For most, aperture is the hard one to grasp.  For one thing its numbered backwards, and its effects on your image are two-fold, exposure and depth of field.  Depth of field is another thing some people seem to have trouble with as well, but its really pretty simply, and I hope to give you some alternative ways to understand it that may clear that up for you… Or make it worse, but I hope not.

First the easy one.  It’s effect on exposure.  The aperture is controlled by a light valve inside the lens.  It is an iris, closing down in the form of a circle in the center of the lens.  It is comprised of a series of curved blades, the more blades the better, but thats another discussion.  Mainly, just know that the higher the F-Stop you set, the smaller the hole in your lens is, restricting light.

The other effect aperture setting has is on the depth of field.  The larger the hole, the smaller the f-stop value, the shallower the depth of field.

What is depth of field?  Well, imagine the distance from your lens to your subject, lets use 10 feet as an example.  Using my 24mm F2.8 lens, with aperture set to 2.8, if I focus on a subject 10 feet away the subject will be perfectly sharp, in focus.  Anything else in frame, from 6.9 feet to 10 feet from the lens, will also be in focus, though a bit less sharp, and behind the subject, from 10 feet to 18 feet, will also be in focus, again, a bit less sharp but still acceptable.  The total depth of field is 11.1 feet, meaning, 6.9 feet from the lens back to 18 feet from the lens.  Past 18 feet is out of focus, and closer than 6.9 feet is out of focus.

Now, everything else is the same, but the subject is only 3 feet away.  Now, the total depth of field is .8 feet.  Everything from 2.65 feet to 3.45 feet will be in focus, everything closer or farther away will be out of focus.  So, the close the subject, the shallower the depth of field.

Another example… Sorry, last one.  Lets move the subject out to 20 feet.  Everything else the same.  So, everything from 10.6 feet out to 190 feet will be considered in focus, for a total depth of field of 179 feet.  No, I don’t have all this memorized, I use a DoF calculator.

You don’t need to memorize this, or learn any clever calculations, just know that the effect is there.  If planning for the shot, and you want to know more precisely, you can use an app on your smart phone or tablet.  There are tons of them, just search for depth of field calculator in your app store.  Some are free, some not, the non free ones can be clever, but usually its the same as the free version but without ads.

So, to sum up.  Aperture, in terms of exposure, controls the opening through which light pours into your camera.  Its a valve.  The larger the opening (small numbers) the more light comes in.  The smaller the opening (big f-numbers) the less light gets in.  In terms of composition, the smaller the F-Stop number the less of your scene, in terms of distance from lens, will be in focus.  Some lenses at close range can be so discriminating that if you focus on your model’s eye it will be crystal clear, but their eyelashes may be out of focus, and their nose and ears all but indistinguishable.