A new group for this: Ipernity Academy.
As a newbie, I'm always searching for great scenes, monuments, expressions and sometimes I study how to compose it.
I gave little attention to light, but photography, as its etymology says, its about "writing, painting with light".
But what are the elements influencing our possibility to paint with light?
They are all encompassed by the term exposure.
I will try to explain something about exposure in the simplest way.
Camera sensors (or films, if you use an analogue camera) are sort of light measurement instruments.
When we press the button, the shutter is open and light photons strike the single pixels of the sensor. Each pixel of the sensors, broadly speaking, "counts" the photons striking it: more photons implies more light, more light implies a lighter scene. (This is a great simplification: what is really counted are different light wavelenghts, corresponding to the different hues).
If we have a given scene (and, consequently, a given light), how can we control the way light strikes our sensor?
Some parameters come to mind:
- how much we open the shutter, called aperture;
- how long we open the shutter, called exposure time;
- how sensible is our sensor, called ISO sensitivity;
- the "whiteness" of the scene, controlled by white balance;
- the quality of our lenses;
- the length of our objective, called focal;
- the presence of filters, changing the wavelenghts that can strike the sensor.
Not all these parameters can be controlled in a point and shoot camera; usually you can change the focal, the ISO sensitivity, the white balance and use exposure compensation and focus/exposure lock.
Some guidelines for P&S users:
- the camera software decides the combination of how much and how long to open the shutter, i.e. aperture and time, to have a good exposition;
- this is made evaluating the light of the scene you see in the viewer or in the LCD;
- this is based on an average lighting and can be totally wrong:
- if you're taking a shot of a scene with a light source, the presence of the light source give to the scene an high average; the same is happening if you have an high reflective surface as snow or sand; in this case the automatic metering fails and you have un underexposed scene: you must give a positive compensation, a + in the EV scale
- in the opposite situation (a large black object, large shadows), you'll obtain an overexposed scene: you must choose a negative compensation, a - in the EV scale
- if you have a camera with focus/exposure lock, you can half press the button pointing to a scene without the object too black or too white and then turn to the scene you want to take and completely press
- white balance has the same function of colored filters; it is used to account for the different composition of light in different situation; I think you observed that the light of a tungsten lamp is more yellow than the light of sun and the light of a fluorescent lamp is more white; in the old times of film cameras, fileters where used to correct the different color of light (called a dominant); now the same function is given by white balance; if you can shot in RAW format, this isn't a problem: you can change the white balance after downloading the shot form the camera; if you don't have a RAW option (as the majority of P&S don't have), you have to choose the white balance before shooting; the automatic white balance (AWB) setting is dangerous (as all the automatic settings do, it subtracts to you the control of what you want to realize); try to make second nature switching from daylight to the other settings when necessary;
- ISO sensitivity is a standard measure for the ability of films/sensors to detect light; the higher the ISO sensitivity, the lighter the light you need to expose the shot; what's the price? with high sensitivity, you capture lots of noise and your shot become more grainy; so: low light --> high ISO --> more grain; more light --> low ISO --> less grain (but you can use it for artistic effects);
- focal length: if you use a tele-lens, the more you zoom, less light you capture, longer will be the exposure or more opened will be the shutter
- when you use a (not neutral) filter, less light strikes the sensor
In one of the next articles, I will complete this exposure analysis, talking about the parameters you can control in a (D)SLR when you shot in manual mode, aperture priority, shutter speed priority and depth of focus (DOF)