The Exposure Triangle Part 3 – Aperture

In this Third post on the exposure triangle, I will be covering aperture, which is in my mind the most interesting part of the exposure triangle. It has two primary affects, depth of field and exposure speed. Like shutter speed aperture works the same for digital and film cameras so will be making no distinctions in this post.


Aperture is the measurement of the size of the hole in your lens that light travels through to fall on your image sensor.  It is measured with f numbers where a low number means you have a large aperture where more light is allowed onto the sensor.  A high f number means you have a small aperture where less light is allowed onto the sensor. Each of the aperture numbers on your camera is called a stop, and when you change from one full stop to the next or previous full stop you’re doubling or halving the amount of light that will fall on your image sensor. This will double or halve the amount of time you need to properly expose your image. I specify full stop because some lenses will allow you to change the aperture by less than a full stop allowing for a finer control. I will explain this in a bit more detail in the technical aside below but, the f stop scale does not go 1, 2 ,3, and so, full stops actually go 1, 1.4, 2, 2.8, etc.

A small f number also means you will have a shallow depth of field and a large f numbers means you will have a wider depth of field. depth of field is basically how much of your picture is in focus. More specifically it is the width of the plain that is in focus around your primary point of focus.  It may seem like having your photo in focus is something that you would always want, but this is not necessarily true. Most portrait photography actually has a shallow depth of field so that the subject of the photo is in focus but the background is blurry and does not distract from the subject.  Similarly with landscape photography you generally want a wider depth of field so that all of the photo is in focus.  something to consider with depth of field is that aperture is not the only factor in determining its width.  The factors are focal length in mm, longer focal lengths will have a shallower depth of field and distance to the subject where the closer you are to your subject the shallower the depth of field will be.  Depth of field examples are shown below, they were shot at 35mm about 1 foot away and from left to right they used f/ 1.8, f/ 11, and f/ 22

DSC_8315_1 DSC_8318_1 DSC_8319_1

Looking at the photos above it’s easy to see that some parts of the image are clearly in focus and some are clearly out of focus.  There is no one single point of transition from sharp to unsharp so this area of transition is called the “area of confusion”. The region that is beyond the area of confusion and out of focus is called bokeh. Bokeh, pronounced boh-kə, is basically the aesthetic quality of the out of focus parts of the image. The variations in lens aberrations and aperture shape can cause the bokeh to be either pleasing to the eye or distracting, this a a very subjective topic so I will leave it to you to decide what bokeh counts as distracting.  Bokeh is most often thought of when you have distance sources of light in your image because they often appear as perfectly spherical and slightly fuzzy points.  With some creativity, you can make bokeh shapes. For now it’s just a fun little bit of possibility, but I’ll do a follow up post on creating bokeh shapes and why they are formed at a later date.  Classic bokeh balls and bokeh shapes are shown below.



You can usually change your aperture setting by just rotating one of the selection dials on your camera if you are in a mode that allows you to change aperture, but check your manual for details.

Technical Details

For those of you still here, welcome to my first the technical section, if you have any additional questions or comments after reading this section feel free to ask below and will do my best to answer.

As discussed earlier the aperture of your lens is measured with f numbers. The f number is the ratio of the focal length of the lens to the effective aperture diameter. This means that the widest aperture that a lens can achieve is dependent on the lens’ length and maximum width. It also explains why quick lenses which have a wide maximum f stop, around 1.4 or 2.8, tend to either be short or very wide at the end of the lens when compared to a similar length lens with a higher maximum f stop. One full stop changes the aperture number by √2 (~1.41), as mentioned above this doubles or halves the amount of light that falls on the image sensor.  This makes perfect sense when you look at what you’re doing mathematically, if you change the diameter of a circle by multiplying or dividing by √2 you’re also effectively doubling or halving the area of that circle as shown by the equations below.

Area=\pi*r^2 = \frac{\pi*Diameter^2}{4}



Unfortunately the technical details behind depth of field, area of confusion, and bokeh are too vast to cover in this technical aside, I will likely be devoting an entire post on the topic at a later date.

End Technical Details

Tune in next post for the exciting finale on the exposure triangle where I will bringing it all together and talking about how I generally choose settings and what to think about when you’re composing a shot.



6 thoughts on “The Exposure Triangle Part 3 – Aperture

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