Today we are going to learn how to use a flash and produce a complete guide that will allow you to improve your knowledge and obtain better photos. We don’t need to complicate our lives on the hardware level. We can be helped by transmitters, flashes, softboxes or umbrellas. However, we need to understand how it all works and the relationship of concepts to practice in order to develop our own visual instinct.
The guide number and the zoom: what are their relationships?
We will delve a little deeper into these two concepts that we have already seen in the previous chapter. To clarify the question a little more, you should know that the guide number is given in relation to ISO 100 and the maximum zoom of the flash. The latter sometimes varies and manufacturers usually specify the guide number of ISO 100 for 105 mm. As we have already said, there are exceptions and it depends on each manufacturer.
The calculation of the guide number is based on the inverse square law that we will see later. It is used for flashes considered as point sources of light (without the use of other accessories) but not for studio flashes, used with diffusers, windows or other accessories.
The inverse square law
The inverse square law, although it is a physical law, also applies in the field of photography. It refers to the intensity of the light which is inverted in proportion to the square of the distance from which it originates (the head of the flash).
It is for this reason that by modifying 2 stops or multiplying the ISO by 2, we manage to illuminate twice the surface:
In practice, this means that if we light our model closer to the light source, the intensity will be more violent. Result: a very bright foreground and a dark background that the light does not reach (it is received by the subject). If, on the contrary, we move away from the light source, part of the light will reach the background. It is for this reason that we must move away from the light when we want to illuminate the main subject of the photograph. Look at the previous table: if the light does not arrive sufficiently, we must open the diaphragm or increase the ISO.
The flash light cone
We can simplify the previous two points by using a cone of light at the flash flash. The light from the flash has certain special characteristics, which ultimately corresponds to daylight.
The flash light does not form a cylinder or multi-directional rays. This type of light usually generates two very clearly distinguishable zones. To observe them, we can trigger a flash facing a wall and capture the catch. We then clearly see the two distinct zones according to the light power: the main light and the secondary light, also called penumbra light. Conclusion: depending on the distance from the center of the base of the cone, the light will be weaker.
The tab-diffuser: more than a soft light
If we pay attention, the flash usually covers a zoom from 24 to 105 mm but there are always exceptions. For example, if we have a prime cine lens, say a 17-55mm, and want to work in a more angular position with just one flash, that will be enough to illuminate the scene perfectly.
The cone of light, as we have seen, can open to 24 mm. We can use in this case, the transparent tab-diffuser which will help us to spread the light over a greater angle, while illuminating the scene perfectly. The light will be softer, but also dimmer, but will generally lose a light stop with a large portion of the flashes.
But, this is approximate and depends on the flash and diffuser used. Therefore the use of a handheld photometer is recommended to measure the values.
Light modifiers and other accessories
Although there are many types of light modifiers, very different from each other, today we will talk about the most basic ones: umbrellas, softboxes or light windows and snoots.