The last part of the exposure triangle: ƒ numbers.

Last week’s post was all about shutter speeds, and if we remember what was all about, it explained how shutter speed affects how moving objects may or not may be seen, stilled or blurry. 

This post is going to be the finale of the triangle exposure, next week I am going to be explaining how to mesure light, so we will be able to know how much light our photograph is going to need.

Before shutter speeds existed, the ƒ numbers were already invented. The photograph camera as an imitation of the human eye, and the knowledge about stenopeic (from greek, stretch opening). But the human eye has something called iris, which, controls the pass through light just by altering the area of light that is passing through, but the eye is capable of maintaining most of the things focused thanks to the ability of having a focal length, while in the stenopeic camera, the little opening is the focal length itself. So adding arrangements of crystals, we are forcing the camera to be able to work almost like the human eye. Also cameras were able to have more sophisticated crystal arrangements, and with it different focal lengths without the need to change the size of the camera. And with it, the lenses gained the ability to have and iris, but in photography are more often called diaphragm or ƒ numbers.

The ƒ numbers are defined by the area formed by the iris, this means the area of light that is going to pass trough and so, also the go by double the area or the half of area, but defining this is not like the ones in time or ISO values. The ƒ number is defined as:

ƒ = √(2ⁿ)_0≤n<∞

Square root of two power of n, in which, n  is equal and greater than cero and less than infinity.

So in this case n is the number that is going to go in increments of 1, starting from zero. Doing this so, is going to give us approximate numbers to the ƒ numbers. The most common are:

ƒ 1.4

ƒ 2.0

ƒ 2.8

ƒ 4.0

ƒ 5.6

ƒ 8.0

ƒ 11

ƒ 16

ƒ 22

ƒ 32

ƒ 64

ƒ 90.5

ƒ 128

From top to below the quantity of light that is passing trough is the half, so ƒ 1.4 lets pass more light than ƒ 2.0. And just like the last article mentions it, they had become with half steps or third steps. Like ƒ 1.8 is half step of ƒ 1.4 and ƒ 2.0.

The second most important of ƒ numbers is the Depth of Field (DoF)

Just like the shutter speed, the ƒ number will modify the end result of a photograph, but in this case the difference is about the sharpness of what the lens is going to see. And that is called the depth of field. The DoF, to abbreviate, is the distance in front of the lens that is going to be sharp, this distance will be around the the focused object, this will apply in front and behind it.

This happens because wider openings will let pass "irregular" light, that is going to make that light coming from places outer the DoF, be projected in the film as blurry or diffused, as we go closing the opening, this will force light to pass more concentrated trough the lens, something like a stenopeic camera, and increases de DoF, making most of things defined.

This diagram shows how the DoF go in front and behind the focused subject.

The next images are going to illustrate how DoF changes, depending on how wide or closed the diaphragm is.

ƒ 1.8

ƒ 2.8

ƒ 4.0

ƒ 5.6

ƒ 8.0

ƒ 11

ƒ 16

As you can appreciate, as the aperture goes smaller the quantity of sharp objects increase, at ƒ 1.8 only the elephant is focused, but everything behind and in front of it is "unfocused", above ƒ 11 the Crystal Frog and the Golden Cat became sharp enough, but if you put attention to the background, the line behind is kind of blurry, maybe the next aperture value will define it.

In more serious photos, controlling the DoF will force the viewer to concentrate in only one thing, as you can see in the first images, even if the Golden Cat shines in the background, your attention concentrates on the Elephant, so mastering this technique for portraits is quite a rule, so your subject will be over all the things.

In the other hand if we try to do a landscape or we want to see the most details on everything we are going to focus on smaller apertures, preferable above ƒ 5.6, as in the example, almost all the toys are seen clearly.

There are other ways to have shallow DoF, but that depends highly in the focal length, and will be discussed on other entry.