the display but rather have to depend on the f-stop ring. However, if you have a digital camera you can change the f-stop and at the same time see a visual more or less a live update of the output instantly. Additionally using digital you can check the shot immediately once you’ve taken it but with the film it is a bit of a pain. I am going through this trouble as I want to get things right the first time – to produce a high-quality image in-camera with minimum tweaking, learning to trust my intuition. This is important to get the exposure I want with confidence when the camera’s being fooled.
In modern SLR cameras you can either control the intensity (aperture size) and let the camera adjust the time (shutter speed) automatically. Alternately you can also change the time and let the intensity be set automatically. So how to control the time? Come back soon to find out…

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