Aperture confuses some photographers, which is why many use the Auto mode and miss most of the best photographs they could’ve ever taken. Aperture is an opening at the end of the lens. We often compare the aperture to your eyelid. It controls how much your eye can see. The aperture controls how much light enters the lens and it also controls the depth of field.
Aperture settings are referred to as f-stops. This is where many photographers stop trying to learn and simply switch their cameras to the Auto mode. F-stop numbers on most lenses range from f/1.8 to f/22. Each f-stop lets in half the amount of light of the preceding f-stop. Before trying to explain further, we’ll include a diagram of the common f-stops.
As you can see in the diagram above, the f/2 aperture is wide open. The f/16 is almost closed. Some people say the f-stop numbering system is backwards but it is what it is. The smaller numbers represent a wider opening. The largest number represents the smallest opening.
When you set the aperture on your camera, you’re telling the lens how wide to open. If you want it wide open, switch to the smallest f-stop. If you want the lens diaphragm closed more, switch to a larger number. Switching to a larger or smaller aperture is called ‘stopping up’ or ‘stopping down.’ Only professional photographers use these terms but we thought we would include them in case you hear someone say you need to ‘stop up.’ That means to switch to a higher f- stop.
The aperture also controls the depth of field. In simplest terms, the depth of field indicates how much of your picture is in focus in front of and behind your subject. Depth of field is important depending on how much of your picture you want in focus. I’ve shot thousands of pictures of roses that were in a private rose garden. By choosing the right f-stop, I was able to isolate each rose while blurring out the surrounding rose bushes. We’ll discuss depth of field in another lesson. First we must explain ISO settings and depth of field.
It was easy with shutter speeds because we could just double the speed. However, to get one more stop with aperture, you shouldn’t multiply by 2 but divide by 1.414 (square root of 2). Since no one actually calculates that, photographers remember instead the usual sequence of f-numbers: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 (and sometimes f/32, f/45, f/64). You don’t have to learn these numbers by heart, but it is helpful to know which number comes before and after each other: to know that if you are shooting at f/4 and want one less stop of exposure, you should go to f/5.6, etc. Thankfully, if you start paying attention to your aperture, you will start remembering them very quickly, as they always stay the same.
But wait, it’s not quite over yet. There is another important factor you should take into account when you are choosing your aperture. If you shoot outdoors, you will often find yourself in a situation where you want depth of field to be as large as possible and you have more than enough light to use any aperture you want (this means that the corresponding ISO and shutter speed to obtain a good exposure will both be within acceptable boundaries). According to what we just talked about, your natural reaction would be to close aperture as much as possible, using something like f/22.
That would be a bad idea. The reason is called diffraction, an optical phenomenon which becomes noticeable as light is forced to go through an increasingly narrow aperture. What this means concretely is that your image will be less and less sharp as you close your aperture. This is usually noticeable only from f/11 or so, however. Most lenses also have to make optical compromises to obtain larger apertures, so won’t be quite perfectly sharp when fully open (low f/stops).
The consequence is that each lens has a sweet spot, an optimal aperture at which its sharpness is optimal. The further you step away from this aperture, the worse the results will be. Depending on the general quality of the lens, it could be hardly noticeable, or it could ruin your images. The exact value of the sweet spot depends on each particular lens, but for DSLR equipment, it is usually around f/8, which makes this a good default aperture (hence the old saying “f/8 and be there”).