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How does aperture affect my photographs?

Why should I care about the aperture with which a photo was taken?

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The aperture is the opening through which light goes to reach the camera's sensor.

The size of the aperture affects not only the amount of time required to take a photo, but also the depth of field within it.

With a wide aperture (so a low number, like f/1.8) gives a shallow depth of field - sometimes less than a millimetre with a macro lens. Because a lot of light is reaching the sensor (be it film or digital), this allows for fast shutter speeds

With a narrow aperture (so a high number like f/22), the depth of field is much greater, which is useful for things like landscape photography - it will limit the amount of light reaching your sensor, so you will get slower shutter speeds, which makes a tripod handy.

  • 11
    But what is aperture? – mattdm Dec 6 '11 at 17:53
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    It's the size of the opening through which light goes to reach the camera's sensor. – Nicolas Bouliane Aug 7 '15 at 13:26
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    @NicolasBouliane If only that comment were in the answer... – mattdm Jul 19 '17 at 17:49
  • Narrow aperature can cause difraction and limit the sharpness in certain cases. cambridgeincolour.com/tutorials/diffraction-photography.htm Wide open aperature may also exacerbate various defects in the lense, such as chromatic aberation, sharpness, etc... I don't pretend I understand fully the physics why, but I think it has something to do with light hitting the edge of the lenses, which bends the light more and makes defects more visible. – Calyth Sep 15 '17 at 21:06
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Aperture simply means "opening", and in photography we use the term to refer to the diameter of the opening in a special adjustable diaphragm within each lens. When this diaphragm is constricted, there's less physical space for light to get in, so naturally the exposure is darker, and a more-open aperture allows more light and results in a lighter exposure.

Aperture has another important property. When the aperture is very small, the admitted light is highly "collimated", which is a fancy way of saying "all the rays are nicely parallel to each other". This results in a sharp focus for all the light that comes in. When the aperture is more open, only the rays which closely match the focus point are collimated — which means that whatever you've focused on is sharp, but farther or closer parts of the scene will be increasingly blurry.

Lens aperture is usually given as an f-number, which is a ratio of the focal length of the lens to the effective size of the aperture. This is more convenient than using physical diameter, because it works out that the actual amount of light gathered for a given physical aperture size depends on the focal length — so, if one uses the ratios, the exposure is the same regardless of lens length. (The counter-intuitive side-effect of this scheme is that smaller f-numbers let in more light.)

These f-numbers are used in photography in a sequence which may seem irrational: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, and so on. However, these are chosen for a simple reason: each one has twice the area of the previous, letting in twice as much light. (There's no mystery to the numbers — the area of a circle is π × radius squared, and you can quickly work out that to double the area, one must simply increase the diameter by a factor of √2.)

Each step in the sequence is called "one stop", presumably because on manual lenses there's a physical catch which makes the dial which controls aperture nicely stop at each of these pre-defined points. However, the term "one stop" is generally also used by analogy for shutter speed and sensor ISO to mean "the amount by which this factor must be adjusted to double or half the recorded exposure".

An important thing to keep in mind is that while a small aperture excludes non-parallel light rays, as the size of the aperture approaches the wavelength of the light being captured, another effect comes into play: diffraction, which is a bending and scattering of waves as they pass through the aperture. In practical terms, this begins to affect APS-C form-factor cameras at around f/8, and so stopping down much beyond may increase depth of field at the expense of decreased sharpness in the in-focus areas. At some point, the effect of diffaction becomes strong enough that stopping down the aperture further doesn't provide any benefit at all.

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    I like the distinction between f-numbers and f/ stops in this answer. It is one of the few that is explicit and correct. Fine job without being pedantic. – Stan Sep 4 '13 at 23:10
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    the admitted light is highly "collimated" +1 – ALH Nov 11 '15 at 11:20
  • Smaller f-numbers letting in more light isn't counterintuitive at all when you consider that the "f-number" is actually the denominator of a fraction. "f" stands for the focal length, so if you have a, say, (for the purpose of this example: perfect) 50 mm prime and use it at f/4.0, then the diameter of the aperture is 50 mm/4.0 = 12.5 mm. If you go to f/2.0, then the aperture is now 50 mm/2.0 = 25.0 mm in diameter. Bigger diameter means bigger area, so it makes sense that it would be admitting more light; compare a room with a single small window with one with a single large window. – a CVn Oct 9 '16 at 10:21
  • I had not read this answer before posting mine. Withdrawing mine and voting this up. Here is a great post explaining the flare and starburst effect: slrlounge.com/diffraction-aperture-and-starburst-effects. – Fabricio Sep 15 '17 at 21:01
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Aperture is how we describe the size of the opening that determines how much light can come into your lens (like an iris on your eye). Assuming two lenses have the same size opening, a wider aperture value (smaller f stop #) will let more light in than the same size lens with a larger minimum aperture value.

A wide aperture will need a shorter shutter speed, which means you can capture high speed motion quicker. More light means a lower ISO setting is needed, which means less graininess in low light. A wider aperture will allow you to take photos in lower levels of light.

A wide aperture will result in a shallower depth of focus (the in focus part of the image will be smaller in the direction directly outward from the camera)

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The aperture of a lens is the opening in the diaphragm that controls how much light is allowed to reach your focal plane (film or sensor). The aperture is usually rated as a fraction of the focal length, hence the f/x.y nomenclature. The wider an aperture is, the more light it allows through, and the smaller the number under f/ is. A common wide aperture is f/2.8, and is called a "fast" lens. A common tight aperture is f/5.6, and is called a "slow" lens. Tighter apertures allow less light through, which requires an decrease in shutter speed to maintain the same exposure.

