What is exposure? The exposure triangle
In short
- Exposure is the total amount of light the sensor records.
- Three settings control it: aperture, shutter speed and ISO.
- Each one also changes how the photo looks, not just how bright it is.
- A “stop” means double or half the light. Trade a stop in one setting for a stop in another and the brightness stays the same.
Light in, picture out
A camera is a dark box with a hole in it. When you press the shutter button, light passes through the lens and lands on the sensor (or film) for a short time. How much light it collects decides whether the picture looks too dark, too bright, or right. That amount is the exposure.
Too little light gives an underexposed photo: dark, muddy, with lost detail in the shadows. Too much light gives an overexposed photo: washed out, with white areas that hold no detail at all.
The three controls
Aperture is the size of the opening in the lens. A wider opening lets in more light, like a wider window.
Shutter speed is how long the opening stays uncovered. A longer time collects more light, like leaving a bucket out in the rain longer.
ISO is how strongly the camera amplifies the signal it receives. Higher ISO makes a dim image brighter, but adds grain (noise).
These three are often drawn as a triangle, because changing one usually means adjusting another to keep the same brightness.
Every setting has a side effect
This is why exposure is more than a brightness knob. Aperture also controls depth of field: how much of the scene is sharp front to back. Shutter speed decides whether motion freezes or blurs. ISO decides how clean or grainy the image is.
So the real question is never only “is it bright enough?” but “which look do I want, and which setting can I afford to change?”
Stops: the common currency
Photographers measure light in stops. One stop more means twice the light; one stop less means half.
Shutter: 1/250 s to 1/125 s is one stop more light. Aperture: f/4 to f/2.8 is one stop more. ISO: 200 to 400 is one stop brighter.
Because all three use the same unit, you can trade them. If you open the aperture by one stop and make the shutter one stop faster, the overall exposure does not change, but the photo will have less depth of field and less motion blur.
Try it: Equivalent exposure
Change one setting and see what the others must be to keep the same brightness.