The aperture--also known as f-stop, is the opening in the back of the lens. An easy way to think of aperture is as a window. The larger the window, the more light you let into the camera.
Aperture is often adjustable by a ring on the lens, a dial on the camera, or both. The aperture ring is common on older lenses and many prime lenses. However many modern lenses lack an aperture ring, so the camera must control aperture. By turning the aperture ring or making the adjustment on the camera (see your manual), you make the opening bigger or smaller, letting in more or less light.
This 50mm prime lens has a ring that can be manually controlled to select the correct aperture. Image courtesy Nikon.
Here’s the sticky part: the f-stop numbers. (what you see on the aperture ring) look like this: 2.8, 4, 5.6, 8, 11, 16, 22, 32. Some lenses have more, some less. You’ll see the same sort of series on your camera’s LCD readouts.
So what in the world do these numbers mean? They’re actually ratios of the lens focal length to the size of the aperture opening. You probably don’t really care about that at this point. The important thing about the numbers is this: Any given aperture will let in the same.
THINGS TO REMEMBER ABOUT F-STOP
Choosing to use lenses with lower f-stops means that you have more control over how much light gets into the camera. Before you decide to use the biggest opening, its important that you consider a few important details:
A lens with a lower f-stop is often more expensive. Most kit lenses have f-stops that range from f/4 to f/5.6. On the other hand, professional zooms can get as fast as f/2.8 and professional prime lenses (fixed focal length) can get even faster. (f/1.2).
Cheaper zoom lenses often change their f-stop as you rotate through the zoom. This can lead to an unwanted exposure or shutter change when you attempt to use the zoom options.
Photographers often speak their own language. There are some important terms you should get familiar with.
“Open up” means you’re making the aperture opening larger, letting in more light.
“Close down” or “Stop down” makes a smaller aperture and lets in less light.
“Wide open” means setting the aperture at its widest setting (smallest f-number).”
“Depth of Field" is the range in a photograph, from near to far, that appears to be in focus.
The aperture’s part in controlling exposure is by opening up or closing down, letting in more or less light. What may not make sense is how the aperture actually affects a picture. Besides having a hand in controlling exposure, the aperture controls depth of field.
When you look through the camera’s eyepiece, everything looks like it’s in focus. However, only a certain portion of the scene is actually in focus. The amount of the photograph that is in focus is controlled by the aperture (and the focus point.) Think of four points. The camera represents point A and infinity is point D. Depth of field is an area somewhere in between – starting at B and ending at C.
The size of the aperture determines the depth of field. The smaller the opening (larger f-stop number), the more depth of field you’ll have. Therefore, f/22 has a lot more depth of field than f/2.8. Think of it this way: A bigger f-stop number equals bigger depth of field.
By using a smaller f-stop number (a wider opening), you’ll get a shallower depth of field and less detail overall, other than what you have actually focused on. This tends to lead to elements in the background blurring when used correctly.
What are some reasons you may want to use one f-stop over another?
If you’re making landscape photos and you want everything sharp from foreground to background, you need lots of depth of field, which means small aperture openings like f/11 or f/16.
Portraits, whether of people, animals, or flowers, generally use wider openings. A wider opening gives you less depth of field—fewer things appear to be in focus. This helps reduce any distractions that take attention away from the main.
A STOP OF LIGHT
Do you remember that doubling and halving formula we mentioned previously? Well, that’s a stop of light. A stop of light isn’t any quantifiable measurement it’s a relative one. We define a stop of light as a doubling or halving of any quantity of light.
For example, f/5.6 lets in twice as much light as f/8. Another way to say this is that f/5.6 lets in one more stop of light than f/8. And 1/60 second lets in one less stop of light than 1/30 second.
The word “stop” is pervasive in photography. Mostly we use it in connection with setting exposures. At workshops, you’ll hear us say, “Open up a couple of stops.” That could mean adding two stops of light to the exposure. It could also mean setting a wider aperture to affect the depth of field.
If you look at all the shutter speeds and apertures available on your camera, you will notice (unless you have an older camera) that there are quite a few in between numbers. These numbers represent fractions of a stop between the main numbers.
It’s important that you understand the concept of a stop. This is one of those BIG CONCEPTS in photography. It allows you to master elements, such as using proper exposure, flash, and filters.
Try this: With a lens attached and your camera in the manual exposure mode, point your camera at anything at all. Now change your shutter speed or aperture until the meter display shows a “correct” exposure, or what we call “zeroing the meter.” You’ll need to consult your manual to know what this should look like. Now change the exposure by one stop by either changing the shutter speed one stop or the aperture by one stop. It doesn’t matter whether you add or subtract light at this point. Observe the change in your meter display. The indicator should move one whole stop. If you’ve added light, it’ll change one stop in the positive direction, if you’ve subtracted light, the change will be in the negative direction.
From back in the film days when the ISO setting was printed on the film box...
THE THIRD PART OF THE EXPOSURE TRIANGLE - ISO SETTING
The ISO rating in your camera simply tells you how sensitive the sensor is to the light. The higher the ISO number, the more sensitive to light it is. This means that it takes less light to get the same exposure than it would take for a lower ISO number setting.
ISO 200 is more sensitive to light than ISO 100. This means that ISO 200 needs less light than ISO 100. In fact, it takes half the amount of light to get the same exposure. That’s one stop. ISO is subject to that doubling and halving we’ve already discussed. A 100 ISO setting needs two more stops of light for any given exposure setting than an ISO 400 setting (100x2x2). An 800 ISO setting needs three stops less light than ISO 100 setting (100x2x2x2).
The main practical effect is that by using higher ISO-rated settings, the photographer can use faster shutter speeds. This is important with sports and wildlife photography where shutter speed is paramount.
The higher the ISO, the “faster” the camera settings. ISO 400 is faster than ISO 100, thus you can use faster shutter speeds.
You now have all the basic tools you need to understand exposure and to take the next step toward becoming a great photographer. The next step is to just go out and shoot. Shoot your brains out. Shoot daily. Look at lots of photos, keep reading your camera manual, handle your camera every day, and re-read this guide as necessary until you feel ready to move to the next level.
ABOUT THE AUTHOR
Scott Bourne is a member of The Board Of Advisors at Macphun, an Olympus Visionary and a professional wildlife photographer, author and lecturer who specializes in birds. He was one of the founders of This Week In Photo, Founded Photofocus.com and is co-founder of the new Photo Podcast Network (photopodcasts.com.)
Scott is a regular contributor to several photography related blogs and podcasts and is the author of 11 photography books.
Scott is available to speak to your birding group, photography group and for both private and small group bird photography workshops. For more information on engaging Scott as a speaker or workshop leader, or for image licensing and print information, e-mail firstname.lastname@example.org.