Slightly correct our leader ...
The amount of light caught on the photosensitive element (film, matrix) is determined by two characteristics: the intensity of the light flux (in the closest approximation - a diaphragm) and the duration of this light exposure (exposure). This, so to speak, is an introduction that was worth mentioning again ...
1. First, let's talk about the shutter speed, more precisely - about the types of valves
If you do not consider all the exotic, all options can be divided into two types: central (FLAP) and shtorochnomu (gap). The central shutter is placed either behind or inside the lens and works like the diaphragm: a few petals open, letting light and immediately shut down. Closures of this type are used in a compact camera or a large lenses for cameras shirokoplenochnyh or formatted. Gap (shtorochnye) closures have been applied in the SLR (film and digital). Among them there are two sub-types: horizontal (earlier - Lined now - metallic) and vertical (metal, sometimes - lyamelnye). Pro operating principle of a horizontal cloth shtorochnogo Shutter colleague
kochrob He described in some detail. Indeed, since Shutter speed - constant, the lighting time can be controlled in only one way: by setting the interval between the movement of the first and second shutters. Those. short exposures by the photosensitive member moves literally slot and a completely open shutter box ( "Zenith", for example) is on only one exposure / 30c. It is for this exposure in such devices and is set synchronization with flash, which is triggered only when the first shutter is fully opened. If you tried to shoot with the flash at a fast shutter speed, then the frame is exposed to only a short section of the frame area, which was open at the time of firing.
The situation with vertical slit gates (currently used in most devices) is somewhat better: the movement time along the short side of the frame is significantly less, the mechanism can be made easier and less inertial, therefore the situation when the frame field is fully opened is achieved at much shorter exposures: 1/200 for example.
The principle of the flap shutter is easy to understand by looking at the horizontal window blinds. Such closures are used, as a rule, in film cameras, when film loading goes through the back cover. In the film mirrors, when it is necessary to gain access to the matrix through the landing hole for the lens, horizontal metal curtain valves are used.
In general, everything is simple with the central closures: there the entire field of the frame is always lit by definition, but here another factor begins to affect ...
2. Flash pulse duration
If you look at the distribution of the amount of light from the flash at the time, it turns out that the form of such characteristics will not be nearly rectangular. Those. there is time until the ionization of the gas in the flask was gaining momentum, and then - until the power applied to the electrodes of the flash lamp is exhausted, and, of course, we will see a decay process. So, the trick is that the flash of the declared capacity is given only for a certain time. If the shutter speed will be much shorter than the pulse time (at the central gate, for example), the photosensitive element will get only part of the light flux necessary for the normal exposure, and as a result we obtain underexposure. This is also the reason that, for synchronization used with the flash exposure is not shorter than 1 / 200c even for the central gate.
By the way, to solve the problem with a flash sync shutter speeds on older cameras used technology "stretched" pulse when using special electronic circuits made lamp glows from the beginning of the first movement of the shutter until the completion of the second movement. Most often, this flash mode called FillIn ( "fill light"). For example, the so-worked (and works well, so far, by the way) built-in flash my OLYMPUS iS-3000 "Portrait" mode, where it is necessary to highlight the shade from the sun.
Well, something like that ... It seems that I did not invent anything "from myself" ...;)
Post has been editedBombilCalabasov - 27.05.08, 08:40