Note 2. The aperture (Depth of Field)

In number 1 we talked about aperture but only in relation with the shutter to obtain a 'correct' exposure.
In 2 we are going to go into aspects of the 'Hole', the 'F' Stops; the APERTURE in the lens and how it relates to an image, that you might visualise.
It really hinges on this: the smaller the hole the bigger the number (F stop), the bigger the hole the smaller the number (F stop).
Big hole, small hole and in between will give you 'depth of field'. How much DOF you want will depend on how you visualise the finished image.
I will explain it this way:
Do you want all the scene to be sharp? Then use a small hole, say F22
Do you want a certain section of the scene to stand out, sometimes called 'differential focus'? Then use a big hole, say F2.8, on your subject you are 'highlighting'.
Try it now/later on your camera lens combination.

The human eye sees everything in front of it, then’ your brain differentiates as what is important to you. You still see the whole scene but 'focus' on the visual bit.
A lens cannot do two things at once. How you set will be what you get.
Now don't let's get bogged down with the detail (although it is important that you understand the principle).
Let us say you are taking a portrait; you generally want the viewer to focus on that person particularly if there is a lot of things in the background, which can DISTRACT from fully seeing what it is you want to convey. Therefore you might want the background out of focus reforming into coloured shapes.
This is where you would look at a big hole.
If you are taking an image of a football match to include the pitch; you would use a small hole to cover the whole scene so that from your minimum focus distance (of your lens) to the end of the image so that all is in focus.

Just to say at this juncture. Whatever size hole, aperture, you use you need to take into account your cameras shutter speed. Aperture and Shutter speed go hand in hand with the light you are shooting by.

To end, that, lens come in varying sizes and minimum apertures.
From Wide Angle lenses to Telephoto lenses, each lens type has a place and a 'job' to do.
Wide Angle and telephoto lenses come in two distinct constructions.
Prime (fixed size I.e. 400mm, that's it).
Zoom (size then multiple choice of length within the construction I.e. 150mm to 600mm).
Lenses come with varying minimum/maximum aperture, I.e. F8/F4.5

Buy the most expensive you can afford. Try to buy 'big' aperture I.e. F2.8 or even F2 these are BIG front end lenses as they need to gather more light than say a F4.5 and very expensive, and heavy (another consideration). If too heavy then you are going to need a tripod and possibly a shutter release to get blur free images.
To help: Ask yourself what do you want to photograph mostly?
Then buy the best one for your pocket and fits your subject profile.
If you occasionally shoot other subject/s then you can always hire then buy if it suites you.