The Exposure Triangle
The aperture defines, as we have seen before, how far open the opening in your lens is to let light pass through. The shutter speed defines how long the shutter curtain is open. The longer you open up the shutter curtain the more motion blur you will get and the harder it will become to handhold the camera without accidentally adding motion blur. The general rule is to keep the shutter speed at 1/(focal length) or faster when you are shooting handhold, and it should in almost all circumstances give you a chance to hold your camera steady while the shutter curtain is open. If you need a longer shutter speed, you should either use a tripod or a beanbag to keep your camera steady1. To photograph a fast moving object–say a racing car–tack sharp you would want a high shutter speed, but when you are photographing landscapes you can often go for a longer shutter speed as landscapes don’t have a tendency to move away.
Since a subject needs a certain amount of light to be exposed correctly onto your sensor or film, one could believe that all you need to take care of for a correct exposure are shutter speed and aperture, but there is one more important setting to take care of: the ISO.
In the analogue era, films were produced with different light sensibilities, the sensibility was marked on the package as an ISO and/or ASA value. In the digital era the sensibility of your sensor cannot be changed–you thankfully don’t have to replace your sensor every time you need a different sensibility–instead a signal enhancer is added, so that when you dial in an ISO of 400 for example the signal enhancer will make sure that the sensor records more of the available light than with an ISO 200. A higher ISO value will however not only enhance the signal that reaches your sensor, but it will also lead to more grain in your photo, so generally you want your ISO as low as possible.
The doubling of the aperture, shutter speed or ISO changes the exposure by what is called 1 stop. So a shutter speed of 1/250sec is one stop faster than a shutter speed of 1/125sec. If you changed the shutter speed from 1/200sec to 1/60sec you would get a two stop slower shutter speed. Changing the ISO from 100 to 800 means a change of three stops (ISO 100 -> ISO 200 is one stop, ISO 200 -> ISO 400 is a second stop and ISO 400 -> ISO 800 is the third stop).
So say you have a perfect exposed photo at F/8, 1/100sec with an ISO 100, but what you would like to do is to have less depth of field. To achieve less depth of field you would want to use a wider aperture, which allows more light to pass at a given time as we have seen in the first article of this series. If you wanted the same subject photographed with less depth of field and you concluded that F/2.8 would do the trick for you, how would that change the shutter speed and ISO? Remember the aperture-value table from the first article?
F/2.8 means adding 3 stops to the exposure, so now you would need to subtract three stops from either shutter speed, ISO or a combination of both. Since we are already on the–for most cameras–lowest possible ISO, in this case we would change the shutter speed. Subtracting three stops from the shutter speed means making it three times faster, which results in a shutter speed of 1/800sec (1/100sec -> 1/200sec equals 1 stop, 1/200sec -> 1/400sec equals the second stop and 1/400sec -> 1/800sec equals the third stop).
Let us collect our findings about changes of one stop for the shutter speed in a table.
|Starting shutter speed||Next slower shutter speed |
(1 stop slower)
And let us do the same for the ISO
|Starting ISO||Next higher ISO |
(1 stop difference)
|ISO 100||ISO 200|
|ISO 200||ISO 400|
|ISO 400||ISO 800|
|ISO 800||ISO 1600|
|ISO 1600||ISO 3200|
|ISO 3200||ISO 6400|
|ISO 6400||ISO 12800|
|ISO 12800||ISO 25600|
|ISO 25600||ISO 51200|
Now let us have a look at some photos that will make use of these changes. I have taken the photo of the pear on the fence always from the same distance, and all of the following photos are taken with the same focal length. So the changes in the depth of field are only related to the changes in the aperture. To make the examples a little easier to follow I have never changed the ISO in between photos, so the exposure is only achieved by changing the aperture and shutter speed.
As you can see the photo is tack sharp from front to back, and it has a high depth of field, caused by the narrow aperture of F/22. But despite this being a bright day, the shutter speed is at a slow 1/15sec–something you would most likely not be able to handhold. So what happens if we widened the aperture by one stop to F/16?
To get the same exposure we had to double the shutter speed. This still gives us a tack sharp photo from front to back, so let us see what happens if we changed the aperture to F/8 instead:
As you can see, with an aperture of F/8 we had to quadruple the shutter speed to get the same exposure, compared to the photo with the F/16 aperture. But we still have a rather high depth of field. What if we wanted a lower depth of field? Yes, we would open up the aperture even more, say to an F/4:
Since we opened the aperture with another two stops we have to quadruple the shutter speed again, so that we now have a shutter speed of 1/500sec. And as you can clearly see in this last photo, the depth of field is significantly shorter, leaving the background and parts of the foreground blurry while our subject–the pear–is still in focus.
So let us collect these findings–and I will add some ISO changes that we haven’t looked at in these example photos–in another table:
|Aperture||Shutter Speed||ISO||Focal length||Depth of field|
|F/22||1/125sec||ISO 800||55mm||high, grain might occur due to the ISO|
|F/1.4||1/4000sec||ISO 100||55mm||very low|
|F/1||1/8000sec||ISO 100||55mm||very low|
Since the correct exposure of a photo is reached by adjusting three values–the aperture, the shutter speed, and the ISO–it is commonly referred to as the exposure triangle.
Next time, on March 24th 2017, we will look into the dependencies of focal length, aperture and depth of field.