There is more to know about aperture than many realize. There is also some hogwash spouted by some. So, by getting to grips with more than the fundamentals, one can more precisely control how your photographs look.
Recently, I wrote an article about shutter speed and how it affects the overall look of an image. It raised questions about the aperture that I hope to answer here, along with some other aspects of it that we should consider with our photography. There's much to learn about apertures, and too much for one article to cover all of the essentials, so this is the first of two parts. Furthermore, it's aimed at beginners. I have therefore simplified some of the explanations to ease understanding. I am not being pedantic and going into in-depth discussions about circles of confusion, for example, as it will befuddle some readers.
What Is an Aperture?
When I look at the dictionary definition of an aperture, it says it is an opening, hole, or gap. In photography, it is that. It is the gap in the lens through which light passes.
Just like the iris of your eye enlarges and shrinks to let more or less light through your pupil, changing the lens's aperture size likewise varies the amount of light passing through it. The aperture is sometimes referred to as the pupil. Also, the blades that open and close the aperture are referred to as the iris.
Understanding Apertures
Each aperture is allocated a number, known as the f-number. This is what befuddles a lot of novice photographers, but it's not hard to grasp.
As the aperture gets smaller, less light gets through the lens. Because there is less light at these smaller apertures, the shutter needs to stay open for longer to collect the same amount of light. That is a slower shutter speed. A wide aperture lets in lots of light, so the shutter gets faster.
There is more to it than that, and it involves a tiny amount of simple math. But keep with me—if you can divide and multiply, then keep reading, because that’s all that is involved.
What Is an F-number?
The f-number is a calculation of the aperture size. Here's what catches people out: a bigger number represents a smaller aperture. That doesn't seem to make sense until you know how it works.
The f-number is not a straight measurement of the aperture's diameter. It is the lens's focal length divided by that diameter. For example, if a lens has a focal length of 50mm and the aperture’s diameter is 25mm, the f-number would be 2, commonly referred to as f/2. A 100mm lens with a 25mm aperture would be f/4. Therefore, a 100mm f/2 lens would need a 50mm aperture.
It's easy to forget. So here’s an memory aid: although the "f" stands for focal, think of it standing for fraction. As ⅛ is smaller than ¼ is smaller than ½, so f/8 is smaller than f/4 is smaller than f/2.
Usually, on a zoom lens, as the lens gets longer, although the size of the aperture remains the same, the f-number gets bigger. (There are exceptions I'll get to later.)
Finding Your Lens' F-numbers
The f-number range is usually written on the exterior of the lens. For example, the lower-quality 18–55mm f/3.5–5.6 kit lenses that are supplied with cheap entry-level cameras typically have an aperture of f/3.5 at 18mm and f/5.6 zoomed in at 55mm. Likewise, a budget 75–300mm f/4–5.6 will be f/4 zoomed out at 75mm and f/5.6 at 300mm.
In the following image of an OM System 150-600mm f/5-6.3 lens, the widest aperture possible at 150mm will be f/5 and will decrease as the lens zooms to f/6.3 at 600mm.
At the other end of the spectrum, professional 12-40mm f/2.8 and 150-400mm f/4.5 lenses have those apertures fixed throughout their range because of some clever design. For example, my favorite lens at the moment is the OM SYSTEM M. Zuiko 150-400mm f/4.5 TC 1.25 IS PRO Lens, which has a fixed f-number throughout its zoom range.
When the lens is on the camera, you should be able to see the f-number through the viewfinder. It will change as you spin the adjustment dial, sometimes called the command dial.
What Is an F-stop?
An f-stop is a term often used interchangeably with f-number. However, it’s not quite that. The f-number is the numerical value we place upon each stop. An f-stop is an interval that halves or doubles the amount of light passing through the lens.
The scale of f-stops looks like this: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32.
f/1 lets through twice as much light as f/1.4, and f/1.4 twice as much light as f/2.
However, lenses exist that have some apertures that don't sit on that scale. For example, you might have f/1.7 like this great little Meike MK-35mm lens that costs around $80.
You will sometimes hear the term stop used when referring to image stabilization. If you can handhold an unstabilized lens at 1/500 second, two stops of stabilization will allow you to double that twice: 1/500 × 2 × 2 = 4/500 = 1/125. You can now handhold it at 1/125 second. Good-quality systems give you 7 or 8 stops of stabilization.
How Changing the F-Number Affects Shutter Speed
To recap, as we have demonstrated, changing the f-number affects the amount of light entering the camera. That, in turn, affects the shutter speed required to achieve proper exposure. We've also demonstrated that a lower f-number (larger aperture) allows more light in, enabling faster shutter speeds. Conversely, a higher f-number (smaller aperture) reduces the light, thus requiring slower shutter speeds to maintain the same exposure.
