In every cell there is a system of automatic focus (except in the most simple “soap dish, fiksfokalnye lenses are always focused on” infinity “).
If AF has not been turned off by force, he is engaged whenever the photographer half utaplivaet shutter button.
By linings autofocus system can be divided into two classes – active and passive.
Active auto focus works on the principle of radar.
With built-in camera transmitter radiates into space a series of infrared pulses, and then tries to light reflected closest object.
On the difference between the number of sent and received radiation (or the difference in time) Automatic apparatus determines the distance to the object and, according to some formula that shifts the lens so that the object was in focus.
This is a very simple and cheap solution, which, moreover, has a rather high speed focusing.
In addition, the undoubted advantage of the active autofocus is that it works even in total darkness.
However, it was not without shortcomings.
Chief among them is the inability to remove objects that are behind glass (eg, coach).
Infrared rays are partially reflected from the glass surface, and as a result of the lens is focused on it, and not on the subject.
In addition, the work of the active autofocus can interfere with extraneous infrared radiation – for example, working heaters, candle flame or fire, etc.
Finally, the pulses of the camera can be almost completely absorbed by black surfaces.
Nevertheless, in most situations the active autofocus is quite reliable, so he found a very wide application in low-cost film “soap”.
Work passive autofocus (or TTL-AF) based on an analysis of the image itself, “catch” the lens.
For this purpose the lens, there is one (central) or more focus points, which are light-sensitive sensors.
Roughly speaking, each such sensor is a rectangle resolution of several tens of pixels in length and width.
It is assumed that a sharp, well-focused image is to be contrasted.
Therefore, the processor camera simply compares the brightness of adjacent pixels in a rectangle and, if all the pixels have approximately the same intensity (ie
they are non-opaque), it is considered that the focus does not hover.
In this case, the lens drive command is a little shift lens on the current situation.
This happens until the sensor is not yet fixed the maximum contrast.
The passive method is more accurate active, but it (for obvious reasons) and somewhat slower.
In addition, often a passive autofocus “strays” too homogeneous subjects (eg, cloudless sky), or, conversely, too heterogeneous (shutters).
However, the main disadvantage of passive autofocus is its precarious work in poor light conditions.
To compensate for this shortcoming, using AF-Assist Illuminator: when focusing the camera illuminates the scene with flash, built-in LED or laser (like in some equipment from Sony).
Passive autofocus systems are used mainly in film and digital SLR cameras (sensors are located between the mirror and viewfinder), as well as in the majority of digital “compact digital cameras.
In this case “soap” role of light-sensitive sensor is a matrix camera itself.
In some vehicles used hybrid AF system, combining active and passive methods.
Coarse pointing (as well as focusing in the dark) is done using an active system, and tweaking – using passive.
This significantly increases the speed and accuracy of autofocus.
As already mentioned, passive autofocus system are as singletons, and multipoint.
With all the one-point clear: the point of focus is located in the center of the frame, and (in some models) can be determined arbitrarily.
Multipoint system measures the contrast in several parts of the image and focuses the lens on the most contrasting sites.
In theory this should provide greater accuracy, but in practice the “intelligent” multi-point systems are often mistaken in poor light conditions.
The situation is exacerbated by the fact that on a small LCD display device, these errors is quite difficult to discern.
In our subjective opinion, in the dark-point focusing system provides a more predictable results, while the well-lit scenes multi-point auto-focus is preferred.
Extremely useful is the ability to independently set the focus point.
Matrix (sensor)
Along with the lens, the matrix is the most important parts of digital devices.
However, the details of matrix, we will not, because earlier in the “Ferret” has the article was our constant author Sergey Aksenov (“His Majesty MegaPixel”), where this issue was covered in some detail.
I would like to emphasize only one point considered in the mentioned article.
Namely: Do not pursue megapixels!
Resolution of the matrix is not a major parameter of a digital camera, and a higher resolution does not necessarily imply better image quality.
Remember that to print photos in the format 10×15 cm or for publishing in the Internet “with the head of” lack of resolution of 3, maximum 4 megapixel camera (and this is with a margin for framing!
Even in semi-SLR cameras are often used sensors with a resolution no higher than 6 megapixels.
Think about whether you are 7-8 in the simple “soap dish?
Most likely, the optical resolution “mylnichnogo” lens will be much lower than the resolution of the matrix itself, so that in practice, “the magic of large numbers” in store for you just a waste of flash memory and a high noise level.
Pay attention not only (and not so much) to “megapixels” sensor, but rather on its physical size.
So, at the same resolution in 5-megapixel advisable to give preference to the cell with a diagonal matrix of 1 / 1, 8 inches, rather than 1 / 2 ,5-inch model.
A simple principle: the more the matrix, the higher the sensitivity, lower noise, less influence of diffraction at the closing of the diaphragm.
Do not buy the advertising promises of the sensitivity of ISO 400 or ISO 800 when the size of the matrix 1 / 1 ,8-1 / 2,7 “- most likely, the noise in these conditions will be such that you admit they are absolutely useless.
Often, the product line manufacturers find two absolutely identical models that differ only in resolution of the matrix.
In such cases, always buy a camera with less resolution – you will not only save, but in most cases get a better picture quality.
At equal the same resolution take that model, which has more physical size of the sensor.
Understand the difference between the types of matrices: a little 6-megapixel sensor in the tsifromylnitse “and 6-megapixel CMOS sensor in a professional” SLR “- are entirely different devices.
