Aperture explained ... or, what is an f-stop?

When photographers begin their journey the first things they struggle with are what equipment to purchase and what all the various settings on the camera mean.

This post won't get into the gear but will begin a series of posts designed to dissect the various setting of the camera. We're going to begin with the aperture.

Why aperture first? Well it does start with the letter A. That seems like a good reason. It's also maybe the most confusing so why not start with the hard stuff first?

The first thing to understand about your aperture is what it actually is. The aperture is an iris housed within your lens that's very similar to the one found in your eye. It opens and closes to allow or limit light entering the camera. In bright light you may need a smaller opening to limit the amount of light entering the camera.

Small aperture

In lower light conditions however, just like your eye, the aperture needs to dilate allowing more ...

... and more light into the camera as the light diminishes.

Large aperture

The aperture is reported as something called an f-stop.  

Here is where a lot of people begin to get confused. Understandably. First of all, when people refer to  apertures and f-stops they are talking about the same thing. Technically, aperture is the opening and f-stop is the measurement of that opening but for all practical purposes, they're used interchangeably. 

It can get really confusing for beginners because the numbers seem to be random and meaningless. Then it gets more confusing when you learn the f numbers seem to be backward. Small numbers equate to large opening and vice versa. 

Actually, f-stop determination isn't random. Like much of the photographic process, physics and math are involved. The numbers assigned to f-stops are based upon a specific formula dividing the focal length of the lens by the actual diameter of the opening. Blah, blah, blah. Does that mean anything to most people? Probably not. And if you're that techie you're likely not reading this blog. So let me explain something, not quite a scientific but accurate enough for our purposes. 

As I mentioned, a small number is actually assigned to a large opening and vice versa. It begins to make a little bit of sense if you think of it as a fraction. 

If you can remember f = fraction this becomes rather easy. Just put a 1 on top of the f-stop number and that's how much light the lens is allowing to pass.

In the example above you can see that f2 is a very large opening compared to f16. If you turn these numbers into fractions f2 becomes 1/2 and f4 becomes 1/4, etc. So, if f2 is letting in 1/2 the light and f8 is only allowing 1/8th of the light to pass, which is the larger opening? Simple right?

Before we go any farther, I need to post a disclaimer or two. This simplified formula is for the purpose of understanding only. While close to accurate, these amounts are not the exact amount of light passing through the lens. Also, the f isn't technically short for fraction. Its short for focal, or the length of your lens in the actual formula. But remembering f = fraction is mighty easy so we're going with it. 

So, when you need more light you will choose a larger aperture like f2 or f4, or "open up."  In other situations when you want to limit some of the light you'll "stop down" your aperture to something smaller like f11 or f16. 

If you happened to learn on an older, film camera, adjusting your aperture was a matter of turning the ring on the lens and, as you can see in the image below, it had a reasonably limited amount of adjustment. 

Things have become a little more complex with electronically controlled cameras. The aperture is now adjusted using controls on the body rather than a manual ring on the lens. Additionally, because smaller, precise adjustment can be achieved electronically, newer cameras have placed 1/3 stop increments between each of the full f-stops shown on the older film lens. It can add some intimidation but it still works exactly the same way. Put a 1 on top and turn it into a fraction. You'll also notice many of the f-stops are shown as decimal numerals. 5.6 for example. Don't be intimidated. Just remember that larger numbers mean smaller openings. 

If you try to open your lens to f2 and it won't get there don't be alarmed. Not every lens will open to f2. Its quite likely yours won't. A quick examination of your lens will determine it's maximum aperture or apertures. Why apertures? Simple. If you have a zoom lens, a lens that changes focal length, such as an 18-55 it's almost assuredly going to have a variable maximum aperture. Only the highest priced zoom lenses have a constantly maintained maximum aperture. Lets look at a standard issue Canon 18-55 "kit" lens.

Next time I need to clean the lens before I take a picture of it. I'm embarrassed. 

See those numbers around the bezel of the lens. See where is shows 1:4-5.6? Don't be mistaken. This is not a 1.4 lens. That's a colon, not a decimal point. The colon is essentially showing my f=fraction trick for you. In relation to 1, this lens will let anywhere from 1/4 to 1/5.6th of the light in. Why does it vary? As you zoom the lens out, or extend it from its shortest focal length of 18mm, some of the light is scattered and lost due to the optics of the lens and the physics involved. So, as you get zoomed all the way to 55mm, the maximum aperture is 5.6. Fixed focal length, or prime, lenses do not have this variable maximum aperture because they don't zoom. 

Why are some lenses like the 18-55 kit lens limited or "slow" and some lenses "fast" or less limited? It really just comes down to the amount of glass involved. "Fast" lenses with larger maximum simply have to have more glass to allow more light in. Here's a visual example. 

The top image is obviously the 18-55mm kit lens and the bottom lens is the monstrous 85mm 1.2. If you're doing the math, that means the 85 allows almost every bit of light to pass through it. But that comes at a cost of both size and, of course, dollars. Looking at them from the back you can clearly see how much more light would be able to move through the 85mm and what the difference between an f4-5.6 lens and an f1.2 lens looks like. But remember, all that glass is not only heavy but also very pricey. The 85mm costs about $1800.00. The 18-55mm came kit bundled with an entry level body for $500. 

I often compare lenses the way I would hoses. Garden hose on the left v fire hose on the right.

So why do photographers want or need to spend the big money for the large aperture lenses. Do we really shoot in super low light conditions all the time? Well, yes and no. Thats part of the answer but there is more to the equation. 

First of all, with extremely few exceptions, no lens performs at its best when shot wide open. While not always practiced, its always best to use a lens stopped down at least a stop or preferably two from its maximum. Each lens has its own characteristics but about two stops down on most lenses will be what's commonly referred to as the sweet spot.  In other words, the kit lens is likely going to hit its sweet spot at around f8 or f11 whereas the the 85mm lens will be in its sweet spot at about f2 which as we have learned is allowing considerably more light into the camera than f8 or f11. And, if you do need to shoot them wide open, higher priced lenses tend to be more forgiving. So sharpness is a factor. There are indeed times all of us are trying to capture images in lighting conditions that simply won't for the limited light transmission of f8 or f11. 

Then there is one final consideration regarding aperture. Each setting on our camera have some type of secondary effect or side effect. In addition to metering the quantity of light entering the camera your aperture controls something known as depth of field. Depth of field is the amount in front and behind your actual plane of focus thats also acceptably sharp. I like to think of it as collateral focus. 

Small apertures such as f16 will yield greater depth of field where larger apertures like f2 provide much shallower depth of field. This can be important depending upon the type of image you're trying to create. If you're capturing portraits, you likely want a shallow depth of field so a larger opening would probably be preferred. However, if you're photographing a group of people with faces of multiple planes, you likely want to increase your depth of field by using a smaller aperture. 

In the following series of pictures, the focus remained on the same specific picket which is apparent in the first image shot at f2. But as you can see, as the size of the aperture decreases, depth of field increases. Learning how these different aperture settings effect not only the light entering the camera but also how they will effect your depth of field can be a game changer.

f2

f5.6

f16

 Choosing a particular aperture to control depth of field rather that simply to control the amount of light entering the camera is very common. Fortunately, since there are a few controls on the camera that control exposure, or the total quantity of light entering the camera, we can sometimes use a large aperture even in bright light or a smaller aperture in low light.

In the next post, we'll look at shutter speed. The following post will address ISO. Once we've looked at these three variables, often referred to as the exposure triangle, we'll look at how to use them together to create a beautiful photograph.