DSLR Breakdown

Technical Deconstruction of a DSLR Camera 

Matthew Fryer

There are three ways to control the amount of light that enters the camera lens. These are shutter speed, aperture and ISO. Below I explore these mechanisms in detail. 

The Shutter

The shutter is a mechanism that opens for a user defined amount of time, to allow a certain amount of light into the camera. It works like a real shutter door would. There are two 'curtains' that open subsequently in order to 'expose' the camera to light. 

The longer the shutter curtains are left open for the more light that enters the lens. For example 1/5 means the shutter is open fully for 1/5th of a second. This would allow a large amount of light to enter the lens. Whereas a shutter speed of 1/2000 means that the shutter is fully open for 1/2000th of a second, meaning very little light enters the lens.

High and low shutter settings each have their own unique properties. A low shutter speed would not only allow a large amount of light to enter the lens but it will also capture any movement in the frame more loosely, causing motion blur. A high shutter speed will freeze the motion capturing a sharper image.

The reason why a shutter uses two curtains is, when the shutter speed is very high one curtain would not be able to move fast enough to only be open for such a short amount of time. Therefore two curtains that open subsequently would allow for a shorter amount of time that the shutter is fully open.

Aperture

The aperture also controls light flow into the camera, but differently to the shutter speed. The aperture is controlled by an iris mechanism unique to each type of lens. Aperture is measured in F-stop. The higher the F-stop number, the smaller the radius of the hole allowing light into the lens. The lower the F-stop number, the larger the radius.

Furthermore the number of aperture blades affects the quality of your photos. Mainly in the quality of bokeh. Bokeh is a japanese word which is used to define the quality of blur in an image. The less blades the aperture had the less circular the hole is, meaning that the blurred parts of the photo may lose their quality and have sharper edges. Higher end lenses use more blades, 6-7 is common.  

Aperture also controls depth of field. Depth of field is the amount of the photo in terms of depth that is in focus at any one time. A larger F-stop will mean that the depth of field is deeper.

In terms of how aperture works technically, see below:

A narrow aperture setting physically closes the aperture blades, meaning that the light coming through the sensor is forced into a narrower radius. This field of focus works like a cross joint. The wider you set the parameters, the shorter the overall area of the field of focus, and the closer you bring the parameters the longer the overall area. This means that the Near focus limit and Distant focus limit are further apart in high F-stops and closer together in low F-stops, defining the depth of field.

ISO

ISO stands for International Standards Organisation and it deals with how sensitive the camera is to light. Changing the ISO on a DSLR is the equivalent of changing the film in a conventional camera to a more or less sensitive film stock. In digital photography the ISO refers to how sensitive the sensor is to light, but it equates to the same output. Moving from one ISO reading to the next either halves or doubles the light sensitivity depending on whether you move lower or higher, respectively.

There are side effects to increasing the ISO. In some cases, for example when the shutter speed needs to be unusually fast (e.g. 1/2000) you will need to increase the ISO to allow enough light into the image to capture a correctly exposed photo. However increasing the ISO also increases the amount of noise in a photograph. When we refer to noise in photography terms we are referring to the amount of grain in a photo. The more grain, the more pixelated your photograph will look which, for most cases, one would want to avoid. The way to do this is to know where and when to use what ISO. Below I have listed the commonplace ISOs and in what situations they would be used.

ISO 100: Should be use on very sunny days, or in places with large amounts of light.

ISO 400: Overcast days, or somewhere with a moderate amount of light

ISO 800: Your location is dimly lit but not so dark as to have to compensate heavily with ISO.

ISO 1600 and higher: Your location is very dark and has almost no light. Aperture and shutter speed can only do so much, to correctly expose an image.

Clearvid CMOS Sensor

The 600D uses a CMOS (Complimentary Metal Oxide Silicon) Sensor, the other type of sensor is called CCD (Charge Couple Device). CMOS Sensors are usually found in lower end cameras. CMOS Sensors use internal photodiodes to convert images through the lens into digital images. Each photodiode in a CMOS sensor has an amplifier that converts electrical charges to voltage, wheres as in CCD sensors electrical charges have to pass along a row of photodiodes before outputting voltage. The main difference between the two is that CMOS is 10-100 times faster (but less sensitive) than CCD meaning that it is great for fast frame cameras. Also they are much more compact CCD sensors meaning that full HD footage can be shot on a relatively small camera. 

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