Understanding Exposure, Part 2: Aperture

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Aperture is the size of the opening in the lens. Some lenses have fixed apertures, but most photographic lenses have variable apertures to control the amount of light entering the lens. This aperture is regulated by a diaphragm made of overlapping blades that can be adjusted to vary the size of the opening through which light passes. The size of the opening also has a secondary effect on the photograph, as the diaphragm also changes the angle at which the light passes through the lens. We will discuss two "side effects" of changing the aperture size after we finish discussing aperture's relationship to exposure.

This article is part of a multi-part series of about photographic Exposure.
1. Introduction: The Exposure Triangle
2. Aperture
3. Shutter Speed
4. ISO

Diaphragm blades open and close to determine the size of the aperture

Like the pupil in your eye, the aperture diaphragm opens and constricts to control the amount of light passing through the lens. To facilitate a properly exposed photograph, we need to quantify the size of the opening so that we can mathematically incorporate this opening into our calculation for exposure+. Luckily, especially if you have my math skills, this has been done for us already!

Graphic representation of apertures at different f-stops

The ratio of the opening of a lens aperture when compared to the size of the lens—not a measurement, but a ratio—is referred to as an f/number, f/stop, focal ratio, f/ratio, or relative aperture. Regardless of the label you use, aperture values are spaced, for mathematical purposes, in exposure values (EV) or stops.

The benefit of mathematically figuring out EVs is that we can apply this measurement to all three adjustments that affect exposure—aperture, ISO, and shutter speed. With three adjustments all speaking the same "language," we can use them simultaneously or independently as needed.

The formula used to assign a number to the lens opening is: f/stop = focal length / diameter of effective aperture (entrance pupil) of the lens.

Written on the barrel of your lens, or digitally inside your camera and displayed in the viewfinder or LCD screen, you probably see f/stop markings at one-stop increments.

The smaller the number, the wider the opening. Therefore, a lens with a larger-diameter barrel and optics will allow a larger opening represented by a smaller f/stop. Your lens/camera might allow you to "dial up" different numbers than what is shown above; older manual lenses usually "click" at 1/2 stop increments. These numbers, seen on a digital display, like f/3.3 for instance, represent 1/2-stop or 1/3-stop ratios.

To keep things simple for this article, let us work with full stops, shall we?

Moving back to physics with some mathematics, here is how the f-stops change your exposure: If you set your camera to f/8 and then widen your aperture diaphragm to f/5.6 you have doubled the amount of light passing through the lens. Changing from f/8 to f/4 quadruples the amount of light. Going from f/11 to f/16 halves the amount of light.

Do you notice something strange? When we go from f/8 to f/4 we are doubling the size of the opening of the lens. Correct? Why then, is the amount of light quadrupled if the opening is only double the size? The return of math and of the Inverse Square Law.

Do the math: Double the radius of the aperture means four times as much light entering the camera

The formula for the area of a circle is: Area = π multiplied by the radius squared. If you crunch some numbers, you will find out that by doubling or halving the radius of the aperture, you will quadruple or quarter the area just like when we were talking about the difference in the intensity of a given light based on distance.

When we bring this numeric data into a system for EVs, it is quite simple. A change in aperture that results in the light being either doubled or halved means you have changed your exposure by one EV, or stop. So, if you widen the aperture from f/16 to f/11, you have a +1 EV result, as you have doubled the amount of light that will pass through the aperture diaphragm. f/16 to f/8 doubles the size of the opening, quadruples the amount of light, and represents a +2 EV shift. Simple, right?

So, now that you know how aperture effects exposure, let us talk about those two "side effects" of aperture that we alluded to above. The size of the aperture diaphragm not only affects the amount of light passing through the lens, it also affects image sharpness and is one of several factors that affect something called "depth of field."

Depth of field is defined as the amount of distance between the nearest and farthest objects that appear to be sharply in focus in an image. Without depth of field, the lens's razor-thin focal plane would cause problems for photography. Take a photo of a person and, for instance, the tip of their nose would be in focus but the rest of them would be completely blurry. Depth of field allows that focal plane to have a perceived depth.

