In photography and video, the relative amount of light is usually measured by light steps (stops or f-stops in English). Here we explain what the light steps are and how they are used.
What is a passage of light?
In photography, we need to measure in some way the amount of light that reaches the photographic film or the sensor.
The total amount of light collected by the camera determines the exposure: how light or dark the image as a whole comes out (concerning the scene, for example)
A light path indicates a two-fold increase or one-half decrease concerning a certain amount of light.
For example, going up a light step is equivalent to having twice the light than the previous state.
Lowering one step of light means that we will have half the morning.
Most of the information on the internet on this topic talks about how the steps are used: aperture (aperture), exposure time (shutter speed) and ISO, how to compensate for exposure, etc. but without going into the basic concepts or the origin of the criteria that are still used today in photography. I like to know the why of things so that the explanations can be a bit technical. If you want to know the practical part, I recommend this other article to discuss light management in photography: aperture, exposure, and ISO.
Why that double and half criterion?
On the one hand, we have that most human senses, including sight, work in a logarithmic way, following the Weber-Fechner Law.
The perception of the brain is not linear concerning the variations of the external stimulus. For example, in human vision, the perceived illumination does not correspond to the eye’s actual light incident.
There are many factors involved, but in general, it follows a more or less logarithmic function, or at least non-linear: small changes in light intensity in a dark environment are perceived as large variations in lighting, and large changes in power in an environment very bright we perceive them as small variations.
We use a logarithmic/exponential scale that fits our brain works very well when working with light steps.
Furthermore, although we could use any other logarithmic scale (for example, the logarithm in base ten is used for sound, and it is measured in decibels), in light, the power scale of 2 is probably the simplest and most intuitive.
Light steps and diaphragm aperture
Historically, the concept of light passage is closely related to the operation of the diaphragm.
The diaphragm works like the iris of the eye. It is a more or less circular opening that can be made larger or smaller at will. Space is the action of opening or closing that gap. In photography, the word aperture is usually used in general, perhaps due to the similarity with the English term ‘aperture.’
The amount of light that can pass through this gap (per unit of time) depends on the area.
The more significant the gap: the more light will pass through, the smaller it is:, the less light. The amount of light that passes through the hole is proportional to its area.
As a curiosity, in the first cameras, there was no diaphragm. As such, a disk or a plate was placed with the necessary hole for the type of photography. For example, if there was much lighting, a container with a small gap was placed (initially by disassembling the camera and screwing the disc with the aperture, later using plates as in the Waterhouse Stop system ). If there was little light, a plate with the gap was placed larger, or merely the entire entry surface of the target was left free.
Modern diaphragms are made up of blades or fins positioned in such a way that their rotation generates a more or less spherical hole in the center:
What is the f-number (F-stop in English)?
To understand the number f (the number that defines the openings of a lens), let’s go step by step.
First, we will see the relationship between the radius of the diaphragm’s inner hollow and its area.
As we can see, to get the hole to have twice the area, we must open the radius (or diameter) by applying a root factor of 2 (that is, we must multiply or divide by approximately 1.41)
Now we are going to see another important question:
Is all the light that passes through the diaphragm reaching the sensor? No
Think that rays of light start (or bounce) from every point in the scene in all directions. The lens focuses part of those rays on building the corresponding point in the image.
Rays with paths that do not pass through the objective’s entrance do not reach the sensor (the aim being an opaque tube, the light can only enter through the entrance hole).
The same goes for the diaphragm. It is an opaque element that prevents the passage of light except for its central hole. The rays of light paths that do not pass through the central hole do not reach the sensor.
When we interpose a diaphragm, we are not altering the image that is formed in the sensor (*), we are merely building it from a smaller number of rays (photons), that is, the same image is formed but with fewer photons, the more we close the hollow of the diaphragm.
Closing or opening the diaphragm does affect the range of the scene that appears in focus; that is, it influences the depth of field, but for what we are trying to explain here, that effect does not matter to us.
To look at it another way: light ray paths that do not reach the sensor for whatever reason will not contribute to image formation. None of those photons will get the sensor and therefore do not count or affect exposure.
