I thought I’d throw up a couple of images to show what you can do with one of the most common lenses photographers of every level have in their kit…the 50mm prime.
There are a few reasons why this is such a popular lens, some of which are the fact that it’s nice and small, super cheap and usually opens up to f/1.4 or f/1.8 which lets in a tonne of light and allows for an extremely shallow depth of field. The Canon system even has a crazy (and inversely crazy expensive!) f/1.2, insane!!
Most people tout about how great this is in low light. And that’s true, it is. But…you can’t simply take as gospel “oh no problem, I’ll just open up to f/1.4 and everything will just work out great!“. Nope, you need to learn to recognize the characteristics of this lens and how to control it and know when NOT to open up as wide as you can.
These two images were taken at f/1.4 and f/1.8 respectively. In the first you can clearly see how Noah’s right eye is completely out of focus. This was intentional, I wanted the left eye sharp with everything else out of focus. I knew that the 50mm at that range (only a couple of feet) at f/1.4 would give me extremely shallow depth of field, so I used that characteristic to my advantage.
This image (f/1.8) is similar. Chelsea’s facing the camera directly so both eyes are lined up to the camera on the same focus plane. This means the end result is that both eyes are sharp, and everything else is out of focus! Again this was intentional. If she had turned her face even a little we would have seen a result like the image above.
Now back to the point I was trying to make, which was really to understand how your lens works at all aperture settings (taking into consideration the distance to your subject).
Imagine you had a group of people, even two people only, and you want to shoot a nice tight portrait of them both. The environment is relatively low light, what do you do? If you answered “open up to 1.4 and fire away” then you’d be incorrect 
Remember the examples above? If you shot a medium close up (head and shoulders) of more than one person at f/1.4 on a 50mm lens you’d likely get bad results. Why? Because the depth of field at this setting/distance is just far too shallow. At f/1.4 from around 5-6 feet away (on a crop sensor body) you’ll get around 7cm that is in focus. This is the combination in front of and behind what you’re actually focus on. Think about it. That is extremely hard to control, and literally impossible to pull off a good image of more than one person at that range.
At f/2.8 this depth of field range increases to around 14cm, still cutting it very close.
f/4 is up around 20cm which is still very tight, but manageable. You need to make sure your subjects are all lined up on the same plane of focus otherwise some of them will be out of focus.
So this is the point, shooting blindly at f/1.4 isn’t always going to give you good results even if that’s what everyone says to do in low light. Instead, try bumping up your ISO and/or slowing down your shutter speed.
Learn the characteristics of all your lenses and how they interact with your camera and importantly…how they work at any given aperture/distance from subject combination!
Happy shooting 
