Explain aperture to me please?

What does it matter what the aperture of your lens is if you can change the aperture via your camera?

Why is the 50mm 1.4 so much more than the 50mm 1.8?
Yeah Im kinda new to all this so any help is appreciated..

The aperture is located only in the lens, the camera controls it electronically in the newer digital camera's. Aperture is one of four things used in combination with one another to control how much light reaches the sensor. The 4 things are as follows in no particular order;
1. ISO - This setting controls how sensitive the sensor is to light with lower numbers being less sensitive and upper numbers being more sensitive. So in bright daylight you would use lower number ISO settings Like 100-200, where as if you are shooting in a dark room and dont want to use flash, then you would want to set higher ISO settings like 400, 600, up to and in excess of 6400 on some