Hi and welcome back, this week we can talk about one of the key points of exposure triangle, Aperture.   Aperture, and for that matter all 3 key elements of the exposure triangle are used in every photograph.  Phone cameras and auto camera modes may alter these for you, even if its in the software on a mobile, but they all impact the taken image.

The first question course is what is aperture? Aperture is a term that refers to the opening of the lens which affects the amount of light that can come through to the sensor or the film if you’re using an older camera. Have a look at this diagram

you can see here that the lens goes from being nearly fully open, which would be in theory a one-to-one relationship, all the way through to a really small opening in the lens. In the diagram these appear as F stops, this is a mathematical formula which relates to what the image plane see in relationship to the opening of the lens, the amount of light that can come through and the focusing of the light.  There’s a whole pile of maths and physics theory behind this, we really don’t want to go into too much of that as there are some things that we do need know to understand how we see the result. Typically what we find is that a lens will start at a small number as low as F1 .2 all the way through to an f-stop of 22 on more common lens.  Strangely though, the smaller the ‘f-stop’ number the larger the opening  and the higher the 'f-stop' number the smaller the opening in the lens.  It all sounds a little bit back to front but don’t worry it will makes sense after you’ve been using cameras and lens for a while we don’t really need to understand the mathematics behind it were just to understand what it means and how it impacts our photographs.

So we looked at the different 'f-stops' and the size of the opening in the image above let’s now look at typically what this might mean, because the biggest thing about the f-stop is the impact it has on what is in focus, what we call the Depth of Field (DoF), and the way that works is by looking at the field of view in relationship to the focal plane of the camera. That sounds like a lot to grasp, lets try and put this into something a little bit easier to understand like a picture, have a look at this diagram:

As you can see with the lower 'f-stop' we have a larger opening and range of view to capture light more quickly and in the higher 'f-stop' we have a smaller opening and the range of view which is extended out further. There are ways of altering this of course but let’s just try and keep this simple for now. So looking at the diagram, with more light coming in means that the focal plane is a lot shorter, and the focus is at a narrower point.  With less light coming through, say at F16 you can see it stretches out further, not the best explanation but it gives you the idea. So, if you have that in mind, you can see the higher the number the more that appears to be in focus because you are actually reaching out further to relation to the focal plane (the imaging point or sensor) and the smaller the f-stop the shorter the range meaning that less is in focus with a broader, wider amount of light coming through.

Look at the diagram again, what we’re looking at here is the point of resolution of the lens. You may have seen images of flowers or maybe even a portrait where the flower itself or the faces are in focus but the background is a blur, this is achieved by using a smaller 'f-stop' which has that large cone (field of view) so as a short relative distance or area on focus, whereas in a large landscape where everything seems to be in focus has a really high 'f-stop' and the reach is that much larger out. So basically you’re altering the way the light is being captured against the sensor, which is turn impacts how much can be seen as in relative focus based on the focal point in the depth of field, the point where you have focus too is the point, the depth of field then is how much in front or behind that point is sharp.

It all seems very back to front and counter intuitive and in some cases even sounds like double-talk. But the big thing to remember is that the small 'f-stop' number has large lens opening and short depth of field and the large 'f-stop' number has a small lens opening and a much wider depth of field, or what looks to be in focus in the image. We can manipulate what aperture we use to get the key elements we want in the photograph in focus.  Some camera phones and some point-and-shoot cameras don’t give you this option and they have an infinity focus basically for everything and use software to simulate shallow depth of field, or pushing the background into a blur.  Being able to control the relative area in focus in a photograph rather than a simple snapshot, allows you to be able to draw attention on what’s important in the image, be it a grand Vista, somebody’s face, somebody’s eyes a flower or if specific object in the scene. The art of a photograph is not capturing everything possible, but capturing the centre of attention, the purpose or vision to tell the story of the image.  Aperture is one of the key tools we use to do this by altering the main points of attention initially seen in an image.

This creative use of aperture is how we alter the depth of field of an image.  That is how to determine how much of the image is actually in focus, based on your chosen point of focus. You’ll often hear people talk about the bokeh, depth of field or what’s in focus and not understand what they’re really talking about is what aperture may created when it has been used in combination with the other key points of the exposure triangle.  Because they all have an impact on the image being captured. So depth of field is simply the term used to describe what is in focus in your photograph, using the primary point of focus within your image.

It is not adjusting the point of focus that alters aperture, the aperture ring on the lens does, or electronically alters the depth field.  Inside the lens, the size of the aperture hole is controlled by opening and closing the lens.  This moves overlapping 'leaves' inside the lens, you can see the opening get larger or smaller when you adjust the aperture ring.   But your eyes also do something similar and automatically by adjusting the pupil  smaller and larger.  In bright sunlight the pupil, which the is little black spot in the middle of your eye, will get really small to lessen the amount of bright light coming into the eye.  But at night the pupil get larger to let more lighting this is all called dilation. So this helps to alter the amount of light that can come into your eyes the other thing that happens is that there are subtle changes when you look at things.  The eyes, being smarter than a camera and lens adjusts both the sensitivity and the amount of light entering when making these changes.

Let’s try a little experiment, look straight ahead and look at something far out into the distance.   It could be a car or a tree, or if inside something on the other side of the room.  Put your finger up in front of your face but still keep looking at the object in the distance.  The finger appears to be blurry, but you can still see the image at in the distance clearly.  Now focus on and look at your finger, is come sharply in the focus but everything past that has become a little blurry you’ve just changed the depth of field in the image with your eyes.  It’s the same sort of thing as what are you choosing a point of focusing on using lens aperture to decide what you want to see clearly.  What you are looking at is the point of interest, you were altering the plane of focus to the image sensors in the back of your eyes, and this controls what you see in focus.  It’s the same sort of thing with the camera, you’re telling it were to focus depending upon the aperture used your then also telling it how much to keep in focus be there at that single point which is shallow with the low 'f-stop' and the wide opening or to expand the depth of field with a high 'f-stop' and a small opening which brings more of image into focus.

Hopefully that helps get the point across about what aperture is and how it affects the depth of field when we are taking photographs. So next time we can talk a little bit about exposure Hope you come back then, see you later.