Understanding Exposure, Part 2: Aperture


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 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 focal length 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.


I wonder if you're still tracking this thread. I understand the discussion and the math, but I'm looking at my Z 70-200 f/2.8 lens and dividing 200 / 2.8 to get a 71.4 mm aperture diameter. Looking at the physical lens it doesn't seem likely that would fit in there. Is there something in lens design subtleties with multiple elements where the actual physical size of the opening is different from the simple formula calculation? In other words, for a simple lens the calculated diameter would match the physical, but when they start stacking multiple elements in series maybe things start to be "effective" focal lengths, and "effective" diameters. I was going to research this some more, but I'm starting with you because I like B&H and I think you guys probably try to get things right:-)

Hi Jeff,

I am no longer at B&H (full time), but, yes, tracking this thread!

Good question and one that requires diving a bit down a rabbit hole of optics to answer...

As I stated above: "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."

The key phrase above is actually that which is in parentheses—entrance pupil.

The entrance pupil is the optical image of the physical aperture stop as seen through the front of the lens. This is different than the front of the physical lens (we know that is 77mm because of our filter ring), nor the opening of the aperture diaphragm (that we cannot get a ruler onto anyway...it is a projection. The location of the entrance pupil is at the lens' no-parallax point (which I mention in my article on panoramic photography).

Clear as a scratched filter?

You can just press the "I believe" button on the side of that Z 70-200 lens and know that some pretty smart folks at Nikon already did all the measuring and math for you so that you can just head out and make some great photos! :)

Let me know if you have more questions and thanks for reading!



A little bit more is needed than the inverse square law. I don't have pen and paper handy for the intermediate steps to bring aperture to the left side of the Exposure Value formula; but since the aperture is squared on the right side, square roots (√) are involved. 


N is apeture and t is shutter speed. log₂(x) can be calculated by log(x)/log(2) or ln(x)/ln(2).

PS: Being a computer programmer, I programmed my Hewlett-Packard programmable calculators, the HP-67 and HP-41C, with the formula so that given two variables, it calculated the other. To be useful for photography, I incorporated ISO into the program as well as Ansel Adam's Zone System to adjust exposure by underexposing or overexposing by whole stops.

As a lament, I miss Hewlett-Packard's calculators that used Reverse Polish Notation for calculating. I've been a fan of RPN when I bought an HP-45 to use in college. Paula brought an algebraic calculator to the household when we married. I couldn't use her calculator to balance the checkbook, so I bought her an HP-16C. That still works 45 years later. HP's advertising slogan was "HP has no equal."

Hey Ralph,

I'll have to take your word for all of that! I tried reading it twice, but my hair (what is left of it) started hurting! :)

Thirty years after high school I am still friends with my high school math teacher. Why? Because I was with her nearly EVERY day after school trying to keep my head above water in her classes!

Thanks, as always, for reading Explora!



Well, for me it was fun doing it. Programming is still fun. Engineering started out as my major in college, but chemistry and physics blew me out of the water. The calculus classes that I had taken transferred over to Computer Science, so that was not a loss. I found that I enjoyed computer programming in my first engineering class using a DEC PDP-8 minicomputer instead of the university's mainframe, which was out of sight. 

Meanwhile...I had 25 credit hours of calculus and can't remember a single thing due to the trauma!




Here is a hypothetical question. It sounds like the diffraction is a problem. wouldn't it be great if we could get rid of it? What if it was possible to replace the aperture with a different device that is always with the same radius, but that can regulate the amount of light that can pass through. for example - suppose there was an opaque container with a liquid whose opacity could be changed very quickly by some device. would that make controlling the result more predictable? better in some cases?

Hey Amit,

I think that technology does exist in the form of built-in neutral density filters that you find on some video cameras and even digital still cameras.

Ironically, if your lens was "wide open" all the time, you would constantly deal with the degraded optics of having the lens with no restricted aperture. Lenses are usually sharpest when the aperture diaphragm is set to mid-range f-stops.

Interesting thoughts! Thanks for reading and commenting!



My brand new camera (Sony A7iii) is using a finer f-stop resolution. It uses 1/3 f-stops. Thus - for example, it has the numbers 4 4.5 5 5.6, and so on. Thinking about it it makes sense to me since it means that the photographer can produce a more precise result (by precise I mean something that is more to his liking)

Hi Amit,

Yes, most modern electronic cameras do 1/3-stop adjustments. I personally change the settings on cameras to do half-stop increments (when the option is available to me). 1/2-stops give plenty of precision (for me). I wish my FUJFIILM cameras would give me the 1/2 stop option!

And, if you shoot vintage lenses on your Sony, you might find yourself with an aperture ring that only does full-stop changes!

Thanks for reading!



I love you Todd :-) you saved my life. Thank you very much and Stay Blessed...

Um...you are welcome...whoever you are. :)

Thanks for reading Explora!



Thank you Todd for your fantastic writing and B&H for keeping this great article available. I think this is the best description of aperture on the internet!

Well, that made my day. :) Thank you for the kind words, Tommy!

Thank you for reading!



[Link removed] This so so useful, thanks for posting this informative article</a>

Thanks, Shoot! I am glad you enjoyed the article!

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.  

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. ;)

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

Nicely done!  Simply put and easy to read.  Thanks

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

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

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

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!

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!


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