Aperture

Aperture is probably the first subject I would discuss with a beginner because that determines how much light you will let in " mechanically " [ before you start adjusting the other settings ] , and the depth of field you choose is often what most photographers worry about when taking pictures .
Aperture is an opening or hole in the lens that lets the light through . The size of the aperture you choose determines how much light you let in , and the side-effect , sometimes the primary objective though , is that it affects the depth of field .
They make it complicated by stating the aperture as a " ratio of the focal length of the lens blah blah blah ......" , nothing you have to know to get good pictures . But knowing that the smallest number is the biggest aperture and the biggest number is the smallest aperture is what's most important , along with knowing the effect they have on depth of field that is .
Here's a 50mm F1.8 lens at various apertures ......
When you are looking through the viewfinder you are always looking through at the widest setting . So with a 50mm F1.8 lens things look bright even at F22 , because the lens stays at F1.8 until you press the shutter release button , then the mirror lifts , the aperture closes to the setting you have selected , it takes the picture , and then goes back to 1.8 and everything still looks bright in the viewfinder . If your camera has a 'depth of field preview ' button then pressing that 'makes everything go dark ' because it closes to the aperture you have selected ... as your eyes adjust to the light you will see more of the scene 'in focus' , similar to what you would see in the final picture , which is why it is called the 'depth of field preview' button of course !


Imagine pressing the 'depth of field preview ' button at f22 - that's how much less light it is letting in ! But look how small that aperture is , now any light reaching the sensor leaves the aperture blades at a smaller angle , so when you are focused on something things in front of and behind the subject look more in focus because it takes longer for the out of focus circles to get bigger .....


Depth of field is best demonstrated with pictures -
F2.8 is a 'wide' aperture { 1/2.8 of the focal length technically } and is the widest on the Nikon 70-200 F2.8 VR lens - it lets the most light in , and has the shallowest depth of field .
I focused on the yellow peg 1/3 of the way into the scene for a reason . [ 1/3 of the way into the scene is a very approximate statement - it can get very technical but it is somewhere in front of halfway into the scene for general photography ] . Watch as the aperture closes how they all start coming into 'focus' , one in front and two behind . Watch the trees in the background as well .

Remember our tap and the glass of water from the 'exposure ' discussion ? When we open a tap really wide the water gushes out at all angles - it does the job faster but things look a bit messy ... but perhaps that is the way you want them to look ! It depends what effect you are after for each particular situation :)
Now one peg in front of the yellow one and two behind it are in focus which is why many people say you should focus approximately 1/3 of the way into the scene - it's the best distance to focus on at these distances to get as much of the scene as possible in focus when you reach the critical aperture , without having to close the aperture any further . Perhaps the bridal party lined up for a picture where you want them all in focus - you wouldn't focus on the closest one , or the middle one , but rather in front of the middle person , approximately 1/3 of the way into the ''subject'' being the line of 10 people - then you would still have to close your aperture down to make them all look in focus . Experts will tell you that there is a special formula to calculate the actual hyperfocal distance ..... update : 13/01/2010 someone has offered to write an article on hyperfocal distance for me to explain that aspect better and I will post it when it arrives because I don't know everything about that topic ;) .
but approximately 1/3 of the way into the scene is good enough for most of us who don't want to carry a calculator to get that extra inch of focus .
Now if we are talking about taps and water again when you have the water coming slowly out of a tap the angle that the water comes out is much narrower and smoother and it looks 'crisper' , much like shooting at smaller apertures ...... it just takes a bit longer but provides sharper results - again , depending on how you want the image to look or whether you can afford to be fussy about the time it takes .


Eventually all 3 in front of the yellow peg look in focus and the 6 behind look in focus . In reality they are not in focus , they're just 'less out of focus' than they were .

Also the trees in the background are starting to look more in focus . If I had focused on the first peg I would have had to go to a much smaller aperture to get the back one in focus as well but focusing part of the way into the scene [ the 'scene' being the row of pegs ] meant not needing to go any further than F13 . realistically with digital you are not supposed to go past F11 because diffraction starts interfering - like looking through squinted eyes , your eyelids [ the aperture blades ] start interfering with the picture but that's another one of those subjects that is nice to know about but not necessary for getting good pictures - as long as you know about it and stick to the guidelines as much as practically possible . [ perhaps someone could submit an article on 'diffraction' ]