FOCUS OF THE LENS
The lenses used in 35mm SLR cameras are multi-lens systems, created by painstaking calculations and trials.
The most important feature of a lens is its focal length. The size of the image of the object depends on its length: the longer the focal length, the bigger the picture. The use of lenses with different focal lengths is then needed, when it is difficult to get close to or away from an object. The close-up effect is achieved by using a long focal length, and the effect of distance — by using a short one. The focal length in centimeters or millimeters is engraved on the lens barrel.
There is a close relationship between the focal length and the type of image, namely:
| Long focal length | Short focal length |
| Large image on the matte | A small picture on the screen |
| Small angle of view of the lens | Large viewing angle of the lens |
| The subject of a large-scale photo, little background around, relatively flat image. It should be used for distant motifs and there, where the fear of an unnatural perspective arises when taking a close-up photo (portraits). | Small scale photo, lots of background around, when photographed close-up, foreground objects appear excessively large in relation to the background, and in the case of photos of buildings, the impression of distortion and dislocation of the edge proportions. These lenses should be used there, where at the longer focal length the object does not fit on the focusing screen (interiors, tall buildings close up). |
This is considered a "normal" focal length, the length of which is approximately equal to the diagonal of the given negative format.
On 35mm SLR cameras as "normal” focal lengths are used 40 mm, 45 mm i 58 mm, usually focal length 50 mm.
A set of interchangeable lenses.
Interchangeable lenses, they are telephoto lenses, long focal length lenses or wide angle lenses.
Telephoto lenses usually have a focal length of 100 mm do 135 mm; focal length lenses 75 mm — 85 mm are not yet considered telephoto lenses, but for universal lenses with a longer focal length; 135 mm - is a type of universal telephoto lens, and then 180, 200 to 500 mm - telephoto lenses for special purposes, and finally 500 mm — 1000 mm - telephoto lenses with a mirror design.
The focal length of universal-type wide-angle lenses is usually 30 mm — 35 mm. Wide-angle lenses with a particularly short focal length between are also built 15 mm — 30 mm.
Following the example of cinematography, zoom lenses appeared on the market, so-called. transfokatory, allowing you to operate focal lengths 35 mm — 85 the mm 85 mm — 200 mm, itd.
Transfocator Zoom Yoigtlandera.
The first zoom lens was the Voigtlandera Zoomar 1:2,8; f=36 mm — 82 mm for 35mm photography. By simply moving the scale on the tube you change the focal length within wide limits without changing the relative aperture brightness, regardless of the chosen focal length. The focal length change is uniform, not jumping from one to the other, and despite this change, the relative aperture of the lens always remains the same.
Transfocters are relatively expensive, but often more convenient than a set of three lenses, tj. of normal focal length, wide-angle lens and telephoto lens. They are specially built for 35mm mirrorless cameras with a focusing screen and a pentaprism.