Camera Lens F-stop & Aperture Explained

F-Stops & Aperture Explained

Because of lots of confusion surrounding what a camera f-stop is and how it works, thought I should write a high level post that explains the concept in an easy (hopefully) to understand way. It's full of generalizations but still a good basis to start from to understand how f-stop & aperture affect your photography.

A f-stop # represents a value assigned to size of opening of the lens aperture. Typical f-stops that you might see on current consumer or professional level camera lenses are as follows but you will see some variation. They may be represented as f/1.2, f/1.8, f/3.5 etc.

Common camera f-stops: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32

From left to right, camera apertures decrease in size with the biggest (widest) on left letting the most light into the lens.  The smaller the number, the larger the opening, the more light that is let in the camera the faster the shutter speed.  Lenses that let a lot of light in are critical for shooting indoors especially if no flash is used. For every increase in full f stop, the camera shutter speed with double (assuming all other variables stay the same). For instance if I can take a picture at 1/500th of a second at f/2.8, changing to f/2 will result in a 1/1000th of a second shutter speed. 

Smaller apertures (larger numbers) like those to the right allow significantly less light in but allow greater depth of field or DOF.  Depth of field allows for a deeper area of the photograph to be in focus which is desirable in landscape photography which is often done using a tripod and long exposures.

A shallow depth of field (sometimes just millimeters deep) is preferred in portrait photography or any time a single object needs to be the center of attention.  Clutter or distracting elements including fade into a blurred background and facial blemishes become less pronounced.

Here's a good example of a shallow depth of field. Notice that only a narrow section of the image is in focus. It draws your eye to the desired subject and affords lots of creative control.

Photograph taken with Canon XSi and 50mm 1.8 lens set at f/2

Sports photographers often need extremely fast shutter speeds to freeze action so they will often be shooting with their lenses "wide open" or near its fastest/largest aperture (left side of scale). Those big lenses on the sidelines of sporting events are often in excess of 400mm which can cost over $7000. What makes them different from a $300 lens is how much light they can gather.  Similarly, photojournalists will often be shooting at larger apertures due to low lighting conditions indoors.

Here is a photo of my son taken on a busy street, notice how the blur removes the busy background and allows you to focus on the subject.  This is achieved by using the Av or aperture priority mode on your camera.

Photo taken with a Canon 7D and 28-70mm L 2.8 lens set at f/2.8

A general rule of thumb, the larger the maximum aperture (left side of scale) the more expensive and heavy a lens will be. Also, you will probably never see a zoom lens with an aperture faster (wider) than 2.8. Trust me on this...the physics would bore you and I'm not qualified to explain them. Lenses available with apertures greater than 2.8 are fixed focal length lenses, also called "prime" lenses. 

I'll bet you wondered why anyone would ever buy a fixed focal length 50mm lens when they could buy a 24mm-70mm zoom lens right? Well, the fastest that zoom is f/2.8 while there are some 50mm lenses that are even wider than f1.4. Even the cheapest sub $100 50mm lens will generally be faster than 2.8. As an example, when you need a fast shutter speed shooting in-doors, perhaps when no flash can be used, the 50mm lens set at f/1.4 may be able to shoot at a reasonable 100th of a second where on the other hand, the 24-70mm zoom would only be able to shoot at 25th of a second which I would venture to say is too slow to hand-hold and even if you could, any movement by people would cause the image to be severely blurred.

Now, to take it a step further, most inexpensive consumer zoom lenses will have a maximum aperture of f/3.5 which turn out to be 2/3 of a stop slower than f/2.8.  So that kit lens that came with your camera will shoot at a maximum shutter speed of a little faster than 1/16th of a second.  Ever wonder why your shots are blurry indoors?  The lens can't give you a fast enough shutter speed to stop any motion.  That kit also has a variable maximum aperture which means if you try to zoom in on your subject the shutter speed will get even slower!  A person with an affordable 50mm f/1.4 lens can keep snapping away with much better results.

OK, but that kit lens I have has Image Stabilization or Vibration Reduction or other brand name shake reducing technology...that makes up the difference right?  The answer is "sorta" because while the stabilization reduces the shake from you holding your camera, it can do nothing to freeze your subject matter so the people in the room (unless stationary) are still blurry.

I talked a friend of mine into purchasing a 50mm f/1.4 lens for his camera so he could capture great shots of his kids. He loved the "bokeh" or blur background that it produced but he was dismayed with the results outdoors on a very sunny day.  As it turns out, a lens can take in too much light and over-expose the image so there are situations where shooting "wide open" may not work without making additional adjustments to the camera.

I'm going to be adding more example photos to this post but thought I'd toss it out there to hopefully start generating questions you might have so I can make the post better and more informative.