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Aperture(f-stop) in a DSLR camera



I have already discussed two methods for altering the exposure in a DSLR camera i.e ISO and shutter speed. Now we’ll look into a little tricky concept called f-stop, also known as f-number, f-value or aperture. While discussing about shutter speed in a DSLR camera,


I had mentioned that the exact value of shutter speed for a situation cannot be decided until you are familiar with the concept of aperture. Aperture determines the amount of light which enters the camera. It is a dimensionless quantity and is expressed as a fraction e.g f/1.4, f/2, f/2.8, f/4 etc, where f is the focal length. Below are the standard aperture values:


1           1.4          2          2.4          4           5.6         8          11           16         22


F-number is a relative measure of the size of the aperture with respect to the focal length of the lens. These numbers will not make sense in the beginning, as the numbers in the denominator do not follow any sequence and appear to be random.


However, there is a little bit of mathematics involved in the derivation of these numbers and fortunately you need not know that for taking beautiful pictures. Geeks who have a bent for mathematics, can click here. For the time being keep the following points in mind:


1. The smaller the f-number (or f-value), the larger the lens opening (aperture). An f-number of f/8 will allow twice the amount of light that an f-number of f/16 will allow and this same relationship holds good for its entire range of values.

This concept of doubling and halving of the amount of light is expressed in terms of ‘stops’. When you decrease the f-number by 1 stop, you are exposing the sensor of the DSLR camera to double the amount of light and vice-versa.


2. The exact value of aperture for a given situation is selected in conjunction with the shutter speed. A combination of shutter speed and aperture is used to get the perfect exposure. A detailed explanation on the combinations to be used for various situations would be provided in a different post. Doubling the shutter speed and decreasing the f-number by 1 stop or halving the shutter speed and increasing the f-number by 1 stop, yield same results. For example: 1/30s f5.6 <=> 1/60s f4 <=> 1/15s f8.


3. Changing the aperture changes the ‘depth of field (DOF)’ of a scene, a very important concept in photography. For the time being just remember that DOF increases with the increase in aperture size (small f-number) and reduces with decrease in aperture size. It needs further explanation and will be explained in another post.


You might have seen pictures having completely blurred backgrounds with the subject alone in focus. The background blur is controlled by adjusting the aperture settings on the DSLR camera.


The concept of aperture and shutter speed is more prominent in a DSLR camera as one can always choose the level of importance one should give to the subject in focus, whereas in most of the point-and-shoot cameras the camera automatically selects the optimum aperture and shutter speed settings, not giving the user freedom to apply his creativity.

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