Understanding Exposure – Part 3: Aperture

by Anthony Morganti

This is part three of our four part series, Understanding Exposure. This series of articles will explain everything a photographer needs to know about how light gets into their camera and how their camera uses that light to record the scene.

Part 1: ISO can be found here

Part 2: Shutter Speed can be found here

Today we’re going to talk about aperture.

Inside the lens of your DSLR is a diaphragm that can open and close letting varying amounts of light pass through the lens to the sensor. This diaphragm and the size of it’s opening is often referred to as the aperture.

The small hole you can see in the center of the lens opens and closes to let varying amounts of light though to the image sensor.

The small hole you can see in the center of the lens opens and closes to let varying amounts of light though to the image sensor.

The size of the diaphragm or aperture is given a number designation called f-stop.  This number designation is counterintuitive in that the smaller the f-stop, the bigger or more wide open the diaphragm is.

So, an f-stop of 1.8 is larger and lets more light pass than an f-stop of 22.

I was going to explain what f-stop means technically but it’s really so over the top technical and won’t, in my opinion, help you take better pictures. If you want to know more about it, check out f-stop on Wikipedia.

Depth Of Field – The key thing about f-stops is depth of field — an easy way to remember it is the bigger the f-stop number, the more depth of field your photo will have. So an f-stop of 22 has much more depth of field than the same shot taken with an f-stop of 1.8.

From a creative perspective, a photographer can drastically alter his image by what f-stop he or she chooses to use. There may be times when you’ll want as much as possible in focus — perhaps in a sprawling landscape — in those cases you’d use a LARGE f-stop number to get a large depth of field which simply means that more of the image will be in focus.

Conversely, you may want a very shallow depth of field — often in portraiture we want the background out of focus. In that case we would choose a SMALL f-stop number to get a shallow depth of field.

 

In this photo I used an f-stop of 5.6. In it, you can see that the background is relatively blurred out.

In this photo I used an f-stop of 5.6. In it, you can see that the background is relatively blurred out.

 

In this photograph, I used an f-stop of 25. The larger number meant a larger depth of field hence the background is more in focus than the previous picture.

In this photograph, I used an f-stop of 25. The larger number meant a larger depth of field hence the background is more in focus than the previous picture.

 

Aperture Priority – Most modern DSLR’s have a feature called Aperture Priority. All this means is that the photographer sets the aperture and the camera will adjust the shutter speed to properly expose the scene. Additionally, If the photographer has Auto ISO set, the camera will adjust the ISO as well.

Whenever you want to control the depth of field in a shot, chose to shoot in aperture priority and pick a small f-stop number for a shallow depth of field or a larger f-stop number for a greater depth of field.

 

That is the basics of aperture. In part four, the final part of this series, I’ll be discussing all three of the exposure triumvirate — ISO, Shutter Speed and Aperture and get a little more in depth of how they work together and offer real life situations where you would choose to give priority to one over the others.