Zoom factor
The zoom factor is defined as the ratio of the longest and shortest focal length of a zoom lens .
There are lenses that have only a very small range of focal lengths and that are used, for example, in video projectors in order to be able to adapt the image size to the projection surface. Lenses with a zoom factor of over five are also known as super zoom lenses in photography . The zoom factor does not allow conclusions to be drawn about the effect of the lens in relation to the angle of view to be achieved ; For example, a lens with triple zoom can be completely in the telescopic or wide-angle range, or it can also cover both the telescopic and the wide-angled range.
Focal length versus angle of view
The following comparison table shows examples of the absolute focal length, the relative image scale and the recorded image angle of a lens with doubling focal lengths for 35 mm photography with a normal focal length of 50 mm . The relative imaging scale and the captured image angle are independent of the normal focal length and can therefore easily be applied to other imaging systems. In addition, these sizes are set in relation to neighboring line entries:
| Focal length in mm | Relative image scale at 50 mm normal focal length | Angle of view ... | Ratio of… Note | ||
|---|---|---|---|---|---|
| for 35mm film in degrees | at normal focal length in relation to the angle of view | Focal lengths and reproduction scales | Angle of view | ||
| 6.25 | 0.125 | 147.8 | 0.317 | 2.00 | 1.23 |
| 12.5 | 0.25 | 120.0 | 0.390 | 2.00 | 1.47 |
| 25th | 0.50 | 81.7 | 0.572 | 2.00 | 1.75 |
| 50 | 1.0 | 46.8 | 1.00 | 2.00 | 1.92 |
| 100 | 2.0 | 24.4 | 1.92 | 2.00 | 1.98 |
| 200 | 4.0 | 12.3 | 3.79 | 2.00 | 1.99 |
| 400 | 8.0 | 6.2 | 7.56 | 2.00 | 2.00 |
| 800 | 16.0 | 3.1 | 15.10 | - | - |
It is worth noting here that the ratio of the focal lengths and the ratio of the image angles are only identical for very large focal lengths. In the wide-angle view, the decrease in the focal length only means a much smaller increase in the angle of view.
See also
literature
- Gottfried Schröder: Technical photography: Basics and applications in technology and science , Vogel-Druck, Würzburg, 1st edition, 1981.