The Three Pieces of Perfect Exposure: F/stops, Speed, ISO

by Kevin Shank | Feb 1, 2024 | 0 comments

F/stops sound high tech, and might even sound like something that will take all the fun out of photography. Stick with me, though, and you will learn they are not quite as scary as they sound.

The term “f/stop” refers to the size of the aperture opening inside the lens. It controls how much light will be let through the lens when you click the shutter. The f-stop of the lens works together with the shutter speed of the camera to allow the correct amount of light into the camera so your photo is properly exposed. When the aperture is made smaller inside the lens (referred to as “stopping down the lens,” or “narrowing the aperture,”) it will take more time for the light coming through the aperture to properly expose the picture. Therefore, the shutter speed is slowed down to compensate.

When the aperture of the lens is stopped down one full stop, the hole in the diaphragm is made enough smaller that twice the amount of time is needed to let an equal amount of light pass through the lens. It is much easier to observe this with the shutter speeds than it is to observe it by the various numbers that designate the f-stop in the lens.

Shutter speeds look like this: 1 second, 1/2 second, 1/4 second, 1/8 second, 1/15 second, 1/30 second, 1/60 second, 1/125 second, 1/250 second, 1/500 second, 1/1000 second, 1/2000 second, and so on.

While not all lenses start and stop at the same place, full f-stops look like this, starting with the widest opening and going to the smallest: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. Many cameras today jump in ½- or 1/ 3-stop increments.

Now let’s do a little exercise. With the ISO locked on 400, if a scene calls for f/16 at 1/125 second, what would the shutter speed need to be if the camera lens were changed to f/11? The lens is opening up one full stop, so the shutter speed needs to be only one-half the time. Therefore, 1/250 second is the correct shutter speed. Now let’s change the aperture to f/22. What happened to the shutter speed? Now the shutter speed is 1/60 second.

So, you might now wonder what difference this makes. Well, more happens than just changing sets of numbers. With a smaller aperture, the depth-of-field (amount of area in focus) increases. This is valuable to know, because sometimes you will prefer a shallow depth-of-field, or just the opposite.

Besides depth-of-field, think about what happens when the shutter speed changes. If something is moving through the photo, a fast shutter speed will stop the action, while a slow shutter speed will blur the action. Again, there will be times when you prefer a fast or slow shutter speed.

Maybe your camera has an automatic mode(s) and you wonder about just using that. It is fine to use an automatic mode; however, there are times it is nice to know how to shoot manually to take advantage of your artistic visions for the photo. For example, if you want moving water to blur, you will want to shoot slowly.

Personally, I combine this knowledge with shooting in an automatic mode—the AV mode, to be exact. Using Aperture Value means I select the aperture on the lens while the camera selects the corresponding shutter speed. If I want moving water to blur, I’ll choose a small aperture, knowing this will also slow down the shutter speed. If I want shallow depth-of-field or a fast shutter speed, I’ll choose a wide open aperture. If the light is low and I am hand-holding the camera, I’ll choose a wide aperture to keep the shutter speed high enough to not blur.

Some cameras have shutter priority as a choice for an automatic mode. I do not choose to use this mode, because the shutter speed range of the camera will easily surpass the working range of the lenses I use. Let’s say I’m on a hike, and I see a deer running across a field in my direction. If I spin the aperture to the widest it can go, I know I am shooting the fastest possible shutter speed that the current light levels, ISO setting, and the lens can handle at that moment. If I’m on shutter priority and spin the shutter speed to 1/2000 second, I may have a dark picture because the lens could not open wide enough to utilize a shutter speed as fast as 1/2000 second.

There is an exception to this with modern cameras, and that is setting the ISO to automatic. Doing so will allow the camera to choose a combination that works for adequate exposure.

So now that you know the basics of f/stops and shutter speeds, lets consider the ISO for a moment. First, ISO stands for International Organization for Standardization. The reason I mention this first is so you can now forever forget about it. ISO (called “gain” in some applications) is all you need to remember.

ISO sensitivity to light is given a numeric rating that resembles a shutter speed, and, like shutter speeds and apertures, can work in full stops, half stops, or one-third stops. The scale for full-stop ISO numbers looks like this: 100, 200, 400, 800, 1600, 3200, 6400, 12,800, and so on.

Now let’s do another exercise. It is midnight. Snow is falling, and the yard light illuminates the back yard, making it look like a Christmas card. It’s time to try capturing the scene. What do we do? How do we set the camera controls? Flash? No, certainly not! We have only three controls to make this happen—f/stop, shutter speed, and ISO.

The f/stop is a no-brainer—wide open, of course—f/4 on this lens. The shutter speed is relatively fixed too, for a reason that is not quite as obvious at first glance. Snow has to look like snow or the photo is a flop. If the shutter speed is too slow, the snowflakes look like streaks of falling rain. On the other hand, if the shutter speed is too fast, a lot of the fine flakes will not make enough of a splash to get captured in the photo. The key is a speed that freezes the flakes just about the time they start to streak.

Just what this speed is will vary depending on how fast or lazily the snow is coming down, and what aperture is wide open on your lens. For my Christmas card shot, a shutter speed of 100 was required.

With both the f/stop and the shutter speed locked down, my only hope for a successful picture would be choosing an ISO that got the sensitivity to a level that looked like an acceptable exposure. I bracketed the ISO so I would have choices. To bracket, I did a range of ISO settings from 5000 to 20,000. The photo I ultimately chose for our card was the one taken at 20,000.

Now you may be wondering why people pay substantially more to buy fast lenses (e.g. A 400 f/2.8 rather than a 400 f/5.6-8 lens) when all they have to do is crank up the ISO. The fast answer is that with high ISO comes a lot of digital noise. Back in the film days we called it grain. A grainy picture, a noisy picture, is just not as beautiful.

Using the 400mm lens as our example again, many wildlife photographers like this lens so they can photograph animals and birds from a distance. A lot of times these opportunities come in the low light of dawn/dusk or shade. The most desirable way to capture adequate light to maintain a shutter speed that does not blur is by opening the lens wider, while increasing the ISO is the last resort.

For our Christmas card, I didn’t have a choice but to use high ISO. There simply was not adequate light to make this come together otherwise.

Photographers today have advantages we did not have years ago. Today’s cameras typically can handle higher ISOs relatively well. Some handle them very well. Additionally, software programs can help to eliminate noise without destroying the picture. Topaz DeNoise is relatively inexpensive, and one I commonly use.

When I started out in photography in 1985, the Christmas card above would not have been possible—we could not buy color film that got anywhere close to 20,000 ISO, and options for enhancing the original simply did not exist.

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