WHAT YOU DON’T KNOW ABOUT APERTURE

WHAT YOU DON’T KNOW ABOUT APERTURE

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The aperture is a small set of blades in the lens that controls how much light will enter the camera.  The blades create a octagonal shape that can be widened (we photogs call it shooting “wide open”), or closed down to a small hole.  Obviously, if you shoot with the aperture wide open, then more light is allowed into the camera than if the aperture is closed down to only allow a tiny hole of light to enter the camera.

So suppose you take a picture that is too bright.  How do you fix it?  Simply choose a smaller aperture.  Simple! Aperture sizes are measured by f-stops.  A high f-stop like f-22 means that the aperture hole is quite small, and a low f-stop like f/3.5 means that the aperture is wide open.

Scale of aperture settings

The aperture also controls the depth-of-field.  Depth-of-field is how much of the picture is sharp, and how much is blurry.  If you want to take a picture of a person and have the background be blurry, you’d use shallow depth of field.  If you want to take a picture of a sweeping mountain vista, you’d want to use a small aperture size (high f-stop number) so that the entire scene is in sharp focus.

Aperture ExerciseThis first photo I shot at a 1.4 aperture.

 

 

 

 

 

Aperture Depth of Field

The second photo I shot at 3.2. Notice how the background is becoming slightly less blurred.

Aperture Light Exercise

This final photo I took at 5.6 and you can see a clear difference between it and the first photo.

With each adjustment to the aperture, the shutter opens less and less wide, letting less and less light in, which changes the depth of field, or how blurry the background is. In aperture priority mode the camera will choose a slower shutter speed to compensate for lack of light. If the camera allows you to see the information on each shot, make note of the shutter speeds and how they change in comparison with how the aperture changes.

Graphic explanation of the aperture for photography basics learners

The images below reveal what a lens looks like at different aperture settings.

apertures copy

apertures-2 copy

apertures-3 copy

As you can see, the physical size of the aperture decreases as the f-number increases.

With cell phones and advanced cameras, everybody’s a photographer these days! But, knowing a little bit about the math, science & engineering that go into capturing a photo will help you perfect your snapshots! aperture6

 Simple geometry plays a central role in the look of your pictures. Consider the opening the lens, or aperture, through which, light passes on its way to the sensor. Your camera’s settings for aperture are represented by what’s called the F number. It’s defined as the focal length of the lens DIVIDED by double the radius of the aperture, so larger apertures correspond to smaller F numbers. The aperture is approximately circular and we know the area of a circle as πr2. To double the amount of light you need to double the area which means increasing the radius by the square root of 2, or about 1.4. This is why the F number settings go in a bit of a strange progression: 8, 5.6, 4, 2.8,…

 

_MG_0112_MG_0111

Large aperture lenses gather more light, usually by using larger and more expensive pieces of glass, but they have an important artistic effect: they only show objects at a certain distance from the camera in sharp focus. Everything closer or further away appears blurred. This is a great way of separating your subject from distracting backgrounds: ideal for portraits, in which the subject is far more important than what’s behind, one would hope. Conversely, a small aperture like F22 blocks out aperture4the out of focus light rays, so it’s ideal for landscapes, where you want as much of the scene to appear in focus.

Photography is about deliberately choosing how, when and from where to take pictures. It’s also about understanding through experimentation. So with this fresh scientific approach, start tinkering with the settings on your camera and take some fantastic photos!

 

Measuring Aperture

Aperture is measured in f stops. The larger the number under the f, the smaller the hole (aperture) and the less light that comes in. A wide open aperture results in a shallow depth of field whereas a smaller aperture results in a greater depth of field. If you want to know why this is see our lesson on the Physics of Depth of Field.

The aperture is measured in f-stops and that the smaller the number, the bigger the opening. The reason for this is because each measurement is actually the fraction of f/(number) IE f/1.4, f/2, f/2.8, f/4, f/5.6, etc. The Aperture F-Number, or f-number, simply stands for the lens’ focal length. That way no matter what the size of the lens, the f-number would be proportional or universally applied across all lenses. In other words, when using the aperture and shutter speed, using one lens will generate about the same exposure as using the same setting on a different lens.

f number

As you can see in this illustration, the opening can have the same size of f/4, while having different physical measurements.

