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    2021-10-21 17:40:50
    Introduction to Optical Zoom and Digital Zoom


    Ⅰ Optical zoom
    Ⅱ Digital zoom
    Ⅲ Hybrid zoom

    Ⅰ Optical zoom

    Optical zoom is produced by changes in the positions of the lens, object, and focus. When the imaging surface moves in the horizontal direction, the angle of view and focal length will change, and the farther scene becomes clearer.

    Obviously, there must be two ways to change the angle of view. One is to change the focal length of the lens. In photography terms, this is optical zoom. The focal length of the lens is changed by changing the relative position of each lens in the zoom lens. The other is the digital zoom. Digital zoom is actually an electronic enlargement of the screen, which uses "interpolation" to enlarge a part of the pixels on the original CCD image sensor. Through the digital zoom, the shooting scene is enlarged, but the quality of the image will inevitably be lost. In fact, digital zoom does not change the focal length of the lens, but only changes the angle of view by changing the angle of the imaging face, which produces an effect that is "equivalent" to the focal length of the lens.

    Therefore, we see that some digital cameras with longer lenses have more room for movement of the internal lenses and photoreceptors, so the zoom factor is also larger. We see that some ultra-thin digital cameras on the market generally do not have an optical zoom function, because the movement of the photosensitive device is not allowed inside the body, while the "long lens" digital cameras such as Sony F828 and Fuji S7000 have an optical zoom function.

    Wide, 5x and 100x zoom.jpg

    Wide, 5x and 100x zoom

    The optical zoom factor of today's digital cameras is mostly between 2x and 5x, which can bring objects 10 meters away to close to 5-3 meters. There are also some digital cameras that have a 10x optical zoom effect. The optical zoom of a home camcorder is between 10x and 22x, which can clearly capture things up to 70 meters away. The use of an extender can increase the optical zoom of the camcorder. If the optical zoom is not enough, we can add an extender in front of the lens. The calculation method is: A 2x teleconverter is attached to a digital camera with 4x optical zoom. Then this digital camera's optical zoom multiples have changed from 1x, 2x, 3x, and 4x to 2x, 4x, 6x, and 8x, which is obtained by multiplying the magnification of the teleconverter and the optical zoom magnification.

    The working of optical zoom

    The principle of camera imaging is that the light on the subject passes through the lens and is projected onto the rear photosensitive element. The point where the light converges is called the focal point. The distance from the focal point to the plane behind the image is called the focal length.

    optical zoom principle.jpg

    optical zoom principle

    From the above figure, we can see that there are three planes behind the lens, and there are three focal lengths respectively "focal length 1", "focal length 2", and "focal length 3". As can be seen from the figure, the distance between the two orange lines is the size of the image. In "focal length 1", because the focal length is relatively small, the light passing through the lens is projected onto the plane before it "diverges", so the image will be smaller. "focal length 2" is slightly longer, and the image will be "larger". By analogy, the larger the focal length, the "larger" the image. The principle of optical zoom is to change the size of the image by changing the size of the focal length. Inside the optical zoom camera lens, there are lenses that can move. The photosensitive element behind the camera is fixed, and the lens moves, which is equivalent to the focus moving. When the focus moves, the distance to the rear photosensitive element also changes, and the focal length also changes. After the focal length is changed, the size of the image also changes.

    So we can understand that the optical zoom only changes the distance of light transmission, the light itself does not change. Since the light has not changed, the image presented remains the same after the focal length has been changed. So the biggest feature of optical zoom is that the pixels of the image will not change after zooming in, that is, the picture will not be affected.

    The focal length of the digital camera optical zoom that we usually contact is actually the length of the focal length in the above picture. For example, the 3x optical zoom lens of Canon A95 has a focal length of 7.8-23.4mm, which refers to the range in which the focal length can be changed, which is actually the range in which the subject can be enlarged.

    Ⅱ Digital zoom

    Different from optical zoom, digital zoom is a change in the vertical direction of the photosensitive device, which gives people a zoom effect. The smaller the area on the photosensitive device, the visually will allow the user to see only a part of the scene. Digital zoom is actually a kind of electronic magnification of the picture, which magnifies a part of the pixels on the original image sensor to the entire picture. Through the digital zoom, the shooting scene is enlarged, but its sharpness is reduced. There is no movable or retractable part of the digital zoom. It uses the processor in the digital camera, and it only zooms in what it can observe, which is actually an electronic zoom of the picture. That is to say, a part of the pixels on the original CCD image sensor is enlarged by means of "interpolation" processing, so as to achieve the purpose of enlargement. This method is like using image processing software to increase the area of the picture, but the program is carried out in a digital camera. A part of the pixels on the original CCD image sensor is enlarged using "interpolation" processing methods, and the CCD image sensor is enlarged. It uses the interpolation algorithm to enlarge the picture to the whole picture. In fact, digital zoom does not change the focal length of the lens.

    The imaging difference between optical zoom and digital zoom.jpg

    The imaging difference between optical zoom and digital zoom

    Although the shooting scene becomes larger, its sharpness will be reduced to a certain extent. The larger the zoom factor, the lower the sharpness. The digital zoom is generally about 3 times, and the digital zoom of the camera is 44 Times ~ 600 times. When using digital zoom, the degree of pixelation is more serious than when using optical zoom. This is because the camera uses "interpolation" processing methods. When zooming in, it can only estimate the shape of the photo.

    Ⅲ Hybrid zoom

    Hybrid zoom is a fairly new concept introduced on smartphones. It combines the optical zoom, digital zoom, and software algorithms mentioned above together. When the zoom requirement exceeds the physical limit of the lens, it can switch from optical zoom to digital zoom to achieve better results.

    Currently, any mobile phone with optical zoom function generally supports 3x or 5x optical zoom. But if your shooting scene requirements exceed the focal length of the optical zoom, then it will cause the picture effect to be distorted. Because in theory, to continue zooming after exceeding the optical zoom range is to use digital zoom. And this combination of optical zoom and digital zoom is the so-called hybrid zoom.

    Although the algorithms and technologies used by each smartphone manufacturer are different, the overall concepts and principles are similar. Hybrid zoom uses software enhancements and algorithms to synthesize shots from multiple photos to create a better image. The principle is somewhat similar to night mode and HDR effects, but the focus is on the focal length instead of exposure.

    Smartphone manufacturers can make use of the different sensors and focal lengths of the mobile phone, and use the details captured by multiple cameras at the same time. All of this information can be used to improve the magnification of photos through smart algorithms. Of course, this is not a true optical zoom, but it is better than ordinary digital zoom, and it can keep the details within a certain distance without missing too many details.

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