Zoom lens is in a certain range can change the focal length, resulting in different width of the viewing angle, different sizes of images and different scene range of the camera lens. Zoom lens does not change the shooting distance of the case, you can change the focal length to change the shooting range, it is very conducive to the composition of the screen. As a zoom lens can take on the role of a number of fixed focus lens, travel not only reduce the number of carrying photographic equipment, but also save the time to replace the lens.
1. expensive; 2. large volume; 3. in any given focal length, the imaging is often not as good as the best fixed focus lens imaging clear.
Regardless of these inherent defects, the optical industry using the computer lens design, has created a lot of very good zoom lens. Many zoom lens imaging has been very clear, and the volume has been quite small, but the price is still very expensive.
When focusing with a zoom lens, it is best to consider focusing the image first to the maximum; that is, focusing with the longest focal length of the lens. Then, the focal length is reduced to the desired focal length at the time of shooting. In this process, all the images on the focal length are always clear. With this technique, it is easier to observe whether the image detail is clear and therefore the most accurate focusing method, because it is focused under the largest possible image. Note that some zoom lenses need to rotate two separate control loops, one ring controls focus and the other ring controls the focal length. The advantage of this structural layout is that once the focus has been completed, the focus will not be accidentally changed by adjusting the focal length.
Zoom lens of the biggest features, or that its greatest value, or is it to achieve the lens focal length can be converted according to the wishes of the photographer.35mm automatic pocket camera or part of the 35mm single-lens reflex camera zoom lens also uses the electric zoom mode, electric zoom is not only effort and convenience, more importantly, to achieve the speed zoom, which is the photographer through the focal length Subtle changes to cut the screen to determine the composition is very favorable. The zoom lens can also be "exploded" by momentary zooming in the camera shutter. Some cameras also rely on the automatic control of the zoom lens focal length conversion to achieve automatic composition function. The most innovative 35mm single-lens reflex camera, also set the function of automatic memory lens focal length, this feature allows the photographer to set the camera to remember one or more of the higher frequency of the lens focal length, at any time in the lens focal length Change to the previous memory of the focal length up.
Of course, relatively fixed focal length lens, the zoom lens structure is more complex, heavier weight. Non-brand-name zoom lens, imaging quality is certainly inferior to the corresponding fixed focal length lens.
Optical zoom is to move the lens through the lens to change the location of the focus, change the length of the lens focal length, and change the lens angle, in order to achieve the image zoom and zoom. In the figure above, the long rectangular edge of the red triangle is the focal length of the camera. The focal length also changes when the position of the focus is changed. For example, if the focus is moved in the opposite direction to the imaging plane, the focal length becomes longer and the viewing angle in the drawing becomes smaller. In this way, the view within the scope of the scene in the imaging side will become larger.
F is the transmittance of the lens. F smaller, the better the light transmission of the lens. f is the magnification ratio of the lens. Such as f = 1.4, that is, in a fixed position, the screen can be enlarged 1.4 times. The lens aperture is used to represent the value, generally from 1.6-2.0, for the use of convenience, a lens set multi-file aperture, the greater the aperture value, the smaller the aperture, the less the luminous flux, the maximum aperture of each lens are With the value marked in front of the lens.
Focal length is also used to represent the value, usually from 50-210, divided into short focus, standard and telephoto, as well as ultra-short and super telephoto. The smaller the value of the smaller focal length, the greater the value the longer the focal length, the projector on the lens focal length requirements are generally in the 50-140, rear projection is generally around 35, the focal length determines the full size of the projector when the projector and the screen Distance, the shorter the focal length, the projector and the shadow of the closer, and vice versa farther. If you want to shoot a large screen in the short distance you need to select the short focus lens projector, and vice versa you need to select telephoto lens. The general projector is a standard lens.
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