Aperture Priority, Shutter Priority and ISO made simple

 Photography is a hobby to many of us. Some want to be very technical about it but to me I would like to be as simple as possible and concentrate on getting better images. Having said that I am of the opinion that we have to get our photography basics right to fully enjoy this hobby. Therefore in the next few postings I hope to share these basics with you starting with this topic. This is specifically referenced to digital photography. 

Our digital camera basically has a sensor that detects light and a CPU (central processing unit) to process the light information. The sensitivity of this sensor can be controlled by adjusting the ISO - the higher the ISO the more sensitive is the sensor. Lets leave ISO aside for the moment.

So what controls the amount of light hitting the sensor? It is the interplay between the aperture and the duration that this aperture is left open which we called the shutter speed. If we open the aperture we allow more light to fall on the sensor or if we open the aperture for a longer time (i.e a slower shutter speed) more light again can fall on the sensor.

The CPU of the camera is programmed in such a way that to get a proper exposure an appropriate fixed amount of light must fall on the sensor. With this in mind lets talk about Aperture priority, Shutter Priority and end with ISO adjustment.

Aperture Priority

In this mode we adjust the size of the aperture and allow the camera's CPU to determine the shutter speed so that the said fixed amount of light to get a proper exposure (as mentioned above) can fall on the sensor. 

Therefore if we widen the aperture the shutter must close faster (faster shutter speed) to get that fixed amount of light. The converse is true.

Aperture size is determined by f over a number e.g f/3.2. Hence the smaller the number the wider is the aperture i.e f/2 is wider that f/3.2. One important point to note is that the Depth of Field (DOF) is also affected by the aperture - the wider the aperture the shallower is the DOF. This is very useful to people who do portraiture where one wants to better isolate the subjects with a shallow DOF. It is also important to photographers who do available light photography where low lighting situations are all too frequent. These are the reasons why I simply love to use Aperture Priority most of the time.

At f/2.8


Shutter Priority

 In this mode we adjust the shutter speed and allow the camera's CPU determine the aperture to get that fixed amount of light through to the sensor.

If we increase the shutter speed the aperture must be wider and the converse is true.

Situations where we want to use this mode is when we want to freeze actions by increasing the shutter speed e.g sports photography. On the other hand we can use a slower shutter speed to do panning.

 Slow shutter speed in panning (1/8s)



ISO is a measure of the sensitivity of the sensor towards light. If we increase the ISO we are effectively increasing the sensor's reception to light. This parameter adds another dimension to how we use the above 2 modes.

I will use Aperture Priority mode as an example. Say we are shooting in available light where we have already increased the aperture to the widest possible and yet the shutter speed is still too slow for handheld photography. In this situation we can increase the ISO till the shutter speed is enough to avoid camera shake. The catch here is that increasing ISO may bring in the problem of noise in the captured image. The rule of the thumb is to use the lowest ISO for a given situation. It is important to know your camera's high ISO performance - my cameras give me a lot of latitude because I can use up to ISO 3200 with minimal noise created in the image.

ISO 12800 (f/2.8 and 1/5s... can't get any slower than that)

High ISO

There you have it. The above is as simple as I can get to explain the various modes. What mode you want to use is your preference. Since I like to do portraiture, travel photography and especially available light photography I almost invariable use Aperture Priority.


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