Understanding Aperture

22 Jun


As I’ve mentioned before, all of your camera settings work together in concert to create a (hopefully) well-exposed image. The aperture (hole through which light passes on its way to the sensor) can be adjusted, with the larger diameters allowing more light to pass through. It is comparable in function to the pupil of the eye. On a bright sunny day, our pupils shrink to pin-pricks, cutting down the harsh light. Similarly, in bright sunlight, a small aperture will generally allow just enough light to hit a sensor for proper exposure. On the other hand, at dusk, you may need to open the aperture wide to capture as much available light as possible.

The somewhat confusing thing about aperture is not the concept, but the labels. The size of the aperture is designated with an “f/” and a number. That number is the ratio of focal length to effective aperture diameter… <— (Greek?)

Now, I know…most of us aren’t math majors. Ratios? Pssshhaaww! I’m going to simplify this a little. What you need to know is that the f-number will tell you how wide the aperture is open, plain and simple.

Phew! Calculator crisis averted!

Now there is one more tricky thing to remember about aperture. The smaller the number, the wider the aperture, the more light you are letting in. It seems backwards, and personally, I found it difficult to remember when I first dove into photography. If you want more information about the hows and whys of aperture math, the interwebs has plenty of resources. For today’s purposes, however, all you need to know is that “small number = big opening”.

Example: f/4 will allow more light to pass through to a sensor than f/22. Therefore…

• f/4 ::: wider opening ::: allows more light through
• f/22 ::: smaller opening ::: allows less light through

Now that you have an understanding of aperture terminology, let’s talk about the practical effect these setting changes will have on your images! Well…beyond the amount of light it lets in, of course.

Sweet Spot

Every lens has an optimal aperture at which a scene will be sharpest with the least amount of blur (tack-sharp in photography terms). Please note, this refers to sharpness, not depth of field.

The blur is considered to be “defocus” blur on images with wide open apertures (f/1.8, etc) and “diffraction blur” for images with closed, or stopped down, apertures (f/20, etc). The sweet spot on every lens varies – generally it is between f/4 and f/8 – and opinions vary on how noticeable the impact this has on images.

Depth of Field

The larger the diameter of your aperture (smaller f-number), the shallower your depth of field. In laymen’s terms, the bigger the opening, the more blurry your background will be. This is important, because your choice in aperture needs to be suited to the type of shooting you are doing.

For landscape images, you generally want to have as much of the scene as possible in focus, so you would shoot with a larger aperture. Obviously, this will vary depending on lighting conditions, but in my experience, when photographing a landscape I generally shoot between f/8 – f/22.  (The tracks below were shot at f/14. For comparison purposes, the chain was shot at f/5.6.)





However, if I am shooting a subject that I want to isolate from the background, or a portrait, a shallow depth of field is preferable. Similarly, if I want to create background bokeh (out of focus bits, such as circular discs of light in the background of images), I will open the aperture up wide (f/2 for example). It’s important to find the best aperture for the effect you want to create.

When shooting portraits wide open (for example with an f/1.8 lens), it’s especially important that the focus is exact. That depth of field is so shallow that you can easily have one part of the face in focus while another is blurry… At such a wide open aperture, the difference between sharpness and blur is only a matter of inches.

Soft skin is not necessarily a bad thing, but the eyes are the focal point of most subjects.  Generally, it’s important for the eyes to be crisp.  With an f/1.8 lens, if a face is angled, it may be impossible to have both eyes in focus.  In that case, I make sure the eye closest to the camera is sharp to give the viewer a focal point.

Of course, these are guidelines not rules…everyone has their own artistic vision. However, knowing the effect an aperture gives you the ability to drastically change, manipulate or fine-tune an image… Understanding apertures will be one of the greatest tools in your artistic bag!


Are delicious! In photography, they are also visually delicious.

Reservoir - Barkhamsted, CT

Reservoir – Barkhamsted, CT

If I am shooting a scene with street lights, or with the sun partially obscured and I want to create a starburst effect (where you can see beams of light, kind of like when you squint while looking towards a street lamp), I will shoot more towards the f/22 end of the spectrum.  Essentially, this does the same thing as you squinting your eyes. The higher the number, the more defined the light rays will be.


Finally, it’s important to keep in mind the effect aperture can have on image distortions. These occur with more frequency at wide open apertures (smaller f-numbers).  At f/1.8 you will see higher incidences of both Chromatic Abberation and Vignetting. You can read about the former here.

The latter – vignetting – occurs because with a larger opening for the light to pass through, the intensity of the light hitting the sensor falls off at the edges of the image. In my mind, I imagine it similar to the intensity of water passing through a hose. If you put your thumb over the end of the hose, the water will hit its target with more intensity.


Obviously, there is more to aperture than just proper image exposure. Even small changes in your aperture settings can drastically affect your final product. Do you want a shallow depth of field, or are you shooting a landscape? Is it a night scene that would benefit from starburst? Are you looking for the sharpest image possible? Understanding the differences between aperture settings is part of mastering your craft…it’s one of the many tools we have to help shape an image.

The best way to become familiar with the various effects that aperture can have on your images t to practice, practice, practice. It’s the only way we make progress!

7 Responses to “Understanding Aperture”

  1. fiztrainer June 22, 2012 at 2:09 pm #

    This has to be the clearest, most understandable explanation of Aperture I’ve read to date. This was really great. I wanted to add something that helped me with the whole f-stop number thing (the smaller the number, the larger the opening and vice versa). I took a class and the teacher explained that the numbers represented are actually fractions. When I think of the number as a fraction, it helps me remember which is which. It made more sense to me once I realized that. Maybe it’ll help others. I would get so annoyed in the beginning because I kept reversing it when I was taking pictures and some pictures you can’t retake if you get that wrong. So, that little tidbit really helped. Thanks so so much for this post. It was really great!! 😀

    • seeingspotsphoto June 22, 2012 at 2:37 pm #

      Oooohhh, great tip! Thank you so much for sharing! Hopefully that helps our fellow creatives out. 🙂

  2. David Pasillas June 22, 2012 at 4:29 pm #

    Back when I tried to have a blog on my own site, I made a post about the exposure triangle that included a couple of graphics that might help people understand aperture. Of course, maybe only a handful of people have ever read that post lol.


    • seeingspotsphoto June 22, 2012 at 5:28 pm #

      Well, hopefully this will get you a handful more! Thank you so much for sharing, David! 🙂

  3. Dave June 24, 2012 at 8:51 pm #

    Hi, Shannon. Very helpful explanation, and I really liked the images you chose to illustrate the points. Years ago my Dad taught me a useful rule of thumb about depth of field (he had lots of thumbs) – about one third of your acceptable depth of field extends in front of your point of critical focus and about two thirds behind, so to get the most out of my depth (say, when photographing a crowd), I would “spread” it through the crowd by focusing one third of the way into it. I have never been displeased with the results.

    • seeingspotsphoto June 25, 2012 at 3:05 am #

      Great tip! I think I’ll have to give this focusing advice a whirl! Thank you for sharing. 🙂


  1. Community Collaborative Project::July « SeeingSpotsPhoto.com - July 13, 2012

    […] First, try pre-focusing on the subject, then metering for the light.  Or, you can make use of aperture to create a larger depth of field, so more of the image remains in focus. ● Flashes need not […]

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