Zoom and focal length
Focal length is a characteristic that determines the angle of view of the lens. The shorter the focal length, the wider the angle covered by the lens, the longer the focal length, the more similar in its action to a telescope.
Often the concept of “focal length” in everyday life is replaced by “zoom”. This is wrong, since zoom is just a factor in changing the focal length. If the maximum focal length is divided by the minimum, we get the zoom ratio.
Focal length is measured in millimeters. Now the term “equivalent focal length” has become widespread, it is used for cameras with a crop factor, of which the majority. Its purpose is to evaluate the angle of coverage of a particular lens / sensor combination and bring them to a full-frame equivalent. The formula is simple:
EGF \u003d FR * Kf
FR – real focal length, Kf (crop factor) – a coefficient showing how many times the matrix of this device is smaller than full-frame (36 * 24 mm).
So the equivalent focal length of an 18-55mm lens on a 1.5 crop would be 27-82mm. Below is a sample list of focal length settings. I will be writing in full frame. If you have a camera with a crop factor, simply divide these numbers by the crop factor to get the actual focal lengths you need to set on your lens.
24 mm or less – “wide-angle”. The coverage angle allows you to capture a fairly large sector of space in the frame. This allows you to well convey the depth of the frame and the distribution of plans. 24mm is characterized by a pronounced perspective effect, which tends to distort the proportions of objects at the edges of the frame. Often, it looks spectacular.
At 24mm, it is better not to photograph group portraits, as the extreme people can get slightly diagonally elongated heads. A focal length of 24mm or less is good for shooting landscapes dominated by sky and water.
35 mm – “short focus”. Also good for landscape, as well as shooting people in the background of the landscape. The coverage angle is quite wide, but the perspective is less pronounced. At 35 mm, you can shoot full-length portraits, portraits in the situation.
50mm is a “normal lens”. The focal length is mainly for shooting people not the closest. Single, group portrait, “street photography”. The perspective roughly corresponds to what we are used to seeing with our own eyes. You can take pictures of the landscape, but not everyone – the angle of the field of view is no longer so large and does not allow you to convey depth and space.
85-100 mm – “portrait”. The 85-100mm lens is well-suited for waist-length and larger portraits, mostly in a vertical frame. The most interesting picture can be obtained with fast lenses with a fixed focal length, for example, 85mm F: 1.8. When shooting at an open aperture, “eighty-five” blurs the background very well, thereby emphasizing the main subject. For other genres, an 85 mm lens, if suitable, is a stretch. It is almost impossible to shoot the landscape on it, indoors most of the interior is outside its field of view.
135 mm – “large-scale portrait”. Focal length for close-up portraits in which the face takes up most of the frame. The so-called close-up portrait.
200 mm and more – “telephoto”. Allows you to take close-up shots of distant objects. A woodpecker on a trunk, a roe deer at a watering hole, a football player with a ball in the middle of the field. Not bad for shooting small objects close-up – for example, a flower in a flower bed. The effect of perspective is practically absent. For portraits, it is better not to use such lenses, as the faces are visually wider and flatter. Below is an example of a photograph taken at a focal length of 600 mm – there is practically no perspective. Near and far objects at the same scale:
The focal (real!) distance, in addition to the scale of the image, affects the depth of the sharply depicted space (together with the aperture). The longer the focal length, the smaller the depth of field, respectively, the blurring of the background is stronger. This is another reason not to use a wide-angle lens for portraits if you want background blur. Here lies the answer and the question is why “soap dishes” and smartphones do not blur the background well in portraits. Their real focal length is several times less than that of SLR and system cameras (mirrorless).