So the point of the f-number is just to convey the illuminance of the aperture opening. It’s done as a fraction of fbecause the physical measurement may change from lens to lens but the light it puts out will be the same, if the f-stop setting is the same.

1) What is Aperture?

Simply put, aperture is a hole within a lens, through which light travels into the camera body. It is easier to understand the concept if you just think about our eyes. Every camera that we know of today is designed like human eyes. The cornea in our eyes is like the front element of a lens – it gathers all external light, then bends it and passes it to the iris. Depending on the amount of light, the iris can either expand or shrink, controlling the size of the pupil, which is a hole that lets the light pass further into the eye. The pupil is essentially what we refer to as aperture in photography. The amount of light that enters the retina (which works just like the camera sensor), is limited to the size of the pupil – the larger the pupil, the more light enters the retina.

So, the easiest way to remember aperture, is by associating it with your pupil. Large pupil size equals large aperture, while small pupil size equals small aperture.

2) Size of Aperture – Large vs Small Aperture

The iris of the lens that controls the size (diameter) of the aperture is called “diaphragm” in optics. The sole purpose of the diaphragm is to block or stop all light, with the exception of the light that goes through the aperture. In photography, aperture is expressed in f-numbers (for example f/5.6). These f-numbers that are known as “f-stops” are a way of describing the size of the aperture, or how open or closed the aperture is. A smaller f-stop means a larger aperture, while a larger f-stop means a smaller aperture. Most people find this awkward, since we are used to having larger numbers represent larger values, but not in this case. For example, f/1.4 is larger than f/2.0 and much larger than f/8.0.

Take a look at this chart (image courtesy of Wikipedia):

F-numbers

The size of the circle represents the size of the lens aperture – the larger the f-number, the smaller the aperture.

3) What is Depth of Field?

One important thing to remember here, the size of the aperture has a direct impact on the depth of field, which is the area of the image that appears sharp. A large f-number such as f/32, (which means a smaller aperture) will bring all foreground and background objects in focus, while a small f-number such as f/1.4 will isolate the foreground from the background by making the foreground objects sharp and the background blurry.

Depth of Field

As you can see, just changing the aperture from f/2.8 to f/8.0 has a big effect on how much of WALL-E is in focus and how visible the background gets. If I had used a much smaller aperture such as f/32 in this shot, the background would be as visible as WALL-E.

Another example:

Mailboxes

NIKON D700 @ 48mm, ISO 200, 1/1600, f/2.8

In the above example, due to the shallow depth of field, only the word “Cougar” appears sharp, while everything else in the front and behind of that word is blurred. If I had used a larger aperture such as f/1.4 and focused on one of the letters, probably only that letter would have been sharp, while everything else would have been blurred out. The larger the aperture, the smaller the area in focus (depth of field).

4) Lens Apertures: Maximum and Minimum

Every lens has a limit on how large or how small the aperture can get. If you take a look at the specifications of your lens, it should say what the maximum (lowest f-number) and minimum apertures (highest f-number) of your lens are. The maximum aperture of the lens is much more important than the minimum, because it shows the speed of the lens. A lens that has an aperture of f/1.2 or f/1.4 as the maximum aperture is considered to be a fast lens, because it can pass through more light than, for example, a lens with a maximum aperture of f/4.0. That’s why lenses with large apertures are better suited for low light photography.

The minimum aperture is not that important, because almost all modern lenses can provide at least f/16 as the minimum aperture, which is typically more than enough for everyday photography needs.

