The basis of photography is the light and how the photographer masters and plays with it. Depending on the result and artistic effects you want to achieve, you will have to expose your image very quickly (to fix an action) or more longly (to show movement and blur).

There are 3 main tools to play with exposure: aperture, shutter speed and sensitivity (ISO). The combination of those 3 measures results in the image exposition i.e. dark or clear (under/over-exposure), fixed or blurred …  Using them also has some side-effects, which are used as creative tools.

Aperture is the opening of the lens, letting more or less light enter into the captor. It is represented by a number like f/2.8; the lesser the number is, the more opened is the lens. This explains why lenses opening at f/1.4 are very expansive, as they allow to shoot images in very difficult lighting conditions. The creative side-effect of opening the lens aperture is a shallow depth of field – the net focus zone will be very shallow at f/1.4 (which is an excellent creative tool for portraits where the face is net while the background is blurred). In the opposite, shooting at f/22 will let very few light in and generate a very large depth of field – the image will be net from your feet to the infinite (this effect is looked for landscape photography, where a tripod is often needed).

Shutter speed is the speed at which the shutter of the camera gear opens and closes to let some light enter into the captor. It is represented by a ratio of time in second like 1/250 (for 1/250th of second) or 1″ (for a 1 second exposure). Shooting an image at 1/2000 will fix a movement (good for sport photography), while shooting a fountain at 1″ will smooth the flow of water. Shooting hand-held below 1/25 is dangerous as a movement of the photographer will result in a blurry image. A tripod is then necessary to capture a movement of light, of water …

Sensitivity (ISO speed) is measured by ISO number (100, 200, 400 …) and determines how sensitive the sensor will capture light. In difficult light conditions, the photographer will have to set a higher ISO number to better exposure his images. At 100 ISO, the image quality is perfect; at ISO 3200, the image has digital grain, which can be used for creativity.

For each combination of 2 of the above measures, the third one will determine the exposition – this is how operate the modes A (for aperture) and S (for shutter speed). For instance, in mode A, the photographer will set his ISO number and his aperture – the camera will then give the shutter speed for the “normal” exposure. In mode S, the photographer sets the ISO and shutter speed measures, while the camera gives the aperture for the normal exposure. To get full control of the exposure, the photographer has to select the M (manual) mode, where he can trigger the 3 measures at his wishes.

I will add some example images soon to give more visual explanations. I hope this helps … Cheers !