Digital Camera Focal Length Format and Angle of Vision and Depth of Field in Digital Photography
On one hand, if we maintain a fixed format and project on it images formed by lenses of different focal lengths, and since the size of the images will be greater the greater the focal length is, the angle covered by a lens in each format will be narrower the greater the focal length. The combination of the lens' focal length (size of the images) and the format give us the angle of vision or portion of the image in the shot. The angle considered is the solid angle (cone) that passes through the four corners of the image, and whose amplitude in degrees is referring to the diagonal plane (from corner to opposite corner). According to the vision angle, the lenses can subdivide in three large categories:
- Normal lens , with a vision angle similar to the focused vision of the eye (45 º).
- Telephoto lenses , with an angle smaller than that of sight.
- Large angle lenses , with a broader field of view than that of the eye.
For every format there is a focal length that provides a "normal" angle of vision. This focal length usually coincides with the diagonal of the format to cover's length. Lenses with a focal length greater than normal in each format will be telephoto lenses, and those with a focal length inferior will be great angles. For this reason, the normal lens for a compact digital camera will be of a focal length between 12 and 16 mm, for the APS (H) format it will be a focal length of 35 mm, in the universal format the normal focal length is 50 mm, for 6 x 6 it is 80 mm, and for 9 x 12 format it is 150 mm. In all cases, the angle covered is approximately the same.
Images of different points of space are distributed, as we saw in 1.5.1, according to it's position to the left or right, top or bottom and front or back. On the other hand, if all the lenses are similar and the only thing that varies with the focal length is the size of the images, a lens of a greater focal length than another distributes the images of the points of the space (in it's three dimensions) on a greater scale: not only the images projected on a plane will be larger (printing), but also the sharp focal planes corresponding to the different distances are more separate to each other. If we go over depth of field (2.1.2), we can reach the conclusion that, for the same focal distances and aperture, the depth of field will be shorter, the greater the focal length of the lens is. This has important consequences in two aspects:
• In the same format, an angular lens will always have more depth of field than a normal, an much more than a telephoto, for each focal distance and aperture. This is evident when we use different lenses or compare the focal extremes of our cameras zoom. Is we wish to selectively apply the possibilities of depth of field, to have most of the planes appearing focused (great depth) or to isolate an only focused plane between others which are very unfocussed (low depth), now we know that the first will be easier no only with a closed aperture and a greater distance between the camera and the scene, but also with shorter focal lengths; to isolate a focused plane and take the rest out of focus not only do we use a large aperture and at a short distance, but we also use a greater focal length.
• Large formats are associated with a lower depth of field than lower formats for similar angles of vision, because, as we have seen, the normal lens will be of a greater focal length the greater the format is. The same thing occurs with any other angle of vision: for example: a great angular of 20 mm in universal format covers approximately the same angle that a 75 mm in 9 x 12 format; for the same focal distances and aperture, it's evident that the depth of field will be greater in the smallest of the two formats. If we want to double the size of the image regarding normal focal, in 35 mm format we will need a focal length of 100 mm, but in a 6 x 6 camera, this same angle and increase is achieved with a 160 mm lens.
The maximum depth of field is in digital photography because the digital camera's sensors are smaller than APS format and much smaller than 35 mm. There are digital reflex bodies with interchangeable lenses compatible with 35 mm format. In a digital SLR the size of the CCD is somewhat larger than in a compact camera but always smaller than a 35 mm; according to manufacturers, the normal lens for these cameras is not a 50 mm but a 32 mm focal length (equivalent relation: approximately 1.5). If we put on a digital reflex a 50 mm lens compatible with a universal format camera, this will not cover the angle of a normal lens but it will have become a short telephoto equivalent to one of 75 mm in 35 mm format. Analogically, a great angular on a digital reflex camera covers a smaller angle when used on a 35 mm reflex. As the format increases , the depth of field decreases for similar angles of vision. If we use the normal focal length of a digital camera, an aperture of f8 provides a great depth of field; on the other hand, for the same angle of vision, in 6 x 6 format this same aperture provides a quite reduced depth of field.