The F-Stop or Aperture

After chatting to a friend today he suggested it might be useful to describe some of the technical aspects of the camera and its settings.  So here is the first of a few blogs on what the different settings do.

In the studio my routine camera setting is usually on the Manual mode, F-Stop 11, ISO 200 and speed 1/125Sec. 

Manual and other modes allow different levels of control over the settings in the camera, which allows everyone from the happy snapper to the professional to set the camera according to their knowledge and skills.  As you get better you will experiment with the other settings and eventually become comfortable with manual.  In layman's terms you cannot go wrong in fully automatic (usually a green rectangle on the dial on the top of the camera body), but then it can be a total disaster on manual if you do not set the rest of the camera up correctly.  I will talk more about modes in a later blog.

The ISO number ranges from 100 up to 3200 or higher on some expensive cameras.  The number refers to film or sensor speed.  What it means is that the lower the number the less sensitive it is to light, the higher the number the more sensitive to light.  Again I will come back to ISO later on in more detail.

The 1/125sec is the shutter speed.  This can be varied from bulb setting which means you hold the shutter open to 1/8000sec.  A speed of 1/60sec is enough to take a photo of a person and avoid camera shake or blurring.  The shutter speed can be set low to allow lots of light in or high to freeze movement or where there is too much light.  Again I will go into more detail later on.

Now the main purpose of this blog the F-Stop or aperture setting, in this case F11.  The F-Stop number refers to the size of the hole that allows light to pass through the lens and onto the sensor.  Bizarrely, the smaller the number the bigger the hole and the more light that will hit the sensor. 

If you look at the lens you have it will have written on it the maximum size of the hole.  On my standard 17-70mm lens it has F2.8-4.5.  This means that when the lens is at 17mm the aperture will be 2.8 but as you zoom in the aperture will close up to 4.5.  This is to do with the characteristics of the lens and its manufacture.  The more expensive lenses have this maximum aperture throughout the zoom range.  The minimum aperture or smallest hole size varies depending upon the lens you have, the lens above has a minimum aperture of F36 when fully zoomed out to F22 when set to 17mm.

So what does this mean?  Well the three characteristics of ISO, Shutter speed and aperture are combined to obtain the correct exposure.  If you adjust the aperture or hole size then you must adjust one or both of the other two to obtain a correct exposure.

There are other effects to adjusting the aperture depending upon what it is you are trying to do.  A small aperture of say F22 will result in what is called a large depth of field - the amount of the picture in front and behind the subject which is in focus.  If you want to capture the foreground and background with your subject you will need to set a small aperture/hole (high number).

Alternatively if you set a large aperture of say F2.8 then only the subject will be in focus and you will have a small depth of field.  Now the added complication here is that the depth of field will also vary depending upon how close the camera is to the subject.

So, summarising, a small hold (F22) in general terms will let less light in and result in more of the picture being in focus.  A large hole (F2.8) in general terms will let more light in and result in less of the picture being in focus.  There are two main uses, letting light in when there is not much about and arranging the amount of the photograph that is in focus.

Just a little more detail and then I will finish off.  The F-stops are 1.0, 1.8, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32, 44 etc.  You will notice that they double every other time, each of these is called a full F-stop.  The F-stops have a linear relationship each one allows twice as much light in as the previous.  So F22 allows twice as much light in as F32.  To add to the complexity or really just to have a finer level of control, most cameras have 1/3 of a stop increments between full stops.  So the sequence between F22 and F32 is 22, 25, 29, 32.

The last line on this is to remember that it is a balance between the three attributes of Aperture, ISO and shutter speed.  It is a lot to think about and therefore it is best to look at the camera mode and use each one depending upon which of these attributes you wish to control, until you master each element.  The camera will always try and compensate or indicate when you have exceeded the limits, except of course in Manual!  You can still take an incorrectly exposed picture in all but fully automatic, if you do not heed the camera's warning (usually a blinking indicator that can be seen when looking through the viewfinder).

I will post some pictures to demonstrate what I have written, it will just take me a bit longer to get that sorted!

As promised a couple of photos to demonstrate the F-Stop and its effect.

This was taken at F22 (small hole), 5 second exposure at ISO 400.  Notice that almost everything is in focus.  The focal point or subject was the word Cochrane on the left.  This is referred to as a large depth of field.

This image was taken at F2.8 (large hole), 1/15 second at ISO 400.  Notice how the right hand side of the image is now out of focus.  The focal point was the same.  This is referred to as a small depth of field.

I adjusted the shutter speed when compensating for the aperture setting.  As I closed the aperture I lengthened the time that the sensor was exposed to the light.  The camera was quite close to the subject here, the further away you are the less the effect.  As you get further away the depth of field to the front and rear of the focal point gets greater.