White Balance
White balance – this is one of the key functions of a digital apparatus, therefore, dwell on this concept a little more detail.
What is its essence?
Even if you have very little experience in color photography, you probably experienced the phenomenon when a shot taken, for example, incandescent light, had a pronounced yellow coloration, while fluorescent light – green.
Why is this happening?
Digress and consider the textbook example of a school physics course.
Take an imaginary absolutely black filament, connecting it to a power source through a rheostat and begin to gradually increase the voltage.
At some point, the thread warms and glows slightly red.
Continuing to operate a rheostat, we note that the emission of threads is not only brighter, but consistently changes its color: first orange, then to yellow, then to white, then blue, etc.
Thus, any light source has a color and this color can be associated with a certain temperature – that is, the temperature to which it is necessary to heat a blackbody to its visible light has been exactly the same spectral composition.
This temperature is called color temperature, which is generally measured in degrees Kelvin.
For comparison: the flame of a candle has a color temperature around 1800 K, the light bulb – 2500 K, the sun – 3800 K, the lamp flashes – 5500 K, and a blue, cloudless sky on a summer day – 11000 K and above.
Now I understand why the objects being lit by incandescent lamps, reflect yellow light.
But why in any light, we usually see a white sheet of paper as white, but not as yellow or blue?
The fact that the human brain quickly adapts to the situation and, based on his knowledge of what the white “automatically” produces the necessary “color correction”.
At the same time, a color film or matrix camera impassively record the scene in exactly “as is”, and the result is sometimes far from the expected.
Of course, since the invention of color photography people tried to deal with this problem.
In the film technology used for this conversion filters and film, calculated at different color temperatures.
Digital technology also offers a more convenient approach based on an automatic color correction at the processor level camera.
This function is called “white balance» (white balance).
How does it work?
Relatively simple.
The logic device based on the assumption that the averaged frame is neutral in color, and therefore more striking elements of the picture should be proportional to the neutral white.
Thus, roughly speaking, the brightest point of the frame conditionally accepted for white, and all other colors are adjusted with respect to them (in fact, everything happens a bit more complicated, but the overall principle is exactly like this).
This algorithm works relatively well in good light, but in difficult conditions, when even the brightest color is very far from white, color correction results can be unpredictable.
So often provides an opportunity to “hint” automation, which light source currently in use.
Typically, the user is available for about five fixed presets (“Daylight”, “incandescent”, “fluorescent lamp”, etc.
), Which can be switched via the menu or by using a special button.
Depending on the user’s choice, the camera makes an amendment on the proper color temperature, color correction and the result is much closer to reality.
Many devices also provides a manual white balance mode.
In this mode, the photographer puts next to the subject a sheet of white paper and, before you start the actual shooting, calibrates the camera on this list.
Of course, calibration requires some time and, consequently, affects the speed of shooting, however, and results in manual mode white balance is usually obtained the best.
Some “advanced” devices allow you to specify the source of light directly in degrees Kelvin – is useful for studio photography, where the color temperature of lighting devices is known in advance of their passports or can be measured by a colorimeter.
When selecting the device is worth remembering that the ideal algorithms for automatic white balance does not exist in principle – can only speak about more or less successful implementations.
If possible, take the camera, take pictures of her in different lighting conditions with appropriate white balance settings, and then carefully review the results on the calibrated monitor.
Part of the “blunders” of the algorithm will mean for you sleepless nights over the bitmap editor in an attempt (usually futile) to bring images to a more or less natural form.
Choose more cameras with a large selection of preset lighting sources, and with manual calibration by the reference white sheet.
Good working white balance – a really important advantage of the apparatus.
Image Stabilization
Any amateur photographer sooner or later confronted with the manifestation of the so-called effect of “shake” – “smearing” the image when shooting handheld at sufficiently high exposures.
Experiments found that the “thirds · Simplicity” begins to appear with a shutter speed equal to one divided by EGF.
In other words, at a focal distance of 60 mm shutter speed longer than 1 / 60, can lead to “sdergivaniyu” frame (the camera is usually warns about this special icon).
Obviously, the greater the focal length, the greater the chance encounter with “Shevelenko.
So shooting with a telephoto lens in hand (and even in low light conditions) becomes a real headache for the photographer, and saves him, only cumbersome tripod to put it mildly, is not always convenient to carry.
It is natural that photographic equipment manufacturers looking for ways to solve this problem, and in 1994 by Canon was presented the technology, dubbed Image Stabilizer (IS).
Its principle is simple enough.
The lens is movable on the vertical and horizontal axes of the stabilizing element, which, depending on their position distorts the path of light.
In addition, special sensors built into the lens, arranged on the principle of gyroscopes.
These sensors continuously determine the angles and speed of movement of the device in space and give teams an electric drive, which rejected a stabilizing element in such a way that the projection of images on film (or matrix) fully repeated oscillation apparatus.
As a result, subject to moderate fluctuations, the projection is always fixed on the film (matrix), which provides the necessary clarity of the picture.
True, the price performance of the lens: the presence of additional optical element affects primarily on aperture.
Nevertheless, optical image stabilization technology has been enthusiastically taken up by other manufacturers and is well proven in a number of telephoto lenses.
Moreover, if more recently, it was relatively expensive possibilities inherent in a shift lens, but now it can be seen even in quite budget digital cameras with built-in optics and the price to $ 500 (Canon, Panasonic).