Zoom principle and lens adjustment method of zoom lens
A zoom lens is a camera lens that can change the focal length within a certain range to obtain different wide and narrow field angles, different size images and different scene ranges. The zoom lens can change the shooting range by changing the focal length without changing the shooting distance, which is very beneficial to the picture composition. Because one zoom lens can also play the role of several fixed focus lenses, it not only reduces the number of photographic equipment, but also saves the time to change the lens when traveling.
1、 Zoom principle
Optical zoom is to change the position of the focus, the length of the focal length of the lens, and the size of the angle of view of the lens by moving the lens inside the lens, so as to realize the enlargement and reduction of the image. In the image above, the longer right angle of the red triangle is the focal length of the camera. When you change the position of the focus, the focal length will also change. For example, if the focus is moved in the opposite direction of the imaging plane, the focal length will be longer and the angle of view in the image will be smaller. In this way, the scene within the scope of view will become larger on the imaging surface
2、 Lens adjustment
The daily operation of the zoom lens is to adjust the aperture and zoom and focus, but in order to ensure the normal operation of the zoom lens, the adjustment of the rear focal length, the use of the magnification lens and the ultra-close lens, the use of the neutral color filter and the adjustment of the aperture tracking are very important.
Adjustment of rear focal length
When the lens is newly installed on the camera or the image sharpness changes with the focal length when zooming, the rear focal length needs to be adjusted. The adjustment method is as follows:
Take an image with different details (preferably a special test card), the object distance is 3 meters to 5 meters, adjust the aperture to the appropriate position, so that a clear image can be obtained on the image finder.
Use the focusing ring to adjust the image to the best definition under the longest focal length. Adjust the rear focal length ring under the shortest focal length to achieve the best image definition. When adjusting the rear focus ring, loosen the locking screw before turning the rear focus ring.
Repeat the first two steps until the focus is accurately tracked, and the image definition remains unchanged when zooming, and then tighten the fixing screws of the rear focus ring.
Use of magnification mirror
The focal length of a zoom lens has a limited range of continuous adjustment, such as a 21 × The 7.5 zoom lens can be changed from 7.5 mm (the focal length of the largest angle of view) to 157.5 mm (the longest focal length), and its zoom multiple is 21. If you change the magnification mirror (EXT) selector switch from × Shift to 1st gear × In the second gear, the zoom factor is still 21, but the range of focus change is changed to 2 × (7.5~157.5)=15 mm~315 mm. The following problems should be paid attention to when using the magnification mirror:
After adding a magnification mirror, the relative aperture of the lens will be reduced, and the luminous flux to the image plane will be reduced to 1/4 of the original value. Therefore, the aperture needs to be increased by two gears. When the light is dark, it is not suitable to use a multiplier lens, otherwise the captured image will be dark.
After using the magnification mirror, the perspective distortion and geometric distortion of the captured image will become larger. After shooting, restore the ultra-close camera ring and lock pin to their original state.
