What is Aperture?

This one is not complicated but the numbers will look a little crazy so bear with me when we get to the numbers, please. In the description of shutter speed I used the analogy of a pipe delivering water and the spigot as the shutter which can open for most any amount of time. Now let's look at that spigot a little more. Instead of just on and off remember that spigots usually can be turned on just a little, just a little bit more and of course, all the way on. Now were looking at the spigot as an aperture, the wider open it is the more water comes through every second. So opening the valve a little way for one second will let in a certain amount of water; opening it twice as far (one stop more) will let in twice as much water in the same one second. Like before we imagine that the water is light. More water means brighter photo so now using our spigot analogy we can control how long the water flows in (shutter speed) and how much flows in at a time (aperture). One more thing. If we replace the pipe with a larger pipe, we've bought a faster lens.

If you're not familiar with the term "faster lens" it refers to the largest aperture of a particular lens. Let's get into those crazy numbers mentioned earlier.

Here they are:

1.4	2	2.8	4	5.6	8	11	16	22	32

This is a list of all the apertures you're likely to find on a DSLR camera lens and more than you'll find on most lenses. In a sort of reverse logic the larger numbers represent smaller apertures. Actually if you think of them as the bottom number in a fraction you can see why this is true. 1/2 is twice as large as 1/4 and if we assume the "1/" and leave it off we are left with 2 and 4 with 4 representing the smaller fraction. What we are actually leaving off, and you will sometimes see it left on, is "f/" so you may see an aperture referred to for example as f/8. "f" represents the focal length of the lens.

Aperture refers to the size of the "hole" we are allowing light to pass through. The "hole" is adjustable. Older lenses have a ring on them that you can turn and stop at numbers like those on the list above. Newer lenses have eliminated the ring and the adjustment is made from the camera Command Dials.

Let's look at a common "kit" lens like the one that comes with the Nikon D3100. It is a zoom lens that zooms from 18mm to 55mm (we'll get to those numbers in a later post) the largest aperture on this lens is f/3.5 or just 3.5. That size is between 2.8 and 4 on the list above and is not an even "whole stop" which doesn't hurt a thing. The adjustable aperture can only get as large as the lens diameter so to get larger apertures (smaller numbers remember) you need to have a larger piece of glass at the front. Larger glass means a larger lens, a heaver lens, a faster lens, and a more expensive lens. The "kit" lens mentioned sells currently for about $125. "Kit", by the way, refers to lenses which are included with the camera. The faster Nikon 24mm to 70mm f/2.8 lens is about $1,900. Faster lenses go up in price quickly.

Just like ISO, and Shutter Speed, changing the aperture (in manual mode) by one stop makes the image one stop brighter or one stop darker by the same amount as the other two. We can depend on "one stop" being the same brightness change no matter how we make it.

Also just like changing ISO added or removed noise from a photo and changing shutter speed changed the amount of blur of moving objects; changing f/stops or aperture has an other effect also.

Both photographs here have the same exposure so the brightness is the same, but a big difference in the look. Also in both shots I focused on the tree branch which was just a few feet in front of me. I was shooting in Aperture Preferred Mode (A or Av) which automatically controls exposure (so they are both the same brightness) but allows me to control the aperture. The photo left was shot at f/22, the one on the right f/2.8. The difference is called "depth of field" and sometimes "depth of focus" (which is technically incorrect, but I ain't gonna split hairs). Small apertures like f/22 keep more of your photo in focus and is commonly used for landscape photography where we usually want to see everything sharp and detailed. Large apertures offer a "shallow depth of field" which focuses attention on your subject and softens the background and makes it less distracting. Large apertures are frequently uses for portraiture.

In the shot here I catch my granddaughter's number and the background is softened to the point where is does not interfere with my intentions of showing my girl, the ballplayer, and the number one. The blond hair tells me who she is.