Nikon 50mm f1.4 AF-S

There are two types of lenses: “fixed” (also known as “prime”) and “zoom”. While zoom lenses give you the flexibility to zoom in and out (most point and shoot cameras have zoom lenses) without having to move closer or away from the subject, fixed or prime lenses only have one focal length. Due to the complexity of optical design for zoom lenses, many of the consumer lenses have variable apertures. What it means, is that when you are fully zoomed out, the aperture is one number, while zooming in will increase the f-number to a higher number. For example, the Nikon 18-200mm lens has a variable maximum aperture of f/3.5-f/5.6. When zoomed fully out at 18mm, the lens has an aperture of f/3.5, while when fully zoomed in at 200mm, the lens has an aperture of f/5.6. The heavy, professional zoom lenses, on the other hand, typically have fixed apertures. For example, the Nikon 70-200mm f/2.8 lens has the same maximum aperture of f/2.8 at all focal lengths between 70mm and 200mm.

How to Choose the Sharpest Aperture

Photographers have a dilemma. If you want your photographs to have the largest possible depth of field – from the foreground to infinity – a small aperture is absolutely necessary. At the same time, though, a small aperture causes your photograph to lose sharpness from diffraction. So, where’s the sweet spot? In this article, I will cover how to choose the sharpest possible aperture for such a photograph, including mathematically accurate charts (free for printing) that are easy to use in the field.

Before that, though, please note that this article only applies if you want everything from the foreground to the horizon (infinity) to be sharp in your photographs. In an ideal world, you would always be able to use the sharpest aperture on your lens. In practice, though, you will find yourself stopping down to smaller, diffraction-prone aperture values if you need more depth of field.

A large portion of this article is simply the background research and calculations that went into the finalized charts. If you prefer to skip the sections two and three, you won’t miss anything particularly important; however, I want to include some background information for readers who are curious.

Overlook

NIKON D800E + 35mm f/1.8 @ 35mm, ISO 100, 6/10, f/11.0

1) What Makes a Photo Blurry?

For the purposes of this article, which only relates to your aperture, there are three elements that cause a photograph to blur.

Lens aberrations: These are what cause your photos to grow less sharp at wide apertures like f/2 or f/1.4. If a lens is considered “less sharp” than another, lens aberrations are to blame. This is something that manufacturers are improving over time, although there is always a balance between the lens’s size, weight, price, and quality.

Diffraction: This is the loss in sharpness that occurs when you stop down to extremely small apertures like f/16 or f/22. Technically, diffraction exists at all apertures, but it only begins to rob noticeable sharpness once you stop down far enough. For more information, read our in-depth article on diffraction.

Defocused Foregrounds and Backgrounds: It is impossible to focus at more than one plane in a single image. So, if you focus partway into a scene, the foreground and infinity will both be blurrier than they would be if perfectly-focused. You can minimize this blur by focusing at the hyperfocal distance, which is twice the distance to the closest object in your photograph. For instance, a scene from five feet until infinity will be as sharp as possible if you focus at ten feet. For more information, read our in-depth article on the hyperfocal distance.

Windswept Valley

NIKON D800E + 105mm f/2.8 @ 105mm, ISO 100, 1/500, f/9.0

2) Airy Disks and Circles of Confusion

Diffraction and defocus effects can be measured by the size of their blur on a camera’s sensor. If the size is large, the photo is blurry. (For now, I will skip over the effects of lens aberrations. They are important in determining your sharpest aperture, but they vary from lens to lens. As such, I’ll cover them in a separate section near the end of this article.)

To quantify the amount of blur in your photograph, you need to combine two different values:

Airy disk: The blur from diffraction that is projected onto your camera sensor. The Airy disk increases in size at small apertures.
Circle of Confusion: The blur from out-of-focus regions that is projected onto your sensor. The more out of focus that a point becomes, the larger its circle of confusion will be. So, the circle of confusion is largest at the foreground and infinity of your photograph (assuming that you have focused at the hyperfocal distance point).

These two variables combine all the time. No matter where you focus, the total blur at a given point in your photograph is the combination of its Airy disk (how much diffraction is present) and the circle of confusion (how out-of-focus it is). Now, the question is simple: in what way do these two variables combine?