Example of deep depth of field

Depth of field is a function of lens aperture size, lens focal length, the distance between the subject and the camera, and something called the circle of confusion. For the purposes of this article, we will keep the depth-of-field discussion relevant to aperture. Depending on your camera and lens, by opening your aperture to its widest settings, you will narrow the range of the focal plane to a very small distance. This can be used in photography for creative compositions with close-up photography and, most popularly, for making distant backgrounds blurry when taking portraits.

Shallow depth of field (large aperture)

It is important to note that some camera/lens combinations will not produce appreciably shallow depths of field, so do not think that by simply opening up your aperture diaphragm to its maximum, you will achieve extremely small depth of field. Adjusting your aperture diaphragm the other way, to its most narrow setting, extends the depth of that focus plane and allows a large range of the image to be in sharp focus. Deep depth-of-field techniques are used commonly in landscape images.

For a varsity-level, three-part depth-of-field discussion, click here.

Large depth of field (small aperture)

Not only does the aperture control the amount of light passing through the lens, it affects the angle of the light rays as they transit the lens. To be clear, we are not talking about how the lenses are bending light, we are talking about how light, when it passes by an object, is slightly bent by that object—in this example, the blades of an aperture diaphragm. This bending of the light is called "diffraction" and is a characteristic of light's wave properties.

When you constrict a lens's aperture diaphragm, you are bringing that diffraction closer to the center of the image. Many photographers, when they are starting to understand aperture, think that the key to maximizing sharpness is a small aperture because of the effect that aperture has on depth of field. However, because of diffraction, this is not true. Although you are increasing your depth of field by constricting the aperture, you are also increasing the amount of diffraction in the image and this causes the image to lose sharpness.

Additionally, even with modern manufacturing precision and computer design, there is no such thing as an optically perfect lens. Because of imperfections in the glass and the way light behaves when it is bent, lenses produce aberrations that have negative effects on an image.

When you open the aperture diaphragm to its maximum size, you allow the maximum amount of light into the lens and, with it, the maximum number of aberrations. By "stopping the lens down," or reducing the size of the aperture diaphragm, you reduce those aberrations and the sharpness of the image created by the lens increases. However, as we discussed above, the downside is that as you make the aperture diaphragm smaller, you will increase the diffraction as the smaller opening causes more bending of the light rays. The middle ground, the region where the aberrations are reduced and the diffraction is manageable, is known as the lens's "sweet spot"—usually in the region between f/4 and f/11 depending on the design of the lens. This sweet spot aperture is where you will get the maximum performance of the lens as far as sharpness and reduced aberrations, as well as getting a middle-of-the-road depth of field.

For more on diffraction, please click here.

So, in summary, aperture not only serves to control the amount of light passing through a lens, it also affects the performance of a lens in terms of depth of field and sharpness. Now it is time to head to the next segment of the eposure series, Understanding Shutter Speed.

317 Comments

Yea, I'm scheduled to have my yearly eye exam this month, so my ophthalmologist will dilate my eyes to f1,8 or f1,2. Fortunately, he provides ND sunshades. I'm hoping for overcast or rain.

Hey Ralph,

Funny, but Wikipedia says that the aperture of the human eye ranges from f/2.1 (or f/3.2) to f/8.3. :)

Great information.  Great writing. Easy reading is hard writing.  I am archiving this and will read over and over.   Glad you have wide margins for my notes and other comments.  B and H are doing a great job of educating its customers as well as increasing loyality.

Thank you for the kind words, Johniegee. I very much appreciate it.

We are glad you stopped by and took time to leave a comment!

Refreshingly articulate and thorough instruction. Giving beginners an intimacy with their creative instrument.  

Thank you very much, Joeseph!

What a terrific resource! I just recently got my very first DSLR (Nikon d3300, and I have had the expected triumphs and tribulations trying to shoot the moon and night sky. I'm reading everything I can get my hands on to help me learn what the heck I'm doing. From an utter newbie, thanks for shedding some light on aperture and for bringing much-needed clarity to the camera's capabilities. ;)

Hey Kaelie,

Thank you so much!

