STOP!

So a word that pops up all the time in photography, but I have hardly ever seen explained in a simple way, is the word 'Stop'.  To put this word in context it is used in phrases like "it has 10 stops of dynamic range", or "it was 2 stops overexposed" etc., but what is a 'stop'?.  I will try to explain it as best I can in simple terms, because that is normally all my brain will cope with on a topic that is as complex as this, but this blog post will still be quite wordy I am afraid.  

To break the blog up slightly I am going to insert 6 random photos that I have never got around to putting out there before now.  These really have nothing to do with the topic, but hey this is a photographic blog after all so it needs photos.

A STOP

In simple terms a 'stop' is defined as the doubling, or halving, of the amount of light in a scene as seen by a camera.

So if you increase an exposure by 1 'stop' then you are doubling the amount of light falling onto the sensor / film in the camera, effectively doubling the brightness of the image.  If you decrease the exposure by 1 stop then you are halving the amount of light.

That is it; simple, eh?

But the complex part starts here; why do we measure this light increase / decrease in this abstract term called a 'stop'?  I will try to explain.

The term "stop" is used because when you want to increase / decrease the amount of light in a scene by a 'stop' there is more than one adjustment you can make to the camera to make it happen, and each of the adjustments uses different measurements / scales / terminology in those adjustments.

There are 3 things you can adjust on the camera that change the exposure of a shot at the time of shooting: ISO, aperture and shutter speed.  Each of these adjustments will alter the brightness of the photo ( as well as creating other desired / undesired effects ) so lets look at how a stop relates to these adjustments.

Just to emphasise the point of using a 'stop' as a measurement to cut across these other measurements, ISO is measured in numbers like 100, aperture is measured in f numbers ( e.g. f8 ) and shutter speed is measured in seconds or fractions of seconds.  Clearly these 3 measurement scales do not appear to relate to each other at all, but they definitely do through the use of 'stop'.

A big caveat before I continue: most cameras are set up with the adjustments set to 1 click on a dial / button adjusting 1 stop, but sometimes they are set to 1/2 stops or 1/3 stops instead.  Because I do not know how your camera is setup I will explain it with the assumption that it is set to 1 stop increments.

I will run through each of the adjustments and then explain how they can be made work together, which is main reason for using the measurement of a "stop".

ISO

I will start with the simplest adjustment to explain. I will not be dealing with the effects of adjusting the ISO other than the effect on the brightness of the photo.

ISO in simple terms is an adjustment to the sensitivity of the sensor / film in the camera.  In regards to a cameras sensor this is not actually what is occurring, but for the sake of this discussion 'adjusting the sensitivity' is good enough.

On most cameras ISO is measured as a number starting at 100 and increasing in doubles up to 12800 or 25600 or even higher ( so 100, 200, 400, 800 etc).  The key is that the number doubles with each adjustment, so that adjustment also equates to a 'stop'. ISO 100 is 1 stop darker ( i.e. 1/2 as bright ) than ISO 200. ISO 1600 is 1 stop brighter ( twice the brightness ) than ISO 800.  The doubling of the measurement number equals doubling the brightness.

And therefore ISO 800 is 2 stops brighter than ISO 200.  That is 4 times brighter.  Hang on, how did that happen? Each stop of adjustment doubles the brightness, so from ISO 200 to ISO 400 is double the brightness, and from ISO 400 to ISO 800 it is double as well. So the brightness at ISO 200, doubled and doubled again gives ISO 800, which is 4 times brighter.  The effects of a "stop" of adjustment are cumulative, and are easiest to understand if you look at the ISO numbers themselves: 800 is 4 x 200, so ISO 800 is 4 times brighter than ISO 200.

That is the easiest system of measurement on a camera, because it is the most modern system used of the 3 that I will cover..

Shutter Speed

Again I will only be dealing with the effects of altering shutter speed as they relate to the image brightness not to anything else.

The shutter speed on a camera is normally measured in seconds, or more likely fractions of seconds.  These numbers normally get adjusted in doubles as well, so if you are at a shutter speed of 1/2 second the next shutter speed offered by the camera is either 1 second ( twice as long as 1/2 second ) or 1/4 second ( half as long as 1/2 second ) depending on which way you turn the dial.  

