What is f/stop… what does it mean..? why should i care..? i just want to take nice photos. You will often hear the photography in-crowd tell you to STOP up or STOP down.. increase by a third of a STOP or push two STOPS…! What are they all talking about……?

Right then no messing about : f-stop, f/stop or FSTOP is a term that has no definitive meaning, some say the ‘f’ stands for focal or functional, but i am not convinced, lets just be happy it is a ‘TERM’ used in photography for increasing or decreasing the magnitude of light entering your camera which then determines the exposure of your picture…. Stopping down by one FULL f/stop will HALVE the magnitude of light whereas Stopping up by one FULL f/stop will DOUBLE the magnitude of light.

thats it sorted….. !

Well, not really …that just tells you what it does but doesn’t really give you enough detail to fully understand… so if you want to know more your going to have to dig deep and read the next few bits….

Lets just chill and take one step at a time….. when you take a picture your aim is to get the exposure to your taste, by this i mean not too dark or not too bright… this is your exposure control.

To get your exposure how you want it, all you are doing is adjusting how much light is entering the camera and falling on your camera sensor… the three main functions to control exposure are the lens aperture, the speed of the shutter and the sensitivity of the camera sensor (ISO). For now lets not make it more confusing, so we shall ignore the shutter speed and ISO (separate blog for these)

Lets talk more about the aperture. This is the hole in the camera lens, which can be adjusted from large to small in a number of standard steps, you may have noticed numbers such as f1.0 , f1.4 , f2.0, f2.8, f4.0 etc on your camera lens. These numbers represent a RATIO… i am going to say it again because this is the key to understanding….. it is a RATIO… not a measurement… a RATIO … between the camera lens focal length and the size of the lens aperture (size of the hole, where the light passes through)

So if we set the lens to f1.0 what we are actually doing is opening the aperture (hole) in the lens to a diameter the same size as the lens focal length…. wow.. bet that hurt your head… lets just recap and look at that again.

i can already hear you shout ‘what does he mean by lens focal length” stay cool.. you have probably been to the camera club and seen Fred in the corner with a bag full of lens.. he will have a 28mm wide angle and 50mm prime portrait lens and probably a huge 400mm zoom plus a load more … well those numbers on the lens represent the lens focal length (there will be a separate blog for focal length, as it does have more to it). Let’s say you fit the 50mm lens, and set the aperture to f1.0. What you have done is open the aperture (hole in the lens) to 50mm diameter

f/stop = lens focal length / lens aperture diameter

f1.0 = 50mm / 50mm

it is going to get a bit harder, so maybe take a breather and have a cup of tea before going on….… just stay with me to the end and it will all come clear (i hope)..

Right then.. i guess if i want to half the magnitude of light coming into my camera i can just half the aperture diameter on my lens to 25mm…. which equates to f2.0

f/stop = lens focal length / lens aperture diameter
f2.0 = 50mm / 25mm

NO that wont work…!!!! OMG i hear you say… why not..!! …… do you really want to know? because this bit is really hard… it even involves mathematics…. dead hard sums..

You can just stop here and accept the fact that each time you increase or decrease your aperture by one full stop you are doubling or halving the magnitude of light entering your camera….!!!

Not satisfied..? you want to know more….

Right you asked for it…. here goes…

To adjust the magnitude of light passing through the aperture you need to adjust the AREA of the hole….. yep the AREA…

To do this we need to use a formula….. the square root of 2… don’t ask why, just accept it.
The square root of 2 is 1.4 (rounded up) and this magic number will half or double the area of a circle…

So if i want to half the aperture (hole) AREA i need to divide the diameter by 1.4..
Lets go back to our 50mm lens.

f/stop = lens focal length / lens aperture diameter

f1.0 = 50mm / 50mm

If i want to half the magnitude of light entering the camera i would need to divide the aperture diameter by 1.4, which equals 35.5 mm.

50 / 1.4 = 35.5mm (this is the new hole diameter, which will halve the area)

Therefore, if i change my lens aperture by ONE FULL STOP (Halving the magnitude of light) i would set it to f1.4

f1.4 = 50mm / 35.5

I think we have used the numbers 1.4 and 50 too much, which may be confusing, so i will fit a different lens… i will try the 28mm lens (focal length).. which i will set to f2.0… i now want to decrease the magnitude of light by one full stop..

f/stop = lens focal length / lens aperture diameter

f2.0 = 28mm / 14mm (aperture diameter)

I know the aperture (hole) size in the lens is 14mm.. so by dividing this by the magic 1.4 the new aperture size will be 10… putting this into our f/stop formula, the f/stop will equal f2.8. so by reducing the aperture size to 10mm this will halve the magnitude of light and equal one FULL stop.

f2.8 = 28mm / 10mm (aperture diameter)

Obviously your camera and lens are configured to work to these magic numbers, so you don’t have to think about it…. or do the mathematics.

If i continue to follow this formula i end up with the following f/stops which are Full Stops and will halve the magnitude of light each time…. the f numbers get bigger the hole size gets smaller… the smaller the hole the less light entering the camera..

f1.0 , f1.4, f2.0, f2.8, f4.0, f5.6, f8.0….. can you see the pattern..?

Every alternative number is doubled…

So this scale refers to FULL stops, but most modern digital cameras give you more flexibility and will allow you to adjust by a third of a F/STOP… so the scale will be:

1.0, 1.1, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.5, 2.8, 3.2, 3.5, 4.0, 4.5, 5.0, 5.6, 6.3, 7.1, 8.0….etc

You can do the maths for the third of a stop…. if a full stop doubles or halves the magnitude of light then a third stop will give you approx. 33% more or less light than the previous f/stop…

What did all that mean…? what do i need to remember…?

Well it did get heavy, but you can ignore MOST of the above, all you need to remember is that the term f/stop if referring to a ratio setting in your camera lens which allows you to control the magnitude of light entering your camera in a uniform standard way regardless of which lens you have fitted.

KEY points:

f/stop is a RATIO between the camera focal length and aperture (hole) size

Changing your camera lens by one FULL STOP UP (example f2.8 to f2.0) will DOUBLE the magnitude of light entering your camera.

Changing your camera lens by one FULL STOP DOWN (example f2.0 to f2.8) will HALVE the magnitude of light entering your camera

The f/stop RATIO is a standard and applies to all camera lens