For some more tips on shooting the moon, check this out: https://www.bhphotovideo.com/explora/photography/tips-and-solutions/14-tips-shooting-moon

Thanks for reading!

Just found this, thank you Toidd for making this more clear.  Excellent article.

 

You are welcome, Beth!

Thanks for reading! Sorry for the delay in replying...we were on break!

Hi, sometimes I see a 400 website error when I browse your site. Just a heads up, best wishes

Thanks, Mury.

Thanks Todd, you really open up my eye's aperture.

Thanks for the kind words, bonnyx!

Excellent article. ...... 

Thank you, Asela!

Nicely done!  Simply put and easy to read.  Thanks

Thank you, Josie!

A beautiful read. Now i have a clear understanding of sweet spot

Thank you, Tope!

A good read and an entertaining and helpful vid too. Well done and thank you.

Thanks for reading, Joe Cool!

Gives clearer picture of the exposure triangle. Thanks for sharing this priceless information....

Thank you for reading, Fred!

Hi, thank you for a really good simple yet expansive explanation!!!!

Thank you for reading, orli! I am glad you enjoyed it! "Simple yet expansive" is my goal!

Thanks for very informative article. :)

Hi TORNIKE! You are very welcome! Thank you for reading!

That was a super explanation, and the video was really great.  Thank you!

I am glad you liked it, Katrina! Thanks for reading!

Luv your vidoes!

I love your music!

Thanks for reading!

again, thanks, you are very good at making things clear.

I really like your clear illustrations, too.

Hey sawa,

Thanks for thanking me and thanks for reading! I will pass the compliment on the graphics to our team of talented graphic artists!

Excellent article! Learned so much! Thanks B & H!

Thank you for reading, Jake!

Great tutorial. Future tutorials ...stars, milky way..night photography..:)

Hi Izabela! I love the suggestions. Now, I just need to get everyone to turn off their lights in NYC so I can get photos to illustrate the articles! 

We do have this older article on star trails: http://www.bhphotovideo.com/explora/content/digital-star-trails-step-step-guide

And this one as well: http://www.bhphotovideo.com/explora/content/epic-battle-between-choosing-star-trails-over-star-points

Our own Gabe Biderman and Matt Hill are night photography experts and teach many workshops. My masters thesis was in night photography and I often join them. Thanks for reading and commenting!

Hi 

i am from India and really like reading your tutors. Many many thanks for keeping them free. i look forward to tracking them and reading them.

a sincere request can you please keep me posted when the next edition is out.

many many thanks and continue the good work.

lalita

Hi lalita! Thank you so much for your comments.

The next editions are already out. Click on these links to see them: Shutter Speed and ISO.

Thanks for reading!

That was very good onhow you explain it and I found it to be better than many books that I have read.

Thank you for reading, Alfred! I am glad you enjoyed it!

Very clear.

Isn't the sweet-spot aperture "rough rule" valid as being about 3 to 4 stops down from wide open no matter the lens? Or maybe this combines with other factors for the actual sweetest spot?

Comments? Thanks.

Hi Erle,

Sorry for the delay in getting back to you!

I, too, have heard about that sweet spot rule. I have also heard that the sweet spot lives at double your maximum aperture (an f/2.8 lens would have a sweet spot at f/5.6, an f/4 lens would have a sweet spot at f/8, etc.).

Honestly, this is a good place to start, but to confirm, you can always test your lenses.

Let us know what you discover!

Thanks for reading!

What a wonderful way to explain this, as a beginner, I am very appreciative for any free material to learn from. We'll done, thank you!

Thank you very much, Martina! I am glad you enjoyed the lesson! 

Awesome! 

Can you forward links to part 2 and 3?

Hey Mike,

Certainly!

Shutter speed and ISO.

Thanks!

  Most intresting enjoyed very much, look forward to more of the same

Thanks for the compliment, richard!

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