Aphragm, shutter speed and ISO are three of the parameters that a good photographer must take into account to take a perfect picture
Aphragms, shutter speed and ISO represent the so-called "exposure triangle". The aperture determines the amount of light reaching the sensor at the moment of shooting, the time is the unit of measurement that determines its duration, while the ISO indicates the sensitivity of the sensor to light: the higher it is, the greater the possibility of taking good photos even in low light conditions.
Exposure is the most important element for the success of your shots. Understanding how it works and how to set it is the first step towards getting the shots that meet your expectations. Exposure is based, therefore, on these three elements - aperture, shutter speed (also called exposure time) and ISO sensitivity - and they work together, if set correctly, to determine the right amount of light that must reach the digital sensor. A faded photo, or one that is too dark, is an image taken with incorrect exposure that is difficult to correct in post-production with a photo editing program.
Manual or automatic exposure?
Digital cameras, in most cases, offer two ways of adjusting exposure correctly: automatic mode, which means the camera does everything itself, or manually adjusting the settings. If you're just starting out in the world of photography, you might opt for automatic adjustment, but as you become more familiar with your camera, there will come a day when you'll want to be in total control of your shots. And it all starts with exposure and the correct setting of the three parameters that regulate it. Let's analyze them in detail.
Diaphragm and light
The diaphragm, depending on its opening, determines the amount of light that reaches the sensor in the period of time that the shutter remains open. It consists of a system of blades that reduces or increases the passage of light depending on the photographer's settings. The apertures use the unit f/stop which ranges from the smallest number that corresponds to the largest aperture, to the largest number that indicates the smallest aperture. Each increment of f/stop corresponds to 1 stop, and each increment equals twice as much light. Be careful because closing the aperture too much produces diffraction phenomena: it is better to stay below f/16. If you have set the diaphragm to f/2.8, to give a practical example, it will be very open - so the sensor will be hit by a lot of light - while a setting of f/11 will let in much less light. In short, low numbers open the aperture, while high numbers close it.
Diaphragm and Depth of Field
The aperture, or rather, its degree of openness, affects the depth of field. Some examples? A very open diaphragm - for example f/1.4 or f/2.0 - is ideal when you want to emphasize a subject by putting it in the foreground compared to the background, as in a classic portrait. A closed aperture - e.g. f/8.0 or f/11 - is preferable when you want to enlarge the area of focus as in a landscape photo. To summarize, the more the aperture is open, the more light reaches the sensor and at the same time the depth of field decreases. A closer aperture, on the other hand, will let in less light but offer greater depth of field.
Shutter speeds. A voi la scelta!
Un’altro modo per variare la quantità di luce in ingresso nella nostra fotocamera è di utilizzare i tempi scatto (o di esposizione). L’otturatore è un meccanismo posizionato davanti al sensore che si apre per una data quantità di tempo di esposizione impostata, per poi chiudersi e non far passare più luce. Un tempo di scatto veloce consente di "congelare" l’azione ed è consigliato per i soggetti in movimento, mentre un tempo di scatto lento offre la possibilità di aumentare, invece, il senso di movimento con risultati, spesso, molto suggestivi. Vediamo nel dettaglio gli effetti prodotti dai vari tempi di esposizione.
- Tempi corti – Sono da utilizzare per cogliere l’attimo in scene con movimento. Non occorre andare a un Gran Premio di Formula Uno e catturare nitidamente l’immagine di una monoposto che sfreccia a 300km all’ora. Una scena in movimento possono essere dei corridori o ciclisti oppure, più semplicemente delle onde che si infrangono sugli scogli. In questi casi sono consigliabili tempi corti che vanno dal 1/500 fino al 1/4000 di secondo. Per una Ferrari che, si spera, corra per vincere, un’impostazione di 1/1000 dovrebbe bloccarne il movimento – si spera solo in foto – mentre per dei bambini che giocano, saltano e si divertono potrebbero bastare anche da 1/320 a 1/500 di secondo.
- Tempi medi – Sono probabilmente i tempi che userete con più frequenza e che vanno, di solito, da 1/30 fino a 1/250 di secondo. Sono l’ideale per scattare il ritratto di una persona in posa, un panorama, il vostro cane che schiaccia un pisolino. Il risultato è un’immagine nitida senza effetto di mosso.
- Tempi lunghi – Se sognate foto a lunga esposizione dovete organizzarvi. Dovete posizionare la fotocamere su una superficie stabile o, ancora meglio, investire in un treppiedi per tenere immobile la fotocamera. I tempi lunghi, partono approssimativamente da 1/15 di secondo fino ad arrivare a 30 secondi o all’infinito. But the effects you manage to create will surprise you every time.
ISO sensitivity: last resort
ISO sensitivity is the sensitivity of the sensor to light. This is its classic definition, but to make you understand its purpose we'll use an example. Imagine that you find yourself in low light conditions and you have decided to immortalize a person without risking blurring, also because you don't have a support with you. You have already set the diaphragm to the maximum aperture - f/2.8 for example - and you cannot decrease the shutter speed any further (1/60s). What else can you do? The answer is to use the ISO sensitivity settings. If you are taking a picture on a nice sunny day, simply set the ISO to 100. If, on the other hand, you are in a low light condition and you do not change the ISO, the picture will be underexposed, i.e. pictures with bright areas where details are missing. You must, therefore, use higher and higher sensitivity values to impress the sensor enough to properly expose the subject. Setting ISO 200 or higher is the solution. ISO sensitivity is, however, a double-edged sword that allows us to get more light, but at the expense of quality. It is not uncommon that, along with the brightness, some sort of noise and, specifically, luminance and chrominance noise, also increases. All you have to do is experiment!