It depends on the look you want in your photograph.
When I started learning photography, my goal was to shoot weddings and engagements. I didn't know much but one thing was for sure: pro photogs were gurus at making their subjects stand out. Yep, I'm talking about those gorgeous soft backgrounds... blurs of color and seamless backdrops.
So what's the trick to creating that look on purpose? Lenses with big apertures. As a reminder, beginner kit lenses normally only open up to f/3.5 or f/4.5 and even when that happens those apertures are only available at certain focal lengths. Why is it like that? Zoom lenses that have bigger apertures available at all focal lengths are really expensive.
But no worries! Even if you only have a kit lens you can still capture background blur and bokeh. That's because the further away your subject is from the background, the more intense the shallow depth of field will be. Granted, you'll have to have your subject stand in front of that pretty wall (or lights) at least 10 feet or more away - but the further away your subject is, the more blur there will be. Yes, even shooting in bigger apertures like f/4.5.
What is depth of field? It is how much of the image is in focus.
Anyway, this is why 99% of the time when beginner photographers learn about aperture they want to add a new lens to their kit. The most popular option? An affordable 50mm f/1.8 prime lens. A lens like that opens up to a whopping f/1.8 aperture. With a lens like that you get tons of opportunity to capture creamy backgrounds and bokeh. You'll also be able to shoot in lower light situations without flash often.
The only downside to shooting wide open? It's really easy to miss your point of focus but with lots of practice you'll get better at hitting focus.
Wide open apertures like f/2.8 are great for capturing stunning detail photos. If you have a blog, Etsy shop, etc. then shooting in f/2.8 or f/2 is going to be something you'll want to try out. Big apertures really make product shots pop.
It's true that most of the time I shoot in wide open apertures (I have a tendency to stay around f/2.8 - f/2) but sometimes I just need to get more of my subject in focus and a wide open aperture won't work. This is when I'll stop the lens down (make the hole smaller.) Smaller apertures = greater depth of field.
Anytime I'm out shooting new places / architecture I like to use smaller or mid-range apertures. Apertures f/3.5, f/4.5, f/5.6 are still considered to be big apertures but they have more depth of field than shooting wide open. Apertures f/7, f/10, f/11 are the true mid-range apertures.
Then of course there are the very small apertures. We're talking f/16, f/22, f/32 and I rarely use those. Those are ideal when you're wanting to capture the details of some grand big picture - like the view from the top of the Royal Gorge Bridge - looking down into the canyon below.
For this view from our cruise ship (above) I actually used f/2.8 - mainly because I didn't want a lot of grain in my photo (from a high ISO) and I wanted to make the most out of the lingering natural light at sunset. This just goes to show that you don't always have to shoot in smaller apertures to get a beautiful landscape shot. It will just always depend on what you want and what you're willing to compromise when capturing a certain image.
While on-location in Vail, Colorado I shot this image of Aspen trees in late spring around Sylvan Lake. I really wanted more of the trees and texture in focus so I chose a smaller aperture.
If you want to use a very small aperture to shoot big pictures you need to set your aperture and adjust your other settings accordingly. Then pick your point of focus on something about 3-4 feet away from you. When you take the shot the details from front to middle to back will all be in good focus (if you're using a fast enough shutter speed.)
Sometimes the subject you'll want to shoot won't have a lot of depth (front to back) in the first place. In that case you can get away with bigger apertures, though it's generally better to use a mid-range aperture.
This image of business cards I collected at a social event was shot with a fairly small aperture. Why? I was using a macro lens and anytime you shoot in a macro situation the shallow depth of field is intensified even more. The closer you are to your subject the more background blur you'll get as well.
So if you're shooting in macro and you're just not getting the depth of field and detail that you want, chances are you need to shoot with a much smaller aperture. The aperture f/4.5 is going to produce a background blur like f/2.8 when shooting in macro. The above image (of cards) was shot at an aperture f/7 with a macro lens.
The next photo was shot in macro. Notice how only the rings are in focus and the focus drops off really quickly past that? I can't remember but I was shooting either aperture f/11 or f/14. When I first started doing ring shots I would get upset because I couldn't get more of the actual ring in focus. I eventually learned that to get more focus in a macro shots, I had to use an even smaller aperture.
Well I hope this post about aperture and depth of field was helpful. Remember that your choice of aperture determines your depth of field in a photo. As always what settings you choose depends on what you want in the photo. It's always your call to get the shot you want.