Now imagine that you have two lenses with different focal lengths. Also, imagine that both have the diaphragm open to precisely the same diameter. Therefore, the same intensity of light passes through the two diaphragms.
Will the same light intensity reach the sensor in both cases?
As the focal length increases, if we keep the hole’s diameter, the cone of light that reaches the sensor becomes narrower, and therefore the light intensity will be less.
When comparing the amount of light that two lenses will deliver to the sensor, it is not enough to know the hole’s diameter in its diaphragm; we also have to consider the focal length.
To take this effect into account and ‘normalize’ the diaphragm opening, a straightforward formula is used:
N (f-number) = F (focal length) / D (hole diameter)
With the f-number to describe the aperture, we will have a more precise idea of the amount of light that reaches the sensor.
The scale of numbers f corresponds to the light steps’ scale (so each step corresponds to letting double or half the light pass).
We take as reference f-number 1 (N = 1)
To get an additional light path (twice the amount of light), you have to double the area, which is equivalent to multiplying the diameter by the root of 2, as we saw above.
But the number f is inversely proportional to the hole’s diameter; therefore, the more extensive the diaphragm’s aperture, the smaller the number f.
That is, small f-numbers indicate a large aperture, while large f-numbers indicate a small gap.
The aperture scale, already normalized with the number f, would look like this (1.41 is approximately the root of 2, each step that we close the diaphragm corresponds to multiplying by the root of 2):
1.4 (1 x 1.41)
2 (1.4 x 1.41)
2.8 (2 x 1.41)
4 (2.8 x 1.41)
Effective f-number / entrance pupil / T-number
In a real objective, made up of a combination of lenses and groups of lenses, the f-number is not determined by the physical diameter of the diaphragm aperture but by the entrance pupil’s diameter.
The entrance pupil image of the diaphragm’s physical aperture, seen from the front of the objective, through the lenses between that position and the physical diaphragm.
To put it simply: the light in the scene does not ‘see’ the actual aperture of the diaphragm; it sees an image of that aperture.
The effective f-number or effective aperture, which is the one that indicates a specific objective, is calculated taking the aperture diameter of the entrance pupil:
N (f-number) = F (focal length) / D (entrance pupil diameter)
For practical purposes, it does not matter because the f-number gives us, in any case, the reference on the amount of light that this objective is capable of collecting.
Therefore 99.9% of the time, if not always, we will talk about diaphragm aperture or f-number, and not effective f-number or entrance pupil.
It should also be borne in mind that the f-number only considers the geometric part (effective area). It does not think that all physical lenses have losses due to absorption and unwanted internal reflections (impurities, reflections between lenses and groups, etc.)
From the point of view of the f-number, two lenses with the same aperture, for example, f / 4, let precisely the same amount of light pass through.
But in reality, the amount of light will depend on each of these objectives’ optical transmittance.
The T number is used to measure more accurately, considering both the effective aperture (f) and the optical transmittance. Two lenses with the same T-number do allow the same amount of light to pass through (at least with a minimal difference)
In photography, the difference that may exist between two objectives with the same aperture is not essential. In any case, they are minimal differences (we are talking about differences of 1/4 of a step or less), and they could be easily corrected from the camera itself or in editing.
In the video, it is more important because, in a sequence created from different shots (with other lenses), you can notice these differences in exposure (remember that in the video, the shutter speed is usually a fixed parameter that depends on the number of frames per second ).
In film and video productions, specialized lenses are often used for video, which uses the T number instead of the f-number.
How does the f-number appear, and how is it generally written?
The f-number has no units; the aperture is usually indicated following this format: f / N
For example: f / 2.8, f / 5.6, f / 22
This notation refers to the fact that if we solve for the diameter in the formula for the number f, it would be:
D = F / N
It is the notation that I like the most because when the number is dividing, it becomes more intuitive to think that the larger that number is, the smaller the gap will be (D)
Another notation that is used a lot is as a 1: N ratio (it usually appears in the silkscreen of the objectives to indicate the maximum aperture of the same)
For example: 1: 2.8, 1: 5.6, 1:22
You can also find it as F2.8, F5.6 …
Working with steps of light
The light paths create a relative scale, not an absolute one.