So again the fact that the shutter speeds are measured in doubling units makes it directly comparable to stops.  1 stop increase in brightness will take your shutter speed from 1/2 second to 1 second.  1 stop decrease in brightness will go from 1/2 second to 1/4 second.

So again this makes sense; a 1 second shutter speed will logically let in twice as much light as a 1/2 second shutter speed, and this is 1 stop difference.  However once you get into the realm of trying to adjust by multiple stops then it is not as easy to calculate as ISO is:  1/60 second is 5 stops darker than 1/2 second ( 1/2 to 1/4 to 1/8 to 1/16 to 1/32 to 1/60 is 5 stops / doubles ).  I personally can only work this out by going through each doubling of the shutter speed in my head to reach the final total.

It is also worth mentioning that the numbers, on the surface, don't always seem to reflect the massive change in the brightness of the final image.  Changing from 1/100 second to 1/200 second will decrease the brightness by the same degree as changing from 15 minutes to 8 minutes.

Aperture

Right, here is where things get a bit confusing.  ISO ( sensor sensitivity ) and shutter speed ( length of the shot ) are reasonably simple to understand and their measuring systems make sense.  I don't know who came up with the idea for measuring aperture, but I think they could have made a better job of it.  The aperture measurements are probably the hardest thing for a beginner photographer to remember, let alone understand.

Aperture is the term used to explain the size of the opening in the lens which lets light into the camera to create the photo.  The size of the aperture is measured in f numbers ( f2.8, f8, f16 etc ), and these numbers do not form a simple linear sequence, or even go in the direction that you would expect.  For example f8 is not twice the size of opening as f4, in fact it is 4 times smaller.

Here is my understanding of the f numbering system: the f number is the ratio of the openings diameter vs the focal length of the lens.  

So an f2 aperture in a 100mm lens has a opening diameter of 50mm ( focal length / diameter of the opening = 100/50 = 2 ).  The same opening diameter on a 300mm lens ( 300/50 ) would be f6, and on a 50mm lens ( 50/50 ) would be f1.

On most lenses the aperture measurements are as follows ( I will start at a common aperture, but the numbers at each end of the scale will vary depending on the lens ) :  f2, f2.8, f4, f5.6, f8, f11, f16, f22.

So these numbers appear to have no relationship to each other, but each number in the above scale is 1 stop darker ( i.e. half as bright ) as the number before it.  The reason they don't seem to relate to the numbers either side of them is because the f numbers are listed in 1 stop increments, and as we have already established 1 stop is a halving / doubling of the amount of brightness ( in this case halving / doubling the area of the opening ) but the f measurement uses diameter instead of area not the area itself.

And the relationship between area and diameter is complicated because we are dealing with the area of circles ( the opening in the lens is generally circular ), and that uses the "π r²" formula. So...

Here is some maths to prove that the f numbers do equate to stops:

Our 100m f2 lens that we used above has a 50mm diameter opening, which using π r² ( i.e. the radius of the opening is 25mm ( half of the diameter ), squared is 625, multiplied by 3.1415 ) is 1963mm²

If we want to change the opening size in the lens to reduce it by 1 stop ( i.e. half the area ) then 1963mm² / 2 = 981mm².

If we then feed 981mm² back through the π r² formula ( but backwards this time so the formula is: square root of ( area / π ) ) then that gives the radius of the opening as 17.6mm or 35mm diameter.

Now if we take our 100mm lens and give it a 35mm diameter opening then we get ( 100 / 35 ) f2.8., which is the next number in the f2, f2.8, f4, f5.6, f8, f11, f16, f22 sequence.  So f2.8 has half the area of opening as f2.

The reason the numbers feel like they go the wrong way ( bigger number = smaller opening size ) is because the formula for working out the f number has the opening size at the bottom, so the bigger the opening diameter for a given focal length lens then smaller the f number ( e.g. 100mm focal length/50mm diameter = f2, but 100mm/75mm = f1.3 ).

