Comparison Canon Speedlite 600 EX II-RT vs Canon Speedlite 580EX II

The guide number is the main characteristic that describes the power of the light pulse of the flash. It is described as the maximum distance (in metres) at which, at ISO 100 and f/1 lens speed (aperture 1), a flash is able to illuminate an "average" subject sufficiently for a normal exposure; in other words, at what distance from the flash it will be possible to normally shoot the scene at the specified ISO and aperture.

There are formulas by which, knowing the guide number, you can derive the practical shooting distance for each specific value of sensitivity and aperture. The simplest formula used to calculate the distance at ISO 100 is: S=N/f, where S is the distance, N is the guide number, f is the aperture value. For example, for a guide number of 56 and an f/2.8 lens, this distance would be 56/2.8 = 20 m. Increasing or decreasing sensitivity by a factor of 2 would increase or decrease the specified distance by approximately 1.4 times, respectively. If you need to calculate the distance as accurately as possible, you should refer to more detailed formulas that can be found in specialized sources.

Separately, it is worth noting that the leading numbers of flashes, usually, are indicated by manufacturers for specific focal lengths of lenses. This is due to the fact that the shorter the focal length and, accordingly, the wider the viewing angle, the more light is needed to illuminate the scene being shot and the more powerful the flash pulse should b...e (at the same distance). Therefore, when choosing by the guide number, it makes sense to pay attention to the focal length indicated by the manufacturer and select a model with a power reserve — especially since guide numbers are often prescribed for rather “long-range” lenses (with a focal length of about 80-100 mm in equiv. 35 mm).

The time it takes the flash or generator (for studio flashes) to prepare for the next flash. The smaller it is, the better. This parameter is especially important for continuous shooting, when the interval between frames is small: if you often shoot in this mode, you should look for a flash with the shortest possible recycle time. Also note that the characteristics usually indicate the shortest recharge time; in some operating modes, it may be significantly more than stated.

The number of flashes that the flash can fire without recharging the battery or changing batteries (see "Power"). This parameter is very approximate, because. in fact, it strongly depends on a number of factors: pulse duration, use of the display and its backlight (if any, see below), autofocus backlight (see "Features"), etc., and with replaceable batteries — even and on their quality. Often, manufacturers indicate in the characteristics the “perfect”, the maximum possible number of pulses — i.e. with their minimum duration, non-use of additional functions and even the optimal temperature regime for the battery. In reality, this figure may be lower. Nevertheless, the data indicated in the characteristics makes it quite possible to evaluate the battery life of the flash and even compare different models with each other.

Flash units with TTL support. TTL is an abbreviation for "through the lens", i.e. "through the lens"; this is the name of the method of measuring exposure by the amount of light that passes directly through the camera lens.

In digital photography, TTL works like a pre-flash: before the main exposure, the flash fires one or more test flashes. The amount of light coming from the object being filmed is measured by special sensors, based on these data, the control electronics sets the necessary shooting parameters, after which the actual exposure takes place. This allows you to fine-tune the camera and get an image of optimal quality. The interval between the test and operating pulses is so small that it may not be visible to the naked eye at all (especially when the flash is synchronized with the front curtain or with a slow shutter speed).

Many modern camera manufacturers have their own developments and varieties of TTL technology, respectively, differing in name: for example, Canon has E-TTL and E-TTL II, Nikon has D-TTL (in early models) and i-TTL (in later ones) , Pentax has P-TTL, etc. Support for one or another variety is directly related to the compatibility of the flash with cameras (see above), and different formats are usually not compatible with each other.

The angle at which the main beam of light from the flash diverges. This parameter is not expressed directly, in degrees, but in terms of the focal lengths of the corresponding lenses, in millimetres: for example, a beam angle of 105 mm corresponds to the angle of view of a lens with the same focal length (35 mm equivalent). This makes it easy to select a flash for specific optics, so that it most effectively illuminates all the space in the frame. And the most advanced modern flashes can have a variable dispersion angle, allowing you to adjust them to different shooting features; this feature is especially useful when using zoom lenses. Changing the angle of dispersion is carried out by a movable lens installed in the flash head, it can be carried out both automatically and manually (for more details, see "Functions and Capabilities").

— Autofocus illuminator. The flash has an auxiliary illumination function for the camera's autofocus system. Modern cameras overwhelmingly use the so-called passive autofocus systems that have one serious drawback: very low efficiency in low light and/or low contrast of the object being shot. The autofocus illuminator is designed to solve this problem: before focus, the scene is illuminated by a separate lamp, placed, in this case, directly in the flash body. This provides enough light for normal autofocus operation. Most often, backlight lamps give light with a characteristic reddish tint, however, in some advanced models, infrared illumination is used — invisible to the eye, but perceived by the camera. In addition, illumination systems can use a special light pattern instead of a solid beam, which further simplifies the task of autofocus systems. Anyway, the presence of this function is especially relevant given the fact that the flash is often used as a light source in low light.

— Control on the camera. The ability to change the settings for the flash using the controls of the camera itself, to which it is connected. In some cases (for example, with a wireless connection), this is much more convenient than switching attention from the camera to the flash.

— Automatic zoom. The ability to automatically change the angle of dispersion of the flash light. For the angle of disper...sion, see the relevant paragraph above, but here we note that this function provides synchronization between the flash and the lens: when the focal length of the lens changes, the angle of dispersion automatically changes. This provides the most effective illumination of the scene and at the same time relieves you of the need to manually re-adjust the flash to a changed angle of view each time.

— Manual zoom. Possibility to change the flash spread angle (see "Light spread angle") manually. This function expands the possibilities for "fine" tuning of work parameters and allows you to set parameters that are not available with automatic zoom (see above). In addition, it will be useful if you have to use several prime lenses with different focal lengths — the flash can be easily adjusted to each of them.

— Wireless control. Ability to wirelessly connect the flash to the camera or to another flash as master/slave (if available, see below). The format and specific features of such a connection may be different: wired connection, IR channel, radio, etc. Wireless control is indispensable if the flash must be placed away from the camera; it also makes it easier to create multiple flash systems for optimum lighting. These features are especially useful for studio shooting (although it's not limited to this).

— Work in master mode. The ability to operate the flash as a master flash for a system of multiple flashes. The entire system is controlled through the master flash, the operation parameters of the slave flashes are set, and the command to fire is given (note that the master flash itself may not fire at all). If you plan to shoot using a multi-flash system, you will definitely need a model with this function — without it, creating a system is impossible. Of course, master and slave flashes must be mutually compatible; this point should be clarified separately.

— Work in slave mode. The ability to work as a flash as a slave in a system of several flashes. In this mode, the device is connected to the master flash and fires on command from it. For more information on flash systems, see "Master Operation" above.

— Diffuser. The colour of the diffuser supplied with the on-camera light source (see "Type"). Colour can be: orange, white, pink or yellow.

A diffuser is a special filter designed to scatter light from individual LEDs, and also, in some cases, to give it a certain colour and change the colour temperature.

— Radio synchronizer. A device designed to wirelessly control a flash or a set of flashes (if each has its own receiver). Usually a separate module installed in the hot shoe; on command to fire, this module sends a radio signal to all receivers tuned to it, ensuring synchronous flash firing. At the same time, some models of luminaires with such a function are capable of receiving via the radio channel not only the start signal, but also the operating parameters (primarily the duration and power of the pulse).