For a much more detail answer about aperture, including the basic mathematics behind how they affect light, you can see my reply to this question:

What does f-stop mean?

  • Niggling edit suggested: The aperture of a lens is the opening in the diaphragm that… s/b The aperture of a lens is the opening using a diaphragm that… – Stan Sep 4 '13 at 23:00
2

Some other frequent questions about aperture:

Is there such a thing as a maximum aperture that a lens can be open to?

Yes, it's contained in the name and marketing materials of, and is one of the most prominent markings on, any camera lens.

If you're asking about lens design in general, the maximum aperture is limited by the size and weight of its lens elements and of certain limitations of the lens design. Also, with bigger, heavier elements and a larger lens body comes greater cost.

What about a minimum aperture that it can be closed to?

This is a less important specification and is not usually provided everywhere the maximum aperture is provided. Unlike the maximum aperture, there is rarely a good reason to use a minimum aperture of any lens. It would be normal to assume that the minimum aperture of a DSLR lens is going to be f/22 or smaller, and yet it would be rare to have a reason to go smaller than, say, f/11.

When you use smaller apertures there is a point at which the diffraction from the aperture blades start to make the image blurrier than any benefit that the small aperture has.

Is there a lens with the narrowest aperture in the world?

Again, it's not a very important statistic so there's no real reason to strive to it.

However, you may be interested in looking into what a pinhole camera is. It is a camera which has a tiny opening for light to pass through. Effectively it is an aperture so small that no lens is needed to focus the light.

Is there one with the widest?

Lenses get bigger, heavier and more expensive with increasing aperture. There are physical limits to maximum aperture. Lenses with a f/0.95 maximum aperture do exist, which means that the effective aperture diameter is a little larger than the focal distance.

See also this discussion thread. There are various interpretations of what "widest aperture" means.

  • <trivia> The theoretical limit is f/0.5 </trivia> – Stan Aug 28 '13 at 19:57
  • Just a note to others that this was originally written as an answer to a question in which these exact questions were asked, that was duplicated to this one. Thanks for the edit mattdm, love your work etc. – thomasrutter Jun 16 '15 at 2:00
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Aperture is often confused and considered backwards - larger hole, smaller number - but this isn't really the case. Aperture is measured in a ratio, so its really a ratio of the diameter of the aperture in comparison to the focal length of the lens. This is why it seems backwards, because a smaller number in the denominator is really a larger number (e.g. 1/ 2.8 > 1/5.6). As said in other answers, the size of the aperture affects the amount of light let in, so changing it forces you to compensate with shutter speed and/or ISO and/or ambient light changes.

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    I find this explanation rather confusing for the following reasons: Aperture is often confused and considered backwards… What follows is generally true: larger hole, smaller number (number being the f/ focal ratio). Then, you say, "But… and go on. The next statement that Aperture is measured in a ratio, so… Aperture is "expressed" as a ratio… is what I was looking for here. What's my point? The OP and the editor have so much experience and intuitively know the subject so well, they have lost the ability to see how confusing this can be for one who doesn't really know. I mean this with respect. – Stan Sep 4 '13 at 22:54
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Aperture is how open your lens is (lack of a better phrase). The lower the number, the more light you allow in, but the less depth of field. The higher the number, the more depth of field and the less light being let in. Example: Taking a shot of something up close might need a lower aperture to capture ONLY that item, person, etc, and leave the background out of focus, while leaving the subject well lit (depending on your other settings). While a larger shot, might require a higher aperture, in order to keep everything in focus.

http://en.wikipedia.org/wiki/Aperture

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One thing to consider is that although some lenses do have low apertures, there can be aberrations at the lower f numbers. This is due to the lenses not being perfect. So to get the best sharpness across an image a more middle-range aperture may be ideal, as it cuts out light that has passed through the slightly imperfect glass at the centre of the lens. It does depend on your subject if this matters to you.

  • This does not answer the basic question: "What is aperture?" – mattdm Jun 15 '15 at 21:37
  • Aberations are not due only to lens imperfections. They're also due to the nature of light and the limits of diffraction limited lenses (basically any lens the refracts light and is larger than a pinhole). Even a single element lens that perfectly matches its blueprint will demonstrate CA, SA, FC, etc. when imaging more than one wavelength of light at the same time. – Michael C Feb 8 '18 at 19:56
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The best answer to your question is a rather a dumb down answer; as I was searching for the same when I was starting out in photography, because everyone was talking about aperture in manual mode.

  • High (narrower) aperture means you will get a sharp foreground and background.
  • Low (wider) aperture means you will get a blurred background and a sharp foreground.

Hope that help.

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    Welcome to Stack Exchange. You're getting downvotes and possibly wondering why. Your recent edit probably helped, but there's more than that. While this answer is generally true, it isn't the whole story — and doesn't even answer the question of what aperture is. Plus, it doesn't seem to add much to existing answers. On top of that, "Beach Life Marketing" as a name is probably making people worried that an increase in site reputation is going to be rewarded with a bunch of future spam posts, even if this post isn't spam. – mattdm Sep 15 '17 at 19:37
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    I think you should read the other answers to be better informed. – Fabricio Sep 15 '17 at 20:04
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    I downvoted not only because of the original wider/narrower mixup, but also because of the statements about foreground sharpness. I see what you're trying to say regarding depth of field, but as written, you don't seem to allow for any possibility to have a blurred foreground. Hint: the subject (or anything at the plane of focus) is not always the foreground, or at least not always the only foreground. Depth of field is described by near and far limits; beyond those limits (closer to the camera for near, further from it for far), things are blurred. – scottbb Sep 15 '17 at 20:18

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