Features photographic lens for cameras and description of the various types that can be mounted on a camera body.
Photo lens function
This important element of each camera has the function of capturing an image by reconstructing it on the film by means of a set of lenses inserted inside.
These lenses are moved by a manual mechanism via a focus ring placed on the lens itself or in the case of autofocus lenses this occurs automatically.
There are cameras with fixed optics and reflex cameras in which it is possible to interchange the lenses by mounting the most suitable optical component for every photographic occasion.
The lenses are divided into two large families, fixed focal length lenses and variable focal length lenses called "zoom".
The focal length of a lens is the distance between the optical center and the focal plane when the focus is positioned on infinity, generally it is a measurement expressed in mm shown on the front of the lens.
Wanting to list the types of fixed focal lenses most used as a kit for SLR cameras we have: the 28 mm also called wide angle or wide angle lens with a shooting angle of 75 °, the classic 50 mm often offered as a basic lens of a camera reflex that has a shooting angle of 45 ° equal to the view of a human eye, the 135 mm called telephoto lens with a shooting angle of 20 °, the 300 mm called telephoto pushed with 8 ° shooting angle.
So we see that as the focal distance increases, the shooting angle decreases. Among the wide-angle lenses there are also lenses called (fish eye) which having focal lengths below 28 mm have a very wide shooting angle but tend to distort the image a lot towards the edges.
Zoom lenses have the great advantage of replacing multiple fixed focal length lenses with a single lens as the focal length can be changed as desired.
The most popular are the 28-80mm and 70-210 which, when coupled together as a reflex camera kit, can cover focal lengths from 28 to 210, favoring a better adjustment of the frame according to the photographic image to be taken.
Among the disadvantages it must be said that a zoom lens has a higher weight and a lower brightness due to a smaller aperture.
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This component of a lens is made up of iris-shaped blades, their opening or closing is controlled by a ring on the lens.
Its main purpose is to increase or decrease the amount of light that will hit the film and to increase or decrease the depth of field.
The diaphragm adjustment is made taking into account the light intensity values given by the exposure meter which has the task of measuring the amount of light that passes through the lens.
The exposure meter will help to set the optimal aperture-shutter speed combination in order to obtain a perfect exposure.
The depth of field, which indicates the ability to focus on objects that are on different planes, depends on the focal length of the lens and the aperture set.
As for the first factor, the focal length, we can say that it will potentially be wider starting from the low focal lengths (wide angle) and will decrease as the focal length increases.
As for the second factor, aperture, let's say that the depth of field will increase by tightening the diaphragm (higher values) and decrease by opening the diaphragm (lower values).
A greater depth of field in looking for cases helps to focus better when, for example, in low light conditions, we cannot perfectly focus using the appropriate ring.
The optics used are also very important in digital as in analogue photography, characterized by more or less good distortion, brightness and chromatic aberration values that affect the final result.
The maximum aperture of a lens is equal to the focal length divided by the internal diameter of the lens.
The larger the maximum aperture, the brighter the lens, thus managing to let more light pass through and impress the film in less time.
The amount of light that passes through the lenses is regulated by a device called a diaphragm, usually located inside the lens.
Its size determines the depth of field and consequently that of focus.
Exposure is automatic in all models, generally with aperture priority; the most advanced models allow corrective interventions.
Lens objectives
They consist of more than one lens because this is the only way to partially correct optical aberrations.
In telescopes, multiple lenses are used to correct chromatic aberration and are called achromatic.
The lenses are built with different types of glass characterized by their refractive index and by the curvature that can be spherical or aspherical.
The curvature of the surfaces characterizes their focal length which will be positive in the case of convergence and negative in the case of divergence.
The use of lenses different in type and focal length, positive or negative, allows the various corrections and defines their general focal length (always positive).
Mirrored lenses
They are called catadioptrics and their construction is similar to the Cassegrain reflector telescope.
Compared to telephoto lenses, they have the advantage of a small footprint and low weight.
In addition to the two mirrors, they are built using low-curvature lenses to correct spherical aberrations and to support the secondary mirror.
Due to the remarkable extra-axial aberrations they are only built with focal lengths from 350mm upwards.
Due to its optical conformation it is not possible to introduce the diaphragm into it. Furthermore, the shape of the blur is a ring instead of a circle.
Normal goal
A lens with a field angle similar to that of the human eye, with a field angle between 43 ° and 45 ° is considered normal.
By extending the range also to wide-angle and moderate telephoto lenses, angles between 20 ° and 59 ° can be considered.
For the Leica photographic format, the most common, called 135 which has a frame of 24x36mm in size, the 50mm focal length lens is considered normal even if the closest lens would be 43mm, i.e. the size of the diagonal of the frame.
In the 120 format known as 6 × 6 the normal has a focal length of 80mm instead of 85mm calculated.
Of course in digital cameras where the sensitive element is generally smaller than 24 × 36 the normal lens is shorter than 50mm.
This type is also called standard, because it was the lens commonly supplied with the new cameras.
They are not subject to aberrations such as wide-angle lenses and telephones, moreover the extensively tested and perfected optical schemes have made them cheap and of good quality. The brightness is always very high, values are normally f / 1.8 and f / 1.4.
Wide angle lens
Image produced by a fish-eye lens in Italian with a fish eye.
Lenses with a larger field of view or shorter focal length than normal are called wide-angle lenses.
The angle of view passes from 60 ° to 80 ° for a wide angle, to also go up to 180 ° in ultra-wide angle and fish-eye lenses.
The latter are so called because because of the extremely wide angle of view the image is round, as if it were captured through a fish eye. For 24x36mm the most classic is 24mm, but 35mm and 28mm are common too.
The pushed wide-angle lenses produce a highly deformed image due to the equidistant projection of the light beams on the film, up to the formation of a circular image.
Their angle of view reaches 180 ° in the 6mm. Distortion can be corrected using straight projection up to a focal length of 14mm.
When the focal length decreases, the body of the lens would be too close to the focal plane, preventing the functioning of some mechanical organs inside the camera.
To overcome this drawback, the retrofocus or inverted telephoto optical scheme has been adopted.
It consists of a diverging front optical group and a converging rear group, it is possible that there are additional central groups.
The wide-angle lenses return an accentuated perspective and are subject to barrel distortions, where the falling lines at the edges curve conspicuously.
This typical wide angle effect allows an exaltation of the subject in the foreground, thus creating interesting creative effects.
Telephoto or long focus
To learn more, see the telephoto lens.
Lenses with a smaller field of view or a longer focal length than normal are called telephoto lenses.
The angle of view varies between 20 ° up to 5 ° or less in extreme cases. It would be more correct to call them long focus when they have a normal optical pattern.
For the laws of optics, the distance between the optical plane and the infinity focus plane is equal to the focal length, further lengthening for short distance focusing.
So a 500mm would become over half a meter long with poor handling and imbalances in practical use especially with free-hand use.
To overcome these drawbacks, the telephoto optical scheme has been adopted.
It consists of a converging front optical group and a slightly diverging rear group, there may be additional central groups.
This focal causes an evident enlargement of the subject and produces a strong compression of the field, that is, it brings the objects closer, apparently reducing the distances.
Focus
In order to clearly visualize the image, the focus is on positioning the lens at a suitable distance between the focal plane and the photographed object.
In some lenses there is no change in its length because the operation is done by moving one or more optical groups inside the lens itself.
The operation is carried out by acting on a special ring placed on the lens barrel.
The focus can be manual or automatic, using an engine placed inside the camera or the lens itself.
