What is the “Aperture” and How It Affects the Depth of Field?

We are constantly bumping on this “Aperture” notion, especially when we are trying to buy a new lens. Well the main function of a camera lens it’s to collect light. But often we need to control the amount of light coming in: some times we need more and other times we need less. Well this is the job of the aperture (which in fact is an iris), to control the amount of light getting in ( the larger the diameter of the iris, the more light gets to the image sensor). Another fact that aperture controls is the Depth Of Field.

So the aperture is measured in F-stop : f/1.8, f/2.8, f/10, etc. The smaller the f number (value), the larger is the diameter of the iris, and the  bigger the  f number (value) the smaller is the diameter of the iris. You might ask “Why it’s so twisted?”. Well because the F Number is represented by the formula F/D (F= focal length, D= diameter of the iris). So as you can see the diameter of the lens varies at different focal lengths.This is because a 300mm lens needs an larger “hole” to collect the same amount of light as a 50mm lens with with the same aperture. In fact that’s the reason why those 300mm, 400mm, 600mm, are so huge, because they need a hole large enough to catch enough light. Also that’s why there aren’t  super telephoto lens like 600mm with small f/number, for example f/1.8 (first of all will have a microscopic depth of field, and second of all will be an enormous).

Also when you buy a Zoom lens there are 2 values of  the aperture. Let’s say 28-135mm with f/3.5 – f/5.6. Well, these are the maximum apertures for the given focal lengths (remember smaller f values means more light coming in). So at 28 mm focal length you can have a maximum aperture of f/3.5 and as soon you change it to 135mm you’ll have the maximum of  f/5.6 without being able to go lower. Also remember that around 50-60mm you’ll have f/4 and around 90-100mm you’ll have f/5. So as you change  the focal length the maximum aperture changes as well. This doesn’t aplly for a lens like 70-200mm f/2.8, because you’ll have the same maximum aperture through all the focal range.

I hope i didn’t bored you to death with these technicalities. Now we get on to the practical part of the aperture:

Depth of Field (DOF)

Depth of field (DOF) is the distance where the object is in focus in front and beyond of it. Depth of field is mostly used to isolate you subject from the backgroound.

The depth of field it’s influenced by three factors: Aperture, Focal length and Distance from the subject. So we will have three images which will exemplify these facts. The region marked in Darker Blue is the region which is in Focus.

1.  Changing the Aperture – when you change the aperture you change the depth of field so when you are at the same distance from the object and the same focal lenght, the DOF will vary with the aperture from f/2.8 (where it will be a smaller DOF) to f/22 (were it will be a larger DOF). You can see this in the image below:

2. Changing the Focal lenght – so as you change the focal lenghts and keep the same distnace and aperture, you will have a very big DOF on wide lens and small DOF on the telephoto as seen in the image below:

3. Changing the distance from the subject – another factor the influences the DOF is the distance from the subject ( the focus distance ). So as we keep the same focal lenght and aperture and get farter from our subject the DOF get larger as seen in the mage below:

So as you can see from the examples above you can use the depth of field to your advantage in every situation having all the flexibility you need. Another important thing you should know is that the depth of field it’s stronger behind the subject  than in front if it ( so in front of the point where you focus is 1/3 of the distance of DOF and 2/3 of the distance behind the pint).

Bokeh

The word Bokeh derives from the Japanese word Boke which means “blur” or “fuzzy”, and that’s just what the term refers to in photography. The out of focus areas in the photograph look very different depending on the depth of field as well as the lens used, some lenses produces much better bokeh than other lenses. The shape of the aperture is one of the most important parts together with the quality of the optics when it comes to how the out of focus areas appear.

The photograph above is meant to illustrate what Bokeh is. The lens used was the Canon 50mm f/1.8 which isn’t considered to be a good bokeh lens due to it’s 5 aperture blades. But, it looks pretty good

Tags: Photography, Tips & Tricks, Tutorials