In its simplest form, exposure is a measure of the amount of light which reaches the sensor in your camera.  Too much light and your image will be overexposed.  Too little light and your image will be underexposed.

Three camera adjustments affect the exposure of your image.  These are:

Lens Aperture, or ƒ/stop - indicates the size of your lens opening

Shutter Speed - is the length of time your shutter is open

ISO - indicates the sensitivity of your sensor to light

The adjustment of one, or all of these parameters, controls the amount of light falling on your camera’s sensor.

In its fullest form, exposure is much more.  Not a blanket of light.  It’s the interaction between light and subject and composition.  Can vary, and is much more interesting to the eye, if different across the photo.  Umbria mountain with and without clouds…

ƒ/stop is a dimensionless parameter which relates lens focal length to the diameter of the lens opening, or aperture.  For example, let’s say you have a 24mm, ƒ/2.8 prime lens.  This means the lens’ aperture diameter is 24 mm / 2.8, or 8.6 mm (0.34 in.).  If you have a 70 mm – 200 mm zoom lens and is ƒ/2.8 throughout the full range, the maximum lens’ aperture diameter would be 200 mm / 2.8, or 71.4 mm (2.81 in.).  This example helps explain why …

ƒ/stop

Ratio of lens focal Length to diameter of lens.

Dimensionless.

f/1, f/1.4, f/2, f/2.8, …

Gotta talk about the doubling or halving of the aperture opening area.

Now, let’s distinguish between “ƒ/stop” and just plain “stop”.  As we said above, the term ƒ/stop is associated with the lens and indicates how much light is allowed to pass through the lens.

A “stop” is used to indicate the doubling or halving of light falling on the sensor.  If we want to double the amount of light being detected by your sensor, we could 1) double the area of your lens aperture opening, or 2) we could double the amount of time your shutter is open, or 3) we could double the sensitivity of your sensor.  This would be for example, changing ƒ/stop from ƒ/8 to ƒ/5.6, or changing shutter speed from 1/250 s to 1/125 s, or changing ISO from 200 to 400.  Each of these three examples doubles the amount of light falling on the sensor.

These three parameters function in an additive manner, so in the previous example, if “and” was used in place of “or”, the net result would have been an eight-fold increase in the amount of light.  Mathematically, this is double • double • double, or 23, or 8.

An interesting question would be, what would happen if we change our ƒ/stop from ƒ/8 to ƒ/5.6 (doubling the amount of light), AND change shutter speed from 1/60 s to 1/125 s (halving the amount of light)?  The answer would be, “no change” in the amount of light falling on the sensor.  This is shown below with other similar combinations of ƒ/stop and shutter speed.

Each of the above combinations results in an identical amount of light falling on the sensor.  So, which exposure setting should you use?  The answer is another question, what depth of field do you want?

Is f/stop written f/stop or f-stop or fstop? The lens manufacturers properly write f/8. The term f-stop has become very popular on-line, we see it often, but we also still see f/stop. I learned to write f/stop, because we also write f/8, to be remindful of the division defining it:

f/stop number = focal length / aperture diameter

f/8 is an aperture diameter, literally = focal length / 8.

Why do all lenses expose equally if all are set to the same f/stop? For two lenses at the same f/8, the lens with 3x longer focal length has an aperture diameter 3x larger. Tricky, but the 3x focal length magnifies the subject 3x, and so it sees 1/3x width and 1/3x height, which is 1/9 area, which only reflects 1/9 the light the wider lens sees. But the longer 3x lens also has aperture 3x larger, which is 9x area, and so now admits 9x more light, which before was 1/9 as much, from a 1/9 area field... so the 9x times 1/9 result is the same exposure in both f/8 lenses. Another argument is the Inverse Square Law over the 3x longer focal length is 1/9 the light, when the image reproduction reaches the sensor plane (which is just repeating the first explanation again). This is why the f/stop system is used. It's good stuff.