welcome to day three of our week of understanding the exposure triangle. today we are going to focus on aperture (or f-stops) in relation to evenly exposed pictures. let’s take a look at our friend the exposure triangle once again. 
first things first, let start with the definition aperture. aperture is a device that controls the amount of light admitted through an opening. in photography, aperture is the unit of measurement that defines the size of the opening in the lens that can be adjusted to control the amount of light reaching the digital sensor. the size of the aperture is measured in f-stops. let’s break that down.
if you take off your lens on your DLSR you will see a mirror, this is your image sensor (see photo). 
the image sensor is the device in a digital camera that converts light into an electrical signal which is then converted into digital data to produce the image.
let’s take what we already learned from yesterday. when you click your shutter button to take a photograph your shutter opens and closes to capture the image you see through your viewfinder using your image sensor. the amount of time the cameras shutter is open is your shutter speed. therefore, the longer the shutter is open (higher shutter speed) the lighter your picture will be. that is if you hold all the other elements of the exposure triangle constant. remember, if you adjust one element, you must compensate by adjusting another.
now that you know how your camera takes the picture, let’s focus on aperture. as stated above, your aperture is nothing more than the size opening of your lens. this determines how much light is let in to reach the image sensor. aperture is measured in f-stops. the more light your lens lets through to the image senor the lower the f-stop, and the less light your lens lets in the higher the f-stop. to make things even more confusing, the higher the aperture the lower the f-stop. just think of aperture and f-stops as an inverse relationship. let’s look at the image below.
again, you will notice all three elements of the exposure triangle listed. the only element not held constant in the images above is the aperture. in the over exposed picture we have a high aperture (low f-stop). the opening of our lens is too large and letting too much light in to the image sensor causing an over exposed picture. in contrast, the under exposed picture has a low aperture (high f-stop). the opening of our lens is too small therefore not letting enough light in, causing an under exposed picture. the evenly exposed picture represents balance among the three elements of the exposure triangle. our f-stop is at 3.5 letting the appropriate amount of light in to the image sensor.
again, as you can see, if you change only one element of the exposure triangle you will cause your image to be either over or under exposed.
we are at the end of day three and well on our way to having an abundance of control over how you shoot. practice again today. pick a set shutter speed and ISO and adjust only your aperture. take notes on what your image looks like with higher and lower f-stops. tomorrow we will explore and combine what we have learned about shutter speed and aperture before exploring our last leg on the exposure triangle on Saturday.
i would love to see your images from today. post below in the comments and let’s see what you come up with.
good luck! and happy shooting!
see life through the lens. 