Digital and optical zoom in CCTV cameras. What is zoom (zoom) in the camera How does optical zoom work

17.08.2013 11885 reference Information 13

We have already talked on our resource about lenses with a fixed focal length -. And quite undeservedly bypassed the most common lenses among photography enthusiasts with a variable focal length, which are regularly equipped with the vast majority of cameras - zoom lenses. These photography optics allow you to zoom in and out of the scene without leaving your spot. With the obvious convenience of this function, you should be aware of the features of their use in order to get the maximum result.

According to the principle of operation, the zoom can be optical and digital. Digital zoom has nothing to do with the lens, and in this article we will not consider it. The focal length range of zoom lenses (FR) is indicated on the body as follows:

Which means: the focal length can vary from 14 to 42 mm in the first image and from 18 to 55 mm in the second. By dividing the second value of the FR (as photographers say in slang - at the long end) by the first, it gets the zoom value or lens magnification. (For example, for the lens in the first picture - 42/14=3.5). However, we are more interested in the angle of view, which determines what is visible in the frame. The angle of view is strictly related to the focal length and the physical size of the matrix (do not confuse it with the number of pixels!). The longer the focal length, the narrower the angle of view and the greater the zoom.:

Now on the market there are a large number of cameras with different physical sizes of matrices: full-frame (FF - "full frame", exactly corresponding to the frame size of 35 mm film), APS-C ("cropped" matrices of amateur DSLRs and prosumer compacts), etc.

Physical dimensions of digital camera sensors

In order to compare the angles of view of the lenses of different cameras, it was necessary to introduce the concept of "focal length equivalent to 35 mm film" or "equivalent focal length" (EFF). At the same time, two focal lengths are indicated in the characteristics of the lens - the real one (which is written on the lens) and the EGF - for comparison with other lenses.

For cameras with sensors smaller than 35mm film, when choosing a lens, you must take into account the factor that increases the focal length written on the lens - Crop Factor. For APS-C sensors, for example, this is usually 1.5 (1.6 for Canon cameras), which means that a lens with a focal length of 50 mm indicated on the body, when mounted on a camera with an APS-C reduced matrix, will work like a 50x1.5 FR =75 mm.

The table below shows the approximate focal lengths by shooting scene. These values ​​are conditional, the boundaries of these ranges should not be taken as a dogma.

Table - Focal lengths are in 35mm film equivalent

How does the focal length change in the lens? By changing the position of the lenses. Because of this, even in small cameras, lenses with a large FR range have to be pulled out of the body a lot. This is not always convenient, as it takes additional time and reduces reliability.

It is impossible to select lenses in the lens that work perfectly when changing position between themselves. Those. a zoom lens can only show acceptable performance in a narrow range of FR, despite its much greater capabilities. In the rest of the range, distortion is inevitable. To ensure sufficient quality for any lens position, the optical formulas of zoom lenses are complex, with more lenses compared to prime lenses. In addition, the extension mechanism can break, especially when sand gets into it, not to mention moisture and its companion - mold. The sealing elements of a zoom lens tend to wear out and lose their elastic properties over time. Also, you probably heard about the backlash of the retractable parts of the lens, which does not contribute to image quality due to the distortion of the geometric axis of the lens group in the lens.

Optical schemes for fixed and zoom lenses. Pay attention to the number of lenses

If the FR range is large - the so-called super-zoom lenses (18-120, 18-200, etc.) - the price for versatility is inevitable: a decrease in light transmission, an increase in internal dispersion, a decrease in image detail and significant geometric distortions of shooting objects at wide corners. You may have noticed that when shooting a group of people in a close room at the wide angle of a zoom lens, the faces of those who stand on the edge are stretched and rounded, and the people themselves become tilted. Who will like this? The same goes for shooting architectural structures. The cheaper the lens and the larger the FR range, the more distortion and other defects listed above when shooting. This is especially true for universal superzooms.

The foregoing equally applies to a camera with a fixed lens that has a large range of FR - the so-called all-reflex cameras (or ultras).

So, you should keep in mind: The greater the range of change in focal length (as they say - the greater the zoom or the degree of increase), the greater the likelihood of distortion at wide angles, internal light scattering and deterioration of image detail

It is worth noting that if you are shooting for uploading to the network or the size of pictures for printing does not exceed 10x15 cm, then you don’t have to worry too much about detailing - the quality of the pictures will be acceptable. But light scattering and distortion cannot be eliminated by any computer modifications.

