As photographers, we all know what f-stop is. It’s the number that appears on your lens, or in a camera menu. But do you really know what it is?
F-stop is the ratio of the focal length to the diameter of the aperture.
That sounds simple enough, but it’s not. It’s just a definition, like “The sky is blue because of Rayleigh scattering.” That’s why I’m writing this blog post: I want to talk about some of the things that go into making that definition useful, and what those things mean for real photography. And I want to include everything I’ve learned from other photographers over the years.
The most important thing to know about f-stops is: f-stops are relative. That means that F/4 isn’t a number. It’s a relationship between numbers. And since every lens has its own numbers, every lens has its own set of relationships between them.
One reason this matters a lot is that two lenses with different focal lengths and diameters can have exactly the same f-stops, even though they will give different exposures. This makes it very hard to learn photography by just looking at numbers on a lens or through an in-camera menu.*
I’ve been meaning to talk a bit more about photography and f-stops, so this seems like a good time.
I’ve noticed that many people (including a number of my students) are confused about what f-stop is, and how it works. I didn’t always understand it either. The concept is simple enough once you understand it, but explaining it can be tricky.
I’ll give it a shot here.
What is an f-stop? It’s the ratio of lens focal length to its effective aperture diameter. This is also sometimes called the “focal ratio,” though that term usually refers to a somewhat different thing. The important thing to realize is that the f-stop is defined independently of any specific measurement units for either the focal length or the diameter. For our purposes here, I’ll assume we’re using millimeters as our measure for both of them, but you don’t have to take my word for that — just remember that if you’re using inches, divide by 25 instead of 100.
First let’s consider a simple lens consisting of just one element: a single piece of glass with two surfaces facing each other. Let’s say the overall length of the lens is 10 mm, and the maximum diameter of its “ap
What is F-stop?
It’s the focal length divided by the diameter of the aperture.
The diameter of the aperture is measured in millimeters, and is referred to as the lens’ “maximum aperture”.
For example, a 50mm lens with a maximum aperture of f/1.8 has an f-stop of 1.8.
Eg: 50/1.8=27mm(aperture) 27/50=0.54 (focal length)/0.54=f/0.9 (f-stop).
And that’s f-stops.
“f-stop”: a term used in photography to refer to the size of the aperture in a lens. Typically expressed as f/2.8, f/4.0, and so on.
The larger the f-stop number, the smaller the aperture (and vice versa).
So what’s this mean? F-stops function much like the pupil in your eye: they control how much light gets into your camera. To understand them, imagine you’re staring at a candle flame across the room. The flame’s brightness seems uniform, but if you squint you’ll notice that it’s actually composed of a bunch of tiny dots of light. That’s because when you squint your pupil is small, restricting the amount of light that can get into your eyes.
If you look at something using a wide-open pupil (such as when taking a picture with an automatic point-and-shoot), you’ll be able to see details in bright or dark areas, but not both; in other words, it will be less sensitive to light overall. If you’re outside on a sunny day and take such a picture, everything will appear washed out and overexposed. If instead you use a small aperture to restrict that wide-open exposure by making
The f-stop value is defined as a ratio of the focal length to the diameter of the aperture. In a simple case, for instance, it could be expressed in terms of millimeters:
If you are using an aperture with a diameter of 22mm (the “f” stands for “focal”) and the focal length is 50mm, then the f-stop will be 50 divided by 22, which is 2.**
T-stops are used in a specific context — when talking about lenses mounted on cameras (film or digital). They are used to describe how much light passes through the lens before being focused on the film or sensor inside the camera. DSLR cameras often have automatic settings that let you choose between different combinations of shutter speed and f-stop to achieve particular effects.
A lot of photographers prefer to use T-stops because they can give you more control over your exposure than just adjusting shutter speed. The reason for this is that as you adjust shutter speed, you’re also adjusting another variable — namely, how long your shutter stays open. If your shutter stays open for too long, you’ll capture too much light, resulting in what’s called motion blur — which means that if an object is moving towards you or away from you
F-stop is a term used in photography to specify the size of the aperture. The lower the f-stop number, the bigger the aperture, and the more light that will be let into your camera.
The usual scale (starting with f/1 on the far right) 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
The larger the aperture, the more blurred your background will appear to be. Many photographers choose a low aperture so they can have a blurry background to make their subject stand out.
Aperture is the opening in a lens that controls the amount of light that passes through to the camera’s sensor. It’s commonly known as f-stop, and is denoted by a number between 1.0 and 22 (although it can be as high as 102). The lower the number, the larger the aperture.
The size of the aperture affects exposure in two ways: first, it affects how much light enters into the camera, and second, it affects depth of field – how much of your picture appears to be in focus.
It’s easy to understand how aperture affects exposure: if there is more light hitting the sensor, you’ll get a brighter image. But how does changing aperture affect depth of field?
