I don't know if this will be of any use to anyone here but I put this together for a friend who is an absolute beginner to this stuff but wants more from their photography results and is talking about learning more/ buying a camera with more functionality.
It's quite basic but might be of some use to someone here.
What is Aperture?
Inside a camera lens is a mechanism called a diaphragm, consisting of a set of curved blades. The blades form a rounded hole called the aperture. This is like the pupil in your eye, which expands or contracts when the light changes. So too can the aperture expand or contract at your command.
This is done to vary the amount of light that gets through the lens and onto your film/ sensor.
The other way of varying the amount of light that hits your film/ sensor is shutter speed - a 1/30th of a second exposure obviously lets in a lot more light than a 1/400th of a second exposure.
Together, the combination of shutter speed and aperture control your exposure.
Any given film type requires a certain amount of light to accurately record the scene so the actual aperture and shutter speed settings used vary hugely depending on the ambient or man-made light of the scene being photographed. Low ISO films need lots of light, fast ISO speed films need much less.
For a digital camera the ISO setting works the same way. A certain amount of light is needed for low ISO/ highest quality images; less is needed for high ISO/ lower quality images.
When using the camera in fully automatic mode the exposure needed is calculated by the on-board light meter and the camera selects an appropriate combination of aperture and shutter speed. You can also use it in manual or priority modes to select these yourself.
For example, let's say my camera suggests an aperture of f11 and a shutter speed of 1/125th of a second.
I can change it manually to half the aperture and double the shutter speed and the overall exposure will be the same - but there will be a difference in my finished photograph.
Why would I do this? To deliberately bring about those differences.
What you need to know in order to make aperture and shutter speed decisions
High shutter speeds will freeze motion - using these tends to mean you have to go for wider apertures (f2.8, f4) to let in more light.
Slower shutter speeds will allow you to capture motion - popular when photographing flowing water for example - you’d need a narrow aperture (f22 etc) to compensate for the slow shutter speed.
Wider apertures result in shallow depth of field - see below for DOF explanation - great when you want, say, an animal in sharp focus, but the grasses and bushes in front of and behind it out of focus.
Narrow apertures result in greater depth of field - where more of the picture from front to back is in focus.
Also worth knowing is that the higher the zoom the lower the depth of field too. Using a zoom also compresses distances between objects.
You have to weight up what you want and make the decisions accordingly. You will seldom be able to have both high shutter speed and narrow apertures (to freeze motion and have everything in focus from front to back) because that usually won't let enough light in to the film/ sensor.
Camera modes
Most modern SLRs/ SLR-like cameras have 4 modes - fully manual, fully automatic and then Aperture priority and Shutter priority. Aperture priority means you select an aperture and the camera works out, using the light meter, what shutter speed it needs to use in order to expose correctly. It will alert you when you’ve selected an aperture which it simply can’t use because there is just too much or too little light.
Shutter Priority means you select a speed and the camera works out the appropriate aperture to expose correctly, according to it's light meter.
Make sure to be aware of what shutter speed it selects if you use Aperture priority because too slow a speed cannot easily be hand held without camera shake.
What is DOF?
A lens can only bring objects at a single distance from the camera into critically sharp focus (at a time). But if you look at photographs, you can see a considerable area of the scene (from near to far) that appears sharp. Even though theoretically only one narrow plane is critically sharp, other parts of the scene in front of and behind that plane appear acceptably sharp to the human eye. Objects become less and less sharp the farther they are from the plane of critical focus. Eventually they become so out of focus that they no longer appear sharp at all. The area in which everything looks sharp is called the depth of field. Where this is only a small area infront of and behind the focal plane it is said that the depth of field is shallow. Where this area is larger it is said that the depth of field is deep/ great.
One can vastly change the image one captures by making a deliberate choice regarding depth of field.
For example, by selecting a wide aperture and focusing on an object in the foreground I know that the object itself, plus a narrow area infront of and behind it will be in focus but that everything else in the further background will be increasingly out of focus. Alternatively I could choose that same wide aperture but focus on something in the background. Then my foreground will not be in my focal plane and will be blurred but my background object will be sharp.
Or I can choose a narrow aperture to deliberately ensure that as much as possible of my scene, from foreground to background, is in focus.
If blurring or freezing motion is my motivator I'll make choices about the shutter speed instead and give less attention to aperture/ depth of field.
Many modern cameras now have shortcut predefined modes that offer a quick way of switching into these modes. An icon of a flower or a face is usually used to refer to wide aperture/ shallow depth of field settings. An icon of a person running refers to high shutter speed settings. An icon of a mountain often refers to a setting which provides a deep depth of field providing focus from foreground through to distant background.
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