Let’s see first of all what are the main components of the camera lens system, bearing in mind that a camera, as well as a film camera, is basically a camera that takes images at a constant rate. This cadence is so fast, the images per second are so many that when we look at them sequentially at a speed of 24 per second or higher we cannot perceive them as single frames and we experience the illusion of movement. The number of frames per second of a video sequence is called the frame rate and is often referred to as frame per second, or fps. In the cinema field, the most used frame rate has been 24 fps for about a century but with the advent of digital 25 fps are finding increasing use.
The lenses have the function of directing the light on the sensor
The diaphragm has the task of reducing the hole through which the light is passed before reaching the sensor. The size of this hole, called the entrance pupil, determines both the amount of light reaching the sensor and the depth of field. The wider the hole, the brighter the scanned image and the shallower the depth of field; the narrower the hole, the darker the scanned image and the greater the depth of field. The diaphragm is always located in the immediate vicinity of the optical center of the lens, which is where the light rays are converged by the lenses. This position allows the diaphragm to close without limiting the angle of view of the lens.
Depth of field is the extent of the area
The distance between the entrance pupil and the focal plane is called the focal length. This measurement, expressed in millimeters, is used to distinguish the various types of lenses, which are therefore called 20mm, 50mm and so on. Check fore more here.
Focal length determines the angle of view that a given lens is able to cover on a given sensor. The shorter the focal length, the greater the angle of view. The smaller the sensor, the smaller the angle of view.
The angle of view is the width of the field of view offered by a given lens on a given sensor. Since it is an angular extension, it is indicated in degrees. Based on the angle of view covered, each lens acquires a particular definition in relation to the sensor in use. Let’s see what these definitions are and consider some examples in reference to the Full Frame format:
|Identification||Field angle||Example focal length
on Full Frame
|Full Frame Angle of View|
|Super TV||Up to 8 °||400mm||6°|
|Normal||51-55 ° (53 ° is the exact measurement)||43mm||53°|
|Super wide angle||91-110°||20mm||94°|
|Ultra wide angle||Over 110 °||14mm||114°|
To get an idea of how much the size of the sensor affects the angle of view covered by a lens, just think that the normal of the Micro 4/3 format is 22mm. The same lens can therefore be a super telephoto as a super wide angle, it all depends on the sensor on which it is used.
The focal plane or plane of focus corresponds to the sensor or film, it is the plane on which the light rays captured by the lens are focused and create the image to be acquired.
Each lens is designed to focus the light rays on a plane that is at a certain distance from its coupling point to the camera body. This distance is called the draft.
The mount, also called the mount, connection or attachment, is what physically connects the lens to the camera body. There are numerous types of mounts, both screw and bayonet.
The draft and the type of mount
The light, exiting from the rear lens of the objective and passing through the mount, reaches the camera body and meets the shutter. The shutter is like an eyelid that opens and closes allowing the light to reach the sensor and expose each single frame only for the time set by the user. The longer the shutter is open, the brighter the scanned image.
While in film cameras a mechanical shutter is actually present and in operation, in the digital field the role of the shutter is played by the sensor itself, which activates and deactivates for the time necessary to acquire each single frame. The sensor can perform the shutter function in two ways: by activating and deactivating entirely or by activating and deactivating in sequence the rows of photosites that compose it. In the first case we speak of global shutter and in the second, much more common case, of rolling shutter.
In addition to affecting exposure, the shutter speed determines motion blur, i.e. the blurring of the image generated by any movement of both the subjects and the camera. The faster the shutter speed, the lower the motion blur. The reason is intuitive: the less time it takes to expose the frame, the smaller the portion of movement that is recorded in it and consequently the more defined the acquired image appears. To obtain a movie with natural motion blur, as we are used to perceiving it in the cinema, the shutter speed during shooting must be set between 1/45 and 1/55 “. Usually 1/48 “is used when shooting at 24 fps and 1/50” when shooting at 25 fps.
Once past the shutter, the light reaches the sensor. This can be set at different sensitivity levels, referred to as ISO. The higher the sensitivity, the brighter the scanned image becomes. It should be considered that the more the ISO is raised above the base level, the worse the quality of the acquired image, in particular as regards the presence of digital noise.
The aperture determines the amount of light reaching the sensor and the depth of field. The further you open it, the more light passes and the shallower the depth of field in the captured image.
The shutter speed determines how long the light must reach the sensor when creating each individual frame and determines the level of motion blur present in the captured image. The faster the shutter speed, the less light the sensor collects and the less motion blur in the captured image.
ISOs indicate the sensor ‘s sensitivity to light: the higher they are, the less light the sensor needs to create the image. For the highest image quality it is necessary to use the basic ISO sensitivity, also known as native, usually specified by the manufacturer or corresponding to the lowest value among those selectable.
The focal length of a lens determines the angle of view it is able to cover on a given sensor. The longer the focal length, the smaller the angle of view. The larger the sensor in use, the greater the angle of view the lens covers.
The frame rate is the number of frames per second that the camera records. If it is 24 fps or more, the captured footage creates the illusion of motion and does not appear as a sequence of frames when played back.
Here is a diagram to visualize the path of light from the outside world to the camera sensor:
By changing the aperture, shutter speed and ISO sensitivity we can make sure that the right amount of light reaches the sensor and that the captured image is correctly exposed.
The most immediate parallel is with water. The diaphragm is the tap and the shutter determines how long the tap remains open. If we have a large tap, it will be sufficient to keep it open for less time; if the tap is small, just keep it open longer to let the same amount of water pass. In this analogy, the ISOs indicate the size of the glass to be filled: the higher they are, the smaller the glass and the less water is needed to fill it.