Monday, 01 January 2018 14:46

The focal length

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One of the advantages of SLR cameras is that it can use a wide range of lenses with different focal lengths and that you can choose the one that best suits the composition you want to create. But what is the focal length? First of all it is measured in mm and represents the distance between the optical center of the lens and the film plane or the sensor on the digital machines, to which a point placed at infinity is focused. The angle of view of the lens depends on the focal length and the sensor size. The lenses with a focal length between 35mm and 50mm, with 35mm sensors, offer an angle of view almost equivalent to the actual field of view, so they are called normal focal lengths. The lenses with focal lengths exceeding 70mm are called telephoto lenses and allow to enlarge the subjects. The telephoto lenses are very useful in sports photography, nature and for close-up portraits. The lenses of less than 35mm are wide-angle lenses, have a wide angle of view, and are very useful for landscape and interior photography.

Digital SLRs and crop factor

Depending on the model of reflex used the angle of field may vary depending on the model of sensor that our SLR mounts. The more our sensor is small the more the angle of view, at the same focal length, will be small. As previously mentioned the focal measurements are referred to the film or to a 35mm (full frame) sensor. Many models of SLR use sensors smaller than full frame, the most common and amateur use the APS-C model that has a multiplication ratio of 1.6x compared to 35mm. A sensor holds an increasingly narrow view compared to that captured by the lens, as shown in the figure, but the lenses are designed to take full advantage of a full frame sensor. Using an APS-C sensor we will cover a smaller surface and proportionally reduce the angle of field equal to the multiplication or crop ratio. This means that it is necessary to recalculate the actual focal length according to the crop factor of the sensor that mounts our SLR. In the case of APS-C sensors a 50mm lens will be equivalent to a 80mm lens (50mm x 1.6). The advantage given by the crop factor of the APS-C sensors is certainly that of decreasing the angle of view for the same lens on FF with the same Effective Focal length (LFE). This can be an advantage in sports and natural photography because of the fact that by decreasing the angle of view we have an effect that magnifies the image. On the contrary, it is negative in photographs where it is necessary to use the large angle. This inconvenience led to the construction of large-scale optics specific for APS-C sensors with focal lengths from 10mm

How to choose the focal length to use.

There is no rule to say which is the right lens to mount to take a picture, but we can give some important indications to make sure you make the right choice for shooting the photograph we have in mind. To recap what has been said up to now we can see what are the main characteristics of the three macro focal categories.

GRANDAGLES Wide angle of view allow to store in the frame a large quantity of elements and has a wide depth of field.
NORMAL or STANDARD. They provide a field angle similar to that of the human eye
TELEJECTIVES. Restricted field angle, reduced field profodity. In addition, the telephoto lenses crush the image by reducing the distances between the various planes of photography.
Knowing the peculiar characteristics of the various types of objectives helps in deciding which one to use to make our shots.
In this paragraph we have mentioned the depth of field that we will see specifically in the dedicated chapter.

Two exercises to see the differences in the field using different focal points on the same subject:

1. Shoot a subject from the same distance using a wide angle, a standard focal length and a telephoto lens.

2. Fill the frame with the same subject by moving closer or closer to the subject and using a wide-angle, standard, and telephoto lens.

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