After that explanation I think you will agree that there must be an easier way to measure the aperture than this, but it is what we are stuck with.  

If you are ever trying to remember the numbers that make up the sequence of 1 stop steps then here is how I do it; if you look at the sequence ( f2, f2.8, f4, f5.6, f8, f11, f16, f22 ) it is made up of 2 interlaced sets of numbers. One sequence is f2, f4, f8, f16 ( each number is double the last ) and the other sequence is f2.8, f5.6, f11, f22 ( which are also doubles ( with some slight rounding )).  So all I need to remember is the 2 measures of f8 and f11 ( each is 1 stop different ) as each number appears in different sequences to the other, and remember to halve each of those numbers alternatively starting at f11.  So f11 halved gives me f5.6, f8 halved is f4, f5.6 halved is f2.8, f 4 halved is f2, etc. giving me the 1 stop sequence from f11 to f2.

Something to notice about apertures is that there is a much smaller number of stops available ( a lens with a maximum aperture of f2 normally only has 7 stops of adjustment up to f22 ) than what is available by using shutter speed ( most cameras go from 1/4000 sec to 30 sec which is 17 stops ) but about the same as ISO ( normally 7 stops from ISO 100 to ISO 12800 ).

Also I will mention that some of the maximum apertures that common lenses can achieve are not in the sequence of whole stops ( e.g. f1.8, f3.5, f5 etc ).  These numbers are 1/3 stop values and sit between the whole stops.

Enough about apertures, because now my head hurts.  

What is the point?

So why do we need to know the 'stop' values for ISO, shutter speed (SS) and aperture (f)?  Basically because the reason we use the 'stop' measurement system is so we can relate the 3 other measurement systems together, and can balance any adjustments to one with an equal adjustment to another.

So hopefully it goes without saying that if you take a photograph with certain settings for ISO, SS and f and you change one of those settings then you will change the brightness of the next image.  But what if you want to change one of the settings but retain the same brightness?  Then that is where the 'stop' measurements come in.

Compare the different camera settings below for ISO, SS and f:

  1. ISO 200, 1 sec, f8
  2. ISO 400, 1/2 sec, f8
  3. ISO 200, 2 sec, f11
  4. ISO 200, 1/8 sec, f2.8

These 4 settings will all produce a photo with the same brightness.  If you look at the first line of settings ( 1 ) and compare to each of the following lines ( 2, 3 and 4 ) in turn you will see that I have changed 2 values in each of these lines.  The 2 changes offset each other to produce the same brightness.

In line 2 I have increased the ISO by 1 stop ( twice as sensitive ) and decreased the shutter speed by the same 1 stop value ( half the length of time )

In line 3 I have added 1 stop to the shutter speed and decreased the aperture opening by 1 stop.

And in line 4 I have increased the shutter speed by 3 stops ( 1 sec => 1/2 => 1/4 => 1/8 sec ) and opened up the aperture by 3 stops as well ( f8 => f5.6 => f4 => f2.8 ).

The photos taken by these 4 settings will look different ( especially the settings of line 4 ), but they will all have the same brightness, which is the point of using the 'stop' measurements.

When would you need to know this?

More often than you think.  Once you have an understanding of the relationship of ISO, shutter speed and aperture, and the halving / doubling of brightness with each stop of adjustment, then your ability to get the camera to produce the shots you want increases, even if you don't necessarily need to use the system to calculate the changes you need you will understand how much effect the adjustments you are making will have on the brightness of the final image.  Each stop of adjustment makes a massive difference to the brightness of an image.

And if you take nothing else from my long explanation of a "stop" then knowing how much you can control the brightness of the shot by adjusting either ISO, shutter speed, or aperture by just 1 click of the dial, then my job is done.

 

So in summary

  • A 'stop' is a doubling or halving of the amount of light hitting the sensor in you camera
  • doubling / halving the ISO value is 1 stop
  • doubling / halving the length of the shutter speed is 1 stop
  • changing from f8 to f5.6, or f8 to f11 is 1 stop.

 

Right, time to stop.  ( sorry )