The answer is not just addition or multiplication, as you may expect. There is no easy way to combine these two fundamentally different effects, but the formula below is most common:
Formula-5

Ignoring lens aberrations (and motion blur), the resulting value is the total blur at a given point in your photograph. Of course, we want this value to be as small as possible. The rest of the article discusses how this is accomplished.

3) Formula

The formula to find the sharpest possible aperture – the one which minimizes the total blur value above – is quite simple:

Formula-4

This formula is not my own work. I gathered the information from three sources, as found in the links below. I encourage you to read these sites if you are interested in the derivation of the formula above, since there is not enough room in this already long article to cover all the background information:

The only change that I made to the formula is to allow “Distance to far object” to equal infinity, which is the case at the hyperfocal distance. When this is done, the formula simplifies to the version shown below:
Formula-3

By plugging in half the hyperfocal distance (“Distance to near object”), along with various focal lengths, I turned this information into the charts you see in the next section.

4) The Charts

Before presenting the charts, there are some points to note:

  1. These charts are huge. In the next section (“Putting It into Practice”), I will show how to take such a large chart and shrink it down to match your equipment. For my 20mm lens, for example, the final chart is only ten rows tall and two columns wide.
  2. Believe it or not, these charts are accurate no matter your camera’s sensor size. The Olympus 7-14mm f/2.8 is why the chart includes such wide angle lenses (there is more information about this in the next section).
  3. This chart stops at any aperture smaller than f/22. If you want more depth of field than that, I strongly recommend focus stacking. I also excluded any apertures larger than f/2.0. I could have stopped before that, but I wanted to cover my bases in case Zeiss introduces a 24mm Otus that is sharpest around f/2.8 (more info for why lens sharpness matters is under “Simplifying the Charts” below).
  4. Although very few people would use a chart like this for telephoto lenses, I included all the major focal lengths up to 200mm. If yours isn’t included, I discuss how to create your own chart in the next section.

Now that those are out of the way, let’s see the charts. Here’s Metric:

Metric Chart

And Imperial:
Imperial Chart

If you click on these charts, the values will be significantly clearer.

5) Putting It into Practice

These charts are not particularly difficult to use. All you need to do is pick a focal length, then find your hyperfocal distance. The intersection on the chart is your ideal f-number. However, there are a few more essential pieces of information before you can use either of these charts. This section is very important; if you use the charts incorrectly, your results will not be as sharp as possible.

Breaking

NIKON D800E + 105mm f/2.8 @ 105mm, ISO 100, 1/3, f/7.1

Hyperfocal Distance?
Yes, unlike a normal hyperfocal distance chart (which gives you the hyperfocal distance), this one requires that you find it yourself. However, this isn’t hard. As I mentioned in my hyperfocal distance article, all you need to do is find the distance to the closest object in your photograph. Then, double the distance.

Say, for example, that you are using a 20mm lens. If you want everything in focus from five feet until infinity, your hyperfocal distance is equal to ten feet. So, on the Imperial chart, find the f-number that corresponds to ten feet at 20mm. In this case, it is f/10.0. Then, simply focus at ten feet, set the lens to f/10.0, and take the picture. Everything from five feet until infinity will be as sharp as is possible for a single frame.

In some of my recent articles, a few readers mentioned that the most accurate way to measure these distances is to use a laser rangefinder measurer. (I tend to estimate the distances instead, although that isn’t as accurate as possible.). If you purchase a measurer, make sure that it also records distances in the one-foot or one-meter range – hunting rangefinders typically do not. Although I tend to think that a laser measurer is overkill, some photographers undoubtedly will want the extra precision.

Carved Stone

NIKON D800E + 20mm f/1.8 @ 20mm, ISO 100, 1/5, f/11.0

Using Different Cameras
As I said above, the two charts are accurate regardless of the camera that you use. A Sony RX100 IV and the Nikon D750 will both use the same chart, even though their sensors are vastly different in size. However, there are two points to keep in mind:

First, and most importantly, don’t use your lens’s equivalent focal length. Instead, always use the actual focal length of your lens. This is absolutely crucial! Say, for example, that you are using the Nikon 24mm f/1.4 lens on a DX camera. Even though the equivalent focal length is roughly 35mm, it is essential that you look at the entry for 24mm instead!