We always talk about going up to N steps of light or going down N steps of light; we still refer to variations in the amount of light, not the absolute amount.
In an analog film camera, there were only two parameters to regulate the amount of light that the frame collected:
- The aperture of the diaphragm
- Exposure time or shutter speed
Each film had its sensitivity once a roll of film was placed, and that parameter could not be modified.
In digital cameras, the sensitivity can be configured as one more parameter:
- Sensitivity (ISO) does not influence the amount of light the sensor receives, but it does affect the apparent exposure of the final image.
Playing with these three parameters, we can get the exposure we need. Furthermore, each parameter has its effect on the final result of the image.
Light steps in the diaphragm
To raise a light step, we will have to lower an f-number in the diaphragm.
For example, if we are at f / 2.8 and we want to double the amount of light that the sensor receives, we would have to open at f / 2
Effect on Image: Depth of Field
Light steps on the shutter
This scale is much more straightforward and more intuitive:
If we want to double the light, we will have to keep the shutter open twice as long.
For example, if we have the shutter in 1 second, we would have to pass it to 2 seconds to raise a light path.
The only thing to keep in mind is that in cameras, for shutter times less than 1 second, only the denominator of the fraction is usually indicated:
Four ″Two ″One ″
2 (1/2 seconds)
4 (1/4 seconds)
Light steps in ISO sensitivity
It is also effortless and intuitive:
To get double the light, we have to double the ISO value.
For example, if we are at ISO 100 and we want to raise a light step, we would have to go to ISO 200
Light steps and exposure value (EV)
Historically (when the sensitivity of the photographic film fixed the ISO), the concept of EV (Exposure Value ) was defined to relate the aperture (f-number) with the shutter time to achieve an individual exposure.
The idea is that different combinations of shutter speed and aperture can achieve an absolute exposure value.
Logically, the formula was not used for each ISO (that is, for each type of film) charts with exposure values (EV charts) were made that related the shutter speed to the aperture for different typical lighting situations.
Bear in mind that the cameras did not have a built-in exposure meter. Using photographic film, you have to try to get the correct exposure right the first time (the trial and error option as in digital cameras is not valid).
For example, a typical scene in broad daylight would be EV = 14 or EV = 15. The photographer could then use the chart to see what combinations of aperture and speed achieved that exposure:
EV = 14 = log2 (8 * 8 / (1/250)): Aperture f / 8 | Speed 1/250
EV = 14 = log2 (11 * 11 / (1/125)): Aperture f / 11 | Speed 1/125
Really from a reference (for example, f / 8 | 1/250), we would adjust aperture and speed to compensate and achieve that same exposure. For example, if we open the diaphragm one step to f / 5.6, we will have to pay by lowering the exposure time to 1/500:
EV = 14 = log2 (5.6 * 5.6 / (1/500))
To give another example, a scene of a snowy landscape in broad daylight would correspond to EV = 16Or for example, in night photography of the sky (stars, Milky Way, etc.), the exposure value would be EV = -8; that is, we would need large apertures (small f-number) long exposure times.
Nowadays, all cameras incorporate an exposure meter, and also in digital photography, we can play with trial and error and compensate the exposure when necessary.
These exposure charts and the EV absolute value scale are no longer commonly used.
However, as each step of the exposure value ( EV ) corresponds to a light step, that notation is widely used as a synonym for light action or f-stop.
What is measured with light steps?
In photography and video, the scale of light steps is used to compare or reference characteristics.
We have already seen that light steps adjust the necessary parameters:
- Shutter speed
What other parameters are measured in light steps:
- The dynamic range (of a camera, a sensor, a medium …) is measured in light steps.
For example, they can tell us that such a sensor has a dynamic range of 12 EV (12 steps)
- The effectiveness of an image stabilizer
That is, how many steps can we lower the shutter speed with that stabilizer compared to not using any to obtain an image without jitter. They can tell us, for example, this objective achieves stabilization of 4 EV (4 steps)