You must have heard of the term “stop” or “stop of light”.  Lets understand what is this and how it affects exposure. we will also see how stop is used to express exposure values.

What is It?
“stop” can be considered as unit of light entering into the camera. In simple terms if we say 1 bulb gives 1 stop of light so 2 bulbs will be 2 stop of light

Stop of Light

so 2 bulbs is 1 stop more than 1 bulb.

How to Calculate it
The focal length of a lens is the distance between the optical center of the lens and the camera sensor. The longer the focal length, the more it “magnifies” the subject.

f-stops are a measure of the aperture of a lens. In other words, f-stops tell us how wide lens opens. Its the ratio of focal length to apparent lens aperture. The smaller the number, the wider the effective aperture, and the more light will go through the lens. Hence f1.0 is a wide aperture, but f8.0 is a narrow aperture. You can read more on Aperture here.

If we have to calculate the amount of light entering in the lens, we should consider the Area of aperture as amount of light will be proportional to the area. Here is the formula to calculate the area:

Area Formula

Here
f  :  Focal length
N  : F-Number

Assuming the Focal length remains same for our comparison. Lets say 50 mm. Here is how the area of aperture will be for different values of F-Number

Area of aperture

Notice that increasing the Aperture (F-Number) from 2.8 to 2 increases the area by double which mean double the light and hence we can say that f/2 is 1 stop more than f/2.8 or in other words f/2.8 is one stop less than f/2. Now since the amount of light exposed to sensor is also dependent on other factors such as ISO, Shutter speed so term “stop” is used in their context as well.

Lets say we change our shutter speed from 1/1000 sec to 1/500 sec thus increasing the shutter time by twice and hence double the light will be exposed to sensor. We can therefore say that 1/500 is 1 stop more than 1/1000 assuming all other settings are same.

Similary, ISO is sensitiveness of camera sensor and is expressed as standard values of 100, 200, 400, 800, 1600 etc. As the ISO number is increased sensitivity of sensor increases and thus less light is optimum for exposure. So we can say that ISO 200 is 1 stop more than ISO 100 and so on…

Here are few more examples to express what we have learnt so far:

f/8 is 1 stop less than f/5.6
ISO 800 is 1 stop more than ISO 1600
1/125 is 2 stop more than 1/500

In Camera Representation
Here is how most of the cameras show builtin meter for showing exposure value

Camera Meter

Center one is optimum exposure
On the left (-) end less exposure values are shown
On the right (+) end more exposure values are shown

The scale is shown with “stop” as a unit. See that on each side 3 units of stop are shown with small lines showing partial stop values. So if you adjust your camera settings like ISO, aperture, shutter speed etc. and the “pointer” is at right side at the “1″ so you can say that you are exposing 1 stop more than optimim exposure.

Stop down & Stop up
These terms are used in photographic communications often. Stop down means reducing the stop value and hence less light and hence under-exposing whereas Stope up means increasing the stop value and hence light and over-exposing.

If I say that “I need to stop down my aperture by 1 stop from f/4″ . It means…  I have to change the aperture value from f/4 to f/5.6 as it will decrease the light by 1 stop.

I hope you understood by now and can relate the stop of light when used in photographic discussions or in your daily use.

Hope it helps! In case you like to share your views on this please leave your comments below…