With each full f-stop, you will need to change the shutter speed by a full stop if you want the same amount of light to hit the sensor and the exposure to remain the same.
The Advantages of Fast Lenses
Lenses with a lower f-number are often referred to as fast lenses, as they enable fast shutter speeds. Fast lenses, with their wide maximum apertures (e.g., f/1.4, f/1.8), offer several advantages:
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Low-Light Performance: They allow more light to enter, making them ideal for photographing in low-light conditions because they enable faster shutter speeds.
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Shallow Depth of Field: Depth of field (DoF) is the distance between the nearest and farthest objects in a photo that appear acceptably sharp. Fast lenses with wide apertures help separate the subject from the background by blurring everything except the subject.
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Faster Shutter Speeds: They enable faster shutter speeds, thus reducing motion blur in action shots.
Why Fast Lenses Are More Expensive
Fast lenses are more expensive because of their complexity, precision, and better construction. Larger, high-quality glass elements with advanced optical designs increase their production costs, as do the other construction materials. Furthermore, the mechanical technology used for changing focusing, zooming, and changing the aperture is more advanced than in cheap lenses.
For example, the Nikon AF-S NIKKOR 800mm f/5.6E FL ED VR is a fabulous, fast, long lens, but it will be beyond the budget of many photographers.
The Aperture's Effect on Depth of Field
I will talk more about depth of field (DoF) in the next article. DoF is how much of the photo is in focus.
The above images were shot with a Sigma 30mm f/1.4 lens. The left-hand image was shot at f/1.4. Meanwhile, the one on the right was shot at f/11 using the same lens.
So, the smaller the aperture (higher f-number), the more depth of field there is.
You will also notice that in the first picture, I focused on a subject much closer to the camera. Proximity to the subject also makes a difference. The closer you get, the shallower the depth of field. I would argue that you can get a shallower depth of field by getting closer to the subject more easily than by opening up the aperture.
The apparent DoF can also be affected by using a longer lens. This gull was photographed with a long lens.
For now, that is probably more than sufficient to take in all at once. Please bookmark this article so you can come back to it, as well as my previous article on shutter speed.nIf you have any questions or comments, write them below and either one of our friendly readers or I will reply. So you have time to digest this, part two will be along in a week or so.
It would be interesting to me if you are able, in the next article, to explain the relationship between aperture and vignette. Why do lenses produce a more extreme vignette at some apertures than they do at other apertures?
Avoiding vignette is of extreme importance to me, as having the deepest corners and furthest edges of my images be bright and clear is something that I strive for.
Unfortunately, I have never found a really detailed explanation as to why some apertures affect the corner of the image more than other apertures. I suspect that a complete explanation would require several paragraphs and quite a bit of research, and a deep dive into optical physics. But if you're up do doing all of that, there is at least one reader who would find it quite valuable reading.
EDIT:
I understand how the size of the image circle that the lens produces had a huge affect on vignetting. But image circle and aperture are very different things. If an image circle is really big, and overlaps the sensor by quite a bit on all sides, then vignetting shouldn't happen at all. But lensmakers want to keep costs down and offer lighter, less expensive lenses, so we get stuck with corners that are a bit darker than the center of the frame. But the size of the image circle should remain the same regardless of which aperture is chosen, so given that, why do some apertures produce darker corners than others?
Excellent question, thanks Tom. I think the simplest way to look at it is by imagining the lens as a flashlight. The projected beam is brighter in the middle than the edges.
If you were to put a smaller aperture behind lens, the spread of light by the lens would be the same, but the dimmer edges of the light sourse, where the photons are more scattered, would be excluded.
Better quality flashlights with bigger and/or better lenses give a far more even beam.
It's clearly more complex than that, but I like simplicity. I am not a physicist, and if anyone who is qualified in the quantum mechanics wants to comment, they are very welcome.
Vignetting is caused by the front of the lens barrel blocking the light at the edges/corners of the frame. As the barrel is so close compared to the point-of-focus, actually being where the lens is, it is very out-of-focus and its vignetting fades very gradually. As you close down the aperture, you increase the depth of field and the blockage becomes sharper, more defined and noticeable.
An excellent explanation of f/stops! Thank you, Ivor.
It's a small, but important correction I think you'll want to make in the section labeled, "What is an f/stop?" In what surely is a typo, the text claims f/4 lets in twice as much light as f/2. It is, of course, f/1.4, not f/4, that lets in twice as much light as f/2.
Details, details!
Your article is an enlightening read. Thanks for your work.
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Thank you. Yes it was a typo. Someone messaged me about it earlier and I have asked the editors to correct it. Hopefully it will be fixed soon. Nice to know that people are reading it and picking up little errors. Thanks again.