The same goes for any camera. The Sony RX100 IV, for example, has an equivalent focal length of 24-70mm, but its actual focal length is 8.8-25.7mm. On this chart, do not use 24-70; use 8.8 (rounded to 9mm) and 25.7 (rounded to 24mm or 28mm) instead.

The second point is also important, though less dramatic. As you can see on the chart, I didn’t include any aperture values smaller than f/22. However, if you use a crop-sensor camera, f/22 will show significantly more diffraction than it would on a full-frame camera, given the same print size. Essentially, if you use a crop-sensor camera, you may need to focus stack even at some of the apertures that don’t say “Stack” in the charts.

Rolling

NIKON D800E + 20mm f/1.8 @ 20mm, ISO 100, 1/4, f/16.0

Simplifying the Charts
As I have said above, those two charts are huge. Although you are welcome to print them out at full size, it is very easy to make a version – specific to your equipment – that is much smaller. To start, you can simply remove any columns that don’t correspond to your focal lengths. For my 20mm f/1.8 lens, that leaves a chart like this:

2016-102-01-02-Simplified 20mm chart

In theory, for an optically-perfect lens, this chart is completely accurate. However, it is plain to see that there are some unusual values once the hyperfocal distance becomes too large. For example, the chart above suggests that the sharpest aperture for a 200-foot hyperfocal distance point is f/2.0. This definitely is not correct! At such a wide aperture, lens aberrations will lower the quality of your image. In this case, a smaller aperture would be much sharper.

To fix this problem, you simply find your lens’s sharpest aperture – based on tests from a review of your lens – and replace any wider apertures on your chart with that value. For example, for my 20mm f/1.8, a sharpness chart is below:

Nikon 20mm f/1.8G MTF PerformanceCenterMidCornerf/1.8f/2f/2.8f/4f/5.6f/8f/11f/1601,0002,0003,000Lens ApertureImatest Score

Aperture Center Mid Corner
f/1.8 2,062 1,462 975
f/2 2,265 1,502 982
f/2.8 2,998 1,891 1,452
f/4 3,097 2,049 1,682
f/5.6 2,961 2,155 1,827
f/8 2,561 2,162 1,835
f/11 2,140 1,898 1,682
f/16 1,640 1,511 1,369

If you care about detail in your corners – as most landscape photographers do – I recommend finding the aperture with the highest “corner” value. In the chart above, that would be f/8, although f/5.6 is close (Remember that the “corner” value only applies if you use a full-frame camera. If you are using a full-frame lens on a crop-sensor camera, pay attention to the “mid” value instead).

Now, after replacing all the wide aperture values with f/8, the chart is a bit simpler:
2016-102-01-03-20mm chart even simpler

By grouping all the identical values together, I can cut it down some more. Also, my 20mm lens does not stop down past f/16, so I am grouping those into the “stack” category. That gives the final version:

2016-102-01-05-Simplest 20mm chart

There we have it! This small chart now shows the mathematically sharpest aperture for my 20mm lens at seven common hyperfocal distances. If you have a zoom lens, just create a table like this for every important focal length in the zoom range.

Rounding
Don’t be afraid to round the values in this chart. If you are very close to the “perfect” aperture, your photos will be all but indistinguishable. This is true both for your focal length and your aperture values. For instance, a 17mm lens at f/11 is not particularly different from an 18mm lens at f/11. There is no reason to stress about small differences. Personally, if I need to round, I tend to lean towards a smaller aperture. Although this does result in additional diffraction blur, it also offers some leeway in case my focus was slightly incorrect. For example, if the chart tells me to shoot at f/8.5 (which my camera does not allow me to set), I am more likely to shoot at f/9 than at f/8. Again, though, this only makes a small amount of difference.

