Aperture

Aperture

Introduction

In this chapter we will analyze another important characteristic of a lens: the diaphragm.

The diaphragm of the lens is the opening through which light passes in order to arrive at the camera’s sensor. This opening has a variable diameter, and it is made up of a certain number of thin mobile plates that move in tandem. The diaphragm is very similar to the iris of the human eye and has been “copied” from the eye. The iris is a membrane that opens and closes depending on the quantity of light present and lets a certain quantity of light inside the eye depending upon external conditions. If the iris remained open where there was a lot of light, we would burn the eyes and would see a completely white image. In photography, we would have a shot that is overexposed.

The diaphragm is useful for increasing or decreasing the exposure of a photo as well as determining the field depth by creating particular effects with light or “freezing” particular moments.

In figures 01, 02, and 03 we see various openings of a typical lens’s diaphragm with 8 thin blades.

01 Diaphragm that is completely open.

02 Diaphragm with an average opening.

03 Diaphragm that is completely closed.

Before shooting a photo, we can decide how much we want to open the diaphragm by using a dedicated command. It is important to keep in mind that no lens reaches the maximum performance at extreme opening values of its diaphragm (in other words, when it is completely open or completely closed). Usually the best definition we can get is with intermediary openings between F8 and F9 but it depends on the lens and on the technical specifications.

On the controls of the camera, the opening of the diaphragm is not expressed in millimeters but is instead indicated with a value of F (luminosity), which is the relationship between the focal length and the maximum opening of the diaphragm. The diaphragm control allows us to choose between various F values, in other words, various openings and diameters. As the “F” gets smaller, it increases the opening of the diaphragm and a greater amount of light will enter. As we mentioned before, the diaphragm regulates the quantity of light that enters and is connected to three different functions (to the first two there is already a chapter dedicated while light effects will be discussed below):

- Exposure

- Depth of field

- Light effects

Light effects

With regards to the function of the diaphragm on light effects, I immediately give you an example. Let’s imagine we are shooting a photo of a cityscape lit up by lamps in the night. If we use an open diaphragm, the lights of the lamps will be point-like, exactly as we see it with the naked eye; if we instead use a diaphragm that is more closed, the light of each lamp will appear like a star with many more points. The number of the points of the star will depend on the number of plates of the diaphragm. The 8 plates of the diaphragm represented in the figure would generated 8-pointed stars.