The use of large focal lengths (strong approximations), regardless of the principle of the zoom, is fraught with two more dangers.

1. The closer the zoom, the less stable the camera position. This is especially felt with the small size of the camera, which is difficult to keep level. The subject begins to jump, dance, disappear from the frame, which, when shooting, will lead to image blur and marriage.

To avoid this phenomenon, fast shutter speeds should be used to obtain a stable image. But here lies another pitfall.

2. As the focal length increases, the light transmission of the lens decreases. That is, the photographer falls into a double hole: the shutter speed should be shorter, and the light transmission is low. As a result, the camera does not have enough light to make a correct exposure. And if there are no problems on a sunny day, then in cloudy weather, at dusk, shooting with long focal lengths is a big problem.

Manufacturers have taken care of users by offering lenses with built-in image stabilizers, which is usually indicated in the description and. These devices allow you to expand the range of illumination by about 3 steps, at which high-quality images are obtained.

When choosing a zoom - is it worth buying universal superzoom lenses, preferring mobility, or fast optics with a moderate FR range, putting quality at the forefront - there is no definite answer. Everyone decides for himself, based on the shooting conditions, the required quality and other criteria.

Explore the capabilities of your equipment by taking test shots with different FRs and for all of the situations listed. This way, you'll know exactly when and how to use different settings to get the best shots.

All you photographic!

Optical zoom is obtained by moving the lens of a camera or video camera to take a close-up picture of an object without getting too close to it. Digital zoom allows you to crop the image to the focal point and then zoom in on the cropped area. What is the difference and which type of magnification is better?

What is optical zoom?

An optical zoom camera zooms in on an image before capturing it. This is done by physically moving the lenses inside the camera lens relative to each other.

For some cameras, the lens literally “pops out” when using the function. So you conditionally “approach” the object you want to capture. Lenses enlarge the image of the object and allow you to capture it without loss of quality.

Camera with optical zoom - the lens extends.

To understand the difference between the two types of approximation, you need to remember two things about optical:

• optical zoom is a "hardware" zoom option;
• approach is happening before shooting.

What is digital zoom?

A digital zoom camera captures an image from the distance where you are physically. And then it zooms in on a certain part of it that you would like to take a closer look with the help of software.

A piece of the picture is literally cut out and stretched to the maximum number of megapixels that your camera sensor has. High-resolution cameras can magnify an image with virtually no loss in quality.

The difference between software zoom and "optics", thus, is the following:

• digital zoom is a "software" zoom option;
• approach is happening after shooting.

Which zoom is better: digital or optical?

Let's figure out which type of approximation gives the best image quality. If you ever had to programmatically enlarge pictures, we think you already guessed it. But let's explain anyway.

With digital zoom, part of the image is cropped and then enlarged to the resolution that your photographic equipment shoots (for example, 4000 x 3000 if you have a device with a 13 MP sensor). It is clear that because of this, the quality of the image is reduced compared to the original - the pixels are stretched, the picture is blurred.

But there is no connection between optical zoom and photo resolution. "Optics" enlarges the subject to a certain size, as if you were pointing a magnifying glass at it. And the camera takes "an image under a magnifying glass." Thus, you get a picture of an enlarged object with a maximum resolution (the same 4000 x 3000).

So the "hardware" version, of course, gives much better pictures than the software one.

How to improve digital zoom?

Digital zoom is used in most compact household cameras - for this they were called "soap dishes", because they seem to "blur" the photo. And also in hand-held video cameras, video surveillance “eyes” and absolutely in all smartphones.

What to do if you want to take beautiful pictures with your phone, but don't buy professional photographic equipment? Let's advise a couple of gizmos that add "optics" to the camera of an ordinary mobile phone.

External lens

It is worn directly on the phone's camera, attached to the body with a clip. So you can take advantage of both the "optics" and the phone's digital zoom to get a super-clear shot in combination. There is a wide range of such accessories on the market with varying degrees of approximation. Their price varies in the region of 500-1500 rubles.

To begin with, I will try to explain photographic terms that are not clear to everyone. Vignetting is the darkening of an image from the center to the edges, especially the corners. Zoom lenses are lenses that have the ability to change their focal length, "bringing closer" or "farther away" the subject. They have become widespread in recent decades; and before that, technically simpler lenses with a fixed focal length were more often used in photography, in photo jargon - “fixes”.