Fjord Horse

NIKON D800E + 50mm f/1.4 @ 50mm, ISO 100, 1/25, f/11.0

Making Your Own Charts
Perhaps the focal length or distance that you use most is not on either of these charts. If that’s the case, and you would prefer not to round, feel free to make your own chart. For Metric charts, the math is exactly what I showed earlier:

Formula-1

Note that, for “half the hyperfocal distance,” your figure needs to be in millimeters! Otherwise, your result will be vastly incorrect. So, if the hyperfocal distance is five meters, that value should equal 2500mm.

Imperial calculations are a bit different:

Formula-2

In this case, make sure that “half the hyperfocal distance” is in feet. So, if your hyperfocal distance is equal to twelve feet, that value should equal six.

If your lens isn’t included in the huge charts above, these formulae should help you create your own.

When you should use Aperture Priority Mode

Situation 1 – Good light / Sunny day

When the light is fairly constant, you could use manual mode, but chances are that you’re just creating extra work for yourself. Why make the small changes that the camera is going to make for you? If you want to change the exposure, you can do so with exposure compensation meter provided in the camera.

When the light is good, you don’t need to worry about blurring your images because the shutter speed is always going to be fast enough to capture the movement. Particularly if the light is good. There’s an old rule ‘sunny 16’, which suggests a narrow aperture for shooting in the sun, and it really works. Just another reason to shoot in aperture priority.

In good light, you’re rich with light, so you might as well use a lot of it by narrowing your aperture to produce sharper images.

Situation 2 – Portraits

When shooting portraits, whether you’re using flashes or natural light, the lighting is usually pretty good. We tend not to make things harder for ourselves than they need to be, so comfortable lighting situations mean that we can take the photos at the aperture of out choosing.

I personally like to shoot at a range of apertures when taking portraits, but f/8 is one of my favourites. I just find that my photos come out the sharpest at this aperture, which works great for portraits.

Situation 3 – Landscapes

Landscapes typically have a foreground and a background, and often a middle ground too. To see all of this in focus, you need a wider aperture, somewhere up to about f/16 works for me. There are times where I will use a tripod, and if I am then I might use manual mode, but more often than not, I’m using my camera handheld so aperture priority works fine as I’m not playing with any longer shutter speeds.This is where depth of field becomes really important.

Situation 4 – Shallow Depth of Field

Shallow depth of field is achieved by opening up your camera’s aperture, which allows more light in at the same time. It’s not a small amount more light either, it’s a lot more light. The jump from f/2.8 to f/1.4 allows four times more light in, which can easily be counteracted by the shutter speed automatically, in aperture priority mode.

When you change the DoF, chances are you will be experimenting quite a bit, so you’re just giving yourself more work if you’re using manual mode. It’s not all about manual mode.

When you Wouldn’t Use Aperture Priority (But think you might)

Situation 1 – Poor light / Darkened room

As I mentioned before, the beauty of a wider aperture is that it allows your camera to see more light, and this is especially true in a darkened room. But that doesn’t mean you should select aperture priority mode. I find it best to use shutter speed priority. Let me explain…

When you’re in low light, the two main worries are about exposure (not getting enough light) and camera shake (blurry pictures). If you set the camera to aperture priority then you’re only really dealing with half of the problem, which is light. When you’re in shutter speed priority, you can account for the camera shake (say, 1/30 or 1/50 of a second) and the aperture will adjust around the speed to produce the exposure. 

Even if there’s not enough light, the aperture will automatically go to it’s widest, and you can play with the photo in post production. At least that way you don’t have a blurred photo, which you can’t fix (yet).

Situation 2 – Night Landscapes

I know I mentioned above that I like to shoot landscapes in aperture priority mode, and this is true, but not when I’m shooting at night. Night photography is a different game, reserved for the likes of manual mode. The lighting becomes so unpredictable, you have to make calculations and estimations in your head, it’s really more a case of trial and error.

 

Comment below if you find this useful.