How open is your lens? Small f-stop = big depth of field and vice versa.
The bigger the hole the more light gets through. This is confusing because the aperture is described in terms of something called an “f-stop” and the smaller the f-stop number the larger the aperture’s opening is, e.g. f1.4 is a big hole and f16 a small one. Don’t be confused by this. It happens because the f-stop number is derived by taking the focal length and dividing it by the actual physical aperture diameter (measured in mm). Because the aperture is the denominator of this fraction (number on the bottom), the larger it gets, the smaller the f-stop number.
Aperture also affects depth of field. The technical reason for this is that it directly affects the “entrance pupil”, a concept we won’t get into here. Just remember that the more open your aperture, the more shallow your depth of field and the more light is hitting your sensor.
The f-stop scale was established as an easy way to quantify the amount of light coming into the camera. Each step on the scale represents a doubling or halving of the light. It’s logarithmic. This means opening your lens from f4 to f2.8 allow twice as much light to hit the sensor. Opening from f2.8 to f2 allows twice as much light again, or 4 times the initial amount of light initially available at f4. Knowing this, you’ll better understand the cost difference between a f4 and an f2 lens. This scale is important to memorize if you want to make manual aperture, shutter or ISO adjustments and maintain an even exposure. It may seem daunting at first, but here’s the thing: notice how, moving from left to right, every other f-number is doubling? If you simply memorize “1” and “1.4”, you can figure out the rest of the numbers in the pattern.