The f/stop number, usually found on the barrel of a lens, indicates the size of the aperture relative to the focal length of the lens. The f/stop number will usually look like one of the examples shown below: f/2.8f/4f/8f/16 The f-number is calculated by dividing the focal length of the lens by the effective diameter of the aperture.

Aperture is the size of the lens opening. It is the opening in which light enters the camera. By using a standardized system, all lenses will transmit the same amount of light if the f/stop number is the same. Here is an example: If the lens is a 50mm lens, meaning its focal length is 50mm, and it has an aperture of 12.5mm, meaning the open hole that allows light to enter is 12.5mm in diameter, the f- number is f/4.

The f-number is a method of measuring the amount of light allowed to enter the camera. Exposure time is the other method used to measure the amount of light allowed to enter the camera. We will discuss both methods and how the two impact each other as well as how the photographer can use the two methods to define and control the photograph. For now, we need to understand f-numbers and how they work.

You may have noticed that 12.5 is 1/4 th of 50. The f- number is actually a ratio. It’s a fraction. When you see an f-number, mentally think of it as a fraction. For example: f/2 = 1/2 f/4 = 1/4 f/8 = 1/8 f/16 = 1/16 As you can see, the bigger the f-number, the smaller the fraction. This means that the bigger the f-number, the smaller the aperture.

Another way to think of the f-number is to understand that the bigger the number, the smaller the aperture (smaller fraction), which means less light enters the camera. The smaller the number, the bigger the fraction, and more light enters the camera. For example: f/2 = 1/2 f/16 = 1/16 1/2 is bigger than 1/16. Over the same amount of time, an f/2 lens lets more light in than an f/16.

Lenses that let in a lot of light require shorter exposure times. They allow for faster exposure settings or they work better in low-light situations. Because of this, they are sometimes called “fast lenses” or “fast glass.” Usually, anything from f/2.8 or faster (such as f/2, f/1.8 or f/1.4) are considered fast lenses. In photography, you will always trade something to get something else. So, what do you trade to get faster exposures when using fast-glass?

Fast lenses are more expensive. A check of some recent lenses prices will show this. For example: A Sigma 50mm, f/1.4 lists for $499 A Sigma 50mm, f/2.8 lists for $299 These two lenses will provide the same field-of-view but the f/1.4 will allow more light to enter the camera over the same amount of time. It’s faster. A Sigma 120 – 300mm, f/2.8 lists for $3,199 A Sigma 120 – 400mm, f/4.5 – 5.6 lists for $899 These are telephoto lenses. The f/2.8 lens aperture will not change when you magnify. The f/4.5 – 5.6 will change aperture, over the range indicated, as you magnify.

Fast lenses have a shorter depth-of-field (DOF). This means that focus becomes critical when using fast lenses. A few feet, even a few inches, will determine whether your subject is in focus or not. More on this later. DOF can be used for artistic expression so it’s not a bad thing. A short DOF will allow you to emphasize your subject while throwing everything in the background out of focus. You can de-emphasize “stuff” in the background and force the viewer to see only your subject in clear, sharp focus.

Fast lenses are heavier. They have more glass, in the form of lens components, and glass is heavy. Because these lenses are heavier and because focus becomes very critical, a tripod is almost mandatory when using them. Of course, if your exposure time is very fast, then you can stop action at high speeds and so a mount may not be needed. Watch any sporting event being photographed. The sports photographers are using very large aperture lenses (fast glass and they are $8,000 $12,000 lenses). They are using tripods or mono-poles to help support the camera and lens. Over the course of several hours, 6-8 pounds of camera and lens can become pretty heavy.

What do you gain with fast-glass? Shorter exposure times. Ability to photograph without flash in low-light situations. Ability to emphasize your subject by using a short DOF. Ability to photograph very fast action without blur.

To determine whether you need fast-lenses you must analyze your photography requirements. Not all photographers need fast-glass. In fact, some photography works better with slower lens and some is impossible with fast-lenses.

You may be wondering about depth-of-field (DOF). Now is a good time to talk about it…a little. It’s the distance between the nearest point and the farthest point of the subject that is perceived to be in acceptable focus. Generally, the shorter the focal length of the lens (compare a 50mm and a 300mm lens), the more shallow the depth of field. Also, the closer a subject is to the lens, the more shallow the depth of field. And the faster the lens, the shorter the depth of focus.

An important question to ask and understand is: What is “sharp focus?” Within a photograph, there is a limit with which the human eye can detect imperfections. This limit is called the “circle of confusion” and defines what is in or out of focus. Any part of the subject that falls in the circle of confusion is perceived to be in focus. Anything outside of the circle of confusion is perceived to be out of focus.

Fast lenses force the circle of confusion to remain very close to the subject. Slower lenses allow the circle of confusion to move farther from the subject. The image of this interesting couple passes through the lens and is focused at a point behind the lens. Any part of the image in front of or behind the focal point is not in sharp focus. Focal point

The circle of confusion projects in front of and behind the focal point. Anything falling between the front and rear circle is perceived by the human eye to be in focus. Focal point Circle of confusion

In the example above, the aperture is open wide. In the example below, the aperture is stopped down.

Circle of confusion The angle of incoming light is steepest in the wide open aperture as shown above. The angle is less steep when the aperture is smaller as Shown below.

Circle of confusion When the aperture is stopped down, decreasing the angle of light, the circle of confusion extends farther in front of and behind the focal point.

Circle of confusion Using a slower lens with a larger f-number (smaller fraction) will produce a deeper depth of focus. How much?

Circle of confusion Sometimes….miles and miles.

Depending on the lens, f-number, distance to the subject and light, the depth of field can extend literally to infinity. This is why landscape photographers use slow lenses. Ansel Adams, famous for his photos of Yosemite Park used an f/64 lens. As you know, this is a very slow lens but the depth of field extended to infinity and allowed him to get very large subjects, such as mountains, in sharp focus. Of course, because the aperture was so small, very little light entered the camera and he needed to use very long exposures.

Landscape photographers, architectural photographers, some nature photographers and fine art photographers are a few of the photographers who use slower lenses. Sports photographers, action photographers, fashion photographers and portrait photographers are a few of the photographers who use fast lenses. There are some tricks to make slower lenses act like faster lenses and faster lenses act like slower lenses, but that discussion will come later.

Soon, we will be discussing the relationship between: Aperture (f/stop) ISO Shutter speed

Together, the three allow a wide range of camera settings. Think of them as the Golden Triangle of Exposure. Aperture (f/stop) ISO Shutter speed