Chromatic aberration (CA) is a type of distortion in photography. They appear as a colored border on the contours of contrasting objects: on the one hand, such a border, as a rule, is red or purple, and on the other, it is blue-violet or, more rarely, green. In the center of the image, CAs are usually absent, but appear more and more strongly as you move away from it towards the edges.

Aperture is a device in the lens that regulates the amount of light passing through it. Usually placed between the lenses, and looks like a resizable hole.

Both vignetting and HA are by no means exclusive to zoom lenses.

The reason for vignetting is the difference in the transmission of light by the optical system of the lens during the "direct" and "oblique" incidence of rays on it. This difference is maximum when the aperture is fully open, and decreases rapidly as the aperture is closed. When the aperture is closed by 2-3 large divisions of its scale (“stop”), vignetting becomes almost imperceptible.

The degree of vignetting at full aperture is one of the important characteristics of the quality of a lens. To reduce it, developers have to increase the dimensions and weight of the structure.

Nowadays, vignetting has become relatively easy and painless to correct by computer processing. In many modern cameras, this is a built-in feature, and the user does not even realize that his camera lens has noticeable vignetting. On the other hand, artificial vignetting is widely used as a visual effect, so that the appropriate filters are included in Photoshop and even Instagram.

Now about HA. The reason for their appearance is a natural physical effect: the difference in the refractive indices of light rays of different colors by the lenses of the lens. Optics designers are fighting against CA with the help of sophisticated techniques: the use of special types of glass and "aspherical" optical elements. Naturally, the better the results of such a struggle, the more complex and expensive the lens. CAs also tend to decrease as the diaphragm closes. Computer suppression of contour HA is applied, and gives good results; but there are also so-called "longitudinal HA" that are not amenable to such suppression.

Now about the "fixes" and "zooms". The optical system of "zooms", due to the need to change the focal length, is noticeably more complicated. When designing them, developers have to solve conflicting technical problems, and at the same time make certain compromises. Therefore, with comparable technical complexity and price, it is much easier to provide high image quality and minimize distortion in "fixes". The general rule is this - the higher the zoom factor, the weaker its optical characteristics, at the same price. But the design of lenses and processing algorithms are constantly being improved, and the best modern zooms with a magnification of 7, 10 and even 18 are able to give a completely acceptable final “picture”.

With a closed aperture, the objective lenses "work" only with their central part, while their characteristics become closer to optically ideal. However, when the aperture is closed strongly (for modern consumer cameras - more than 1 / 5.6-1 / 11, depending on the size of the sensor), the sharpness of the image deteriorates somewhat due to the optical effect of diffraction.

Reply

Usually the lens is designed in such a way that it cannot be apertured to such an extent that the image quality becomes completely unacceptable. The deterioration in sharpness is noticeable, but in reality you have to choose a trade-off between sharpness and depth of field.

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Comment

I'll add to Maxim Petrenko's answer that the image quality of a zoom lens depends on its focal length. For example, a standard Nikkor AF-S DX 18-105 / 3.5-5.6 G VR lens:

Sharpest at focal lengths from 18 to 24 mm and apertures up to 5.6;

Least distorts straight lines at a focal length of 24 mm;

Vignetting is the least at any focal length greater than 24 mm and aperture 5.6 or more closed;

It shows the least chromatic aberration at 70mm at aperture 8, but in fact it is not so important;

In general, it is best at focal lengths from 18 to 24 mm and aperture from 3.5 to 5.6.

In practice, knowing these nuances is useful, but you can use such knowledge only if the situation allows. For example, in a report, no one cares what kind of vignetting or XA the lens has: if the picture is not successful, they simply will not watch it.

Aberrations are inherent in absolutely all lenses, since this is a consequence of the action of the optical laws of refraction of light passing through the lens. The color perceived by the eye depends on the wavelength - it is more for red shades and less for blue ones. Blue rays are refracted more than red ones, so the contours of any image passing through the lens consist of a kind of thin rainbow. The whole history of lens construction is the history of the fight against aberrations. The simplest lens has no lens at all, but only a tiny diameter hole, a pinhole (English pinhole - a pin hole). Behind it is a regular convex lens. To correct chromatic distortions, negative (concave or convex-concave) lenses began to be glued to it - this is how the lens appeared - achromat. Then two achromats of different optical power were mounted in one frame - this design was called aplanat. Then they learned how to calculate lenses from three lenses - two positive and one negative (triplets), in which most of the distortions were corrected to an acceptable level. This class of distortion-corrected lenses came to be called anastigmata. In the lens, the distance between the lenses is carefully calculated and respected in order to minimize distortion. Inside the zoom lens, the distance between the lenses and lens blocks changes, so it cannot be corrected by standard means of distortion. The calculation of a zoom lens is complicated and almost impossible without a computer, and in order for it to work more or less acceptable in all modes, aspherical (non-circular) lenses and lenses made of special types of optical glass are often used in its design. Summary: if the goal is to get the highest quality images when shooting, you should strive to use a lens with a fixed focal length. But in many situations, a modern high-quality zoom is sufficient.

