For a long time now I have struggled to understand what photographers are on about when they talk about increasing or decreasing the number of stops. The Steve Sint book finally explains it in a way that I sort of understand. So this post is for the following reasons
1. To check if I can explain it in a way that I can remember, and will be a useful post for me in future
2. For you to tell me if I have got it wrong or
3. If you are in the same boat as me and struggling to understand this could be useful, or it might not. Depends if I have got it right or not :-/
But before I start explaining it I think it is useful to see where all the confusion starts. If I understand correctly the old cameras only had limited flexibility and when you changed the aperture or the film or the shutter speed you could only change each aspect by one stop. Modern cameras now allow photographers more flexibility and allow 1/2 stop changes and 1/3 stop changes. So while this gives extra flexibility and fine tuning, for the newbie it adds to the confusion.
So the best place to start would be show what a whole stop each for each of the three items that control light. So for each of the element below as you go from one number to the next is an increase or decrease of one stop.
Aperture:
f1.4 – f2 – f2.8 – f4 – f5.6 – f8 – f11 – f16 – f22
Shutter Speed:
1/4000 – 1/2000 – 1/1000 – 1/500 – 1/250 – 1/125 – 1/60 – 1/30 – 1/15
basically you just half or double the speed to achieve a change of one stop
ISO:
50 – 100 – 200 – 400 – 800 – 1600 – 3200 – 6400
as with shutter speed you just need to half or double the ISO to achieve a change of one stop.
So the only thing you need to memorize is the aperture which doesn’t seem to follow any logic that I can see. But I do know it is linked to mathematics, so we won’t go there!!! 
Right so now we know the increments how can we use it. I think the description in Steve Sint’s book Digital Portrait Photography is perfect. I have provided a shorten version of Steve Sint’s description, for the full description I do recommend buying Steve Sint’s book.
Situation:
You are taking a portrait photo with a camera that has the following specs
- Aperture range f1.4 – f16
- ISO range 200 – 1600
- Shutter Speed range 1 sec to 1/1000 sec
Camera ISO is set to 800
Light meter reading gives you f/8 and 1/250
You decide f8 is too narrow an aperture and want a larger aperture to increase the bokeh affect and through the background out of focus. So you choose f2
If we look at the scale given above and count the decrements from f8 to f2 it will give us 4 stops, which means you will need to gain 4 stops from else where.
The easiest way would be to increase the shutter speed by 4 stops (using the scale above) gives us a shutter speed of 1/4000. Perfect, but you realise that the camera only has a shutter speed range of 1/1000 which is 2 stops, leaving you two stops short.
You can get the extra two stops by changing the ISO down by two stops, by using the above scale gives us an ISO of 200.
So the final settings for the camera would be
ISO:200
Shutter Speed: 1/1000
Aperture: f2
This will give the same exposure as (Providing the light source is the same)
ISO:800
Shutter Speed: 1/250
Aperture: f8
However the picture will give a different mood.
This information is particularly useful when doing flash photography, because the flash can sometimes be restricted to 1/500 as the fastest speed. Ultimately there is only one correct exposure but by varying any of the three elements you can achieve different effects, and gives you greater control over the final image.
Now my problem is how do I remember whether to go up or down the scale for each element. I think the way to do this is by thinking of the light sensitivity as you change each variable. So the light sensitivity for each element is as shown
Aperture: the greater the aperture the more light that passes through the lens and thus increases the sensitivity.
Shutter Speed: the slower the shutter speed the more light that hits the film or sensor and thus increases the sensitivity
ISO: the higher the ISO the more sensitive the pixels become, thus increases the sensitivity
So if you increase the sensitivity in one area you need to decrease the sensitivity in another area. So in the above example we increased the sensitivity by opening up the aperture and decreased the sensitivity by increasing the shutter speed and lowering the ISO.
In one of my previous posts, I mentioned the See-Saw principle given to me by Mark Cleghorn when I went on his training course. He stated you should keep one element fixed and vary the other two. This seems to be a practical way of working. Think about the image you want to achieve, if you are after the option then fix the shutter speed, if you are after bokeh or lack of bokeh then fix the aperture. How to choose the ISO I do not know, except that the lower the ISO the better the quality, however if you are a sport photographer, then you know you will need a higher ISO, or if you are working in low light.
Once you understand that, you can talk about light in terms of stops.
If you increase the sensitivity by:
1 Stop – you get 2 times more light
2 stop – you get 4 times more light
3 stop – you get 8 times more light
4 stop – you get 16 times more light
5 stop – you get 32 times more light
and so on.
We have talked about changing the settings on your camera and how that affects the number of stops, but that is not the only way of changing the number of stops. Changing the light source will also increase or decrease the light source and can be referred to in the number of stops.
I think Mark Cleghorn mentioned in his training that moving the light source one meter away from the subject is roughly a one stop decrease. (Don’t quote me on this)
So if you take a picture and the image looks overexposed, you can now quickly use the principles of stops to make changes.
Hope this helps you, it has certainly helped me and I hope I do have the right end of the stick.
