A fundamental requirement for a photograph is LIGHT. This light can be of various kinds. Some important ones are:
Hard light- Perceived as a point source, gives well defined, contrasty shadows. e.g. the sun on a clear day
Soft light- A large source, gives soft shadows. e.g. An overcast sky(the sun is hidden behind clouds)
Transmitted- When the source of light is in the frame. e.g. A sunset
Each type of light has its uses (for portraits, soft light is generally preferred), and you cannot say one type of light is 'better' than another(in the absolute sense).
There are many instances when we need to artificially add light into the frame, for various reasons(some of which I will discuss in later parts of the article). One way of adding light is using a flash.
Flashes may be built into the camera (e.g. a pop-up flash) or may be an external unit. The power of a flash is expressed as a "guide number" in meters or feet(@ISO 100).
This formula will tell u how far a flash can expose a subject.
f-number x distance x sqrt(100/ISO)= Guide number
*sqrt= square root for those who dont know.
My canon 400D flash has a guide number of 12m... lets see what dat looks like in a practical situation:
Say I use a reasonable aperture of f/4 and ISO 400... then
4 x D x sqrt(100/400) = 12m
4 x D x 1/2= 12m
D=6m
So the flash can effectively expose a subject upto a distance of 6 meters, with the above settings.
If I keep the aperture constant and change the ISO to 100, the max distance the flash will expose is 3m.
for ISO 800, it will be 6 x 1.414= 8.5m
The reason for the square-root is because of the way light intensity varies with distance (inverse-square rule).
Say I have a constant source of light. Suppose, when I stand at a distance of 10 m from it, I receive 100 lux of light intensity, if I go another 10 meters away from the source (total of 20m from the light source now), I will receive 25 lux.(1/4th the original intensity when at double the distance)
#Remember that doubling the f-number (say from f/4 to f/8) reduces the light by a factor of the square of the ratio(in this case (8/4)^2 or by a factor of 4), that is why there is no square root function required for the f-number.
For 'ordinary lenses', the maximum aperture available varies with zoom. Take an 18-55 f/3.5-5.6 lens. When zooming in therefore, the maximum range of the flash is reduced. (With a constant aperture lens, this does not happen)
------------ More coming later in Part Two---------------------------
Hard light- Perceived as a point source, gives well defined, contrasty shadows. e.g. the sun on a clear day
Soft light- A large source, gives soft shadows. e.g. An overcast sky(the sun is hidden behind clouds)
Transmitted- When the source of light is in the frame. e.g. A sunset
Each type of light has its uses (for portraits, soft light is generally preferred), and you cannot say one type of light is 'better' than another(in the absolute sense).
There are many instances when we need to artificially add light into the frame, for various reasons(some of which I will discuss in later parts of the article). One way of adding light is using a flash.
Flashes may be built into the camera (e.g. a pop-up flash) or may be an external unit. The power of a flash is expressed as a "guide number" in meters or feet(@ISO 100).
This formula will tell u how far a flash can expose a subject.
f-number x distance x sqrt(100/ISO)= Guide number
*sqrt= square root for those who dont know.
My canon 400D flash has a guide number of 12m... lets see what dat looks like in a practical situation:
Say I use a reasonable aperture of f/4 and ISO 400... then
4 x D x sqrt(100/400) = 12m
4 x D x 1/2= 12m
D=6m
So the flash can effectively expose a subject upto a distance of 6 meters, with the above settings.
If I keep the aperture constant and change the ISO to 100, the max distance the flash will expose is 3m.
for ISO 800, it will be 6 x 1.414= 8.5m
The reason for the square-root is because of the way light intensity varies with distance (inverse-square rule).
Say I have a constant source of light. Suppose, when I stand at a distance of 10 m from it, I receive 100 lux of light intensity, if I go another 10 meters away from the source (total of 20m from the light source now), I will receive 25 lux.(1/4th the original intensity when at double the distance)
#Remember that doubling the f-number (say from f/4 to f/8) reduces the light by a factor of the square of the ratio(in this case (8/4)^2 or by a factor of 4), that is why there is no square root function required for the f-number.
For 'ordinary lenses', the maximum aperture available varies with zoom. Take an 18-55 f/3.5-5.6 lens. When zooming in therefore, the maximum range of the flash is reduced. (With a constant aperture lens, this does not happen)
------------ More coming later in Part Two---------------------------
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