The term zoom comes from the English verb zoom, which literally means “enlarge an image”. When choosing a camera, many are guided by the number of pixels of the matrix, although this indicator is not the main one. Like decades ago, the dominant factor in image quality is still optics.

optical zoom

Optical zoom is a way of zooming in on an object using a lens system, which is why it's called that. Optical zoom has existed in photography for more than half a century, and the high-quality effect of image enlargement is achieved using a special device called a zoom lens, which is a complex optical system with the ability to change focus using numerous lenses. Enlarging the subject with a zoom helps to get a picture at the output without loss of quality. There are two types of zoom: with manual mechanical focus or with automatic focus adjustment, which is convenient for amateur photography, but useless when creating a professional picture, for example, in macro mode. Autofocus does not provide for high sharpness with multiple optical zooms of the object being photographed. In a word, in cameras with auto focus, the magnification, as a rule, is no more than three times, while when using a mechanical zoom, you can zoom in on the object by more than ten times. Among the disadvantages of cameras with manual adjustment of sharpness and zoom (optical zoom), the most common risk is the risk of dust getting inside the lens.

When choosing photographic equipment with digital zoom, you need a middle ground between the number of megapixels of the matrix and the zoom zoom, because even with a zero and five-fold zoom, the image is fixed at the same number of megapixels.

digital zoom

The definition of digital zoom appeared along with the spread of compact digital cameras, which were popularly nicknamed "soap dishes". Digital zoom is not associated with the approach of the object, since the image displayed on the display is only stretched using the camera's matrix, being a framing method with a significant loss of quality.

With digital zoom, the picture loses its sharpness, since in this case there is simply no focus, because focusing and sharpening can only be done if the camera has an additional optical system. As a rule, the use of digital zoom is justified to increase the object by no more than 40-50%, because with a larger zoom, the image clarity will be very low.

Articles and Lifehacks

Our material is for those who want to figure out what the digital zoom actually is, which is available in your smartphone, according to the characteristics given on the seller's website.

In fact, there is absolutely nothing complicated in it: it is just a software scaling of the image received from the camera sensor before saving it to a file.

And what else is zoom

The camera lens has sufficient dimensions so that designers can provide for changing the focal length. It is this parameter that determines how “close” or “far” in relation to the photographer the object will turn out in the picture.

The range of focal length in specialized cameras is from 10 to several hundred millimeters. Moreover, for them, the developers provide for interchangeable optics, both short-focus (wide-angle) and long-focus (telescopic).

The design of the smartphone camera does not allow anything like this. Therefore, their creators have to get out, compensating for weakness through the use of software.

What are the disadvantages of digital zoom

Therefore, in order to get an image of a larger scale with the same image size, the image has to be “stretched” programmatically. At the same time, the clarity of details deteriorates, and digital noise appears.

How to deal with it

• Buy an expensive model with an optical zoom.
• Choose a device with a high-quality dual-module camera.
• Save the picture in RAW format in order to edit it on a PC.

Smartphones with good dual-cameras can't be called cheap either. However, their device allows you to get better photos due to the difference in the focal length of each of the modules.

One of them can be wide-angle, and the second - telephoto. By combining the information received from the two sensors, the camera application is able to digitally zoom much more accurately.

The last option, oddly enough, is also found only in top-end, in extreme cases - sub-flagship models.

Unlike saving photos in JPEG format, which leads to serious losses in quality when they are further edited, the RAW format saves much more data and allows you to get a much better image.

Unfortunately, such files take up many times more space than JPEG.

The conclusion from the above is unequivocal: cheap phones are not designed for high-quality photography. Those who do not want to purchase a separate camera, but want to get impressive photos, need to choose a model from the top segment.

In it, even the digital zoom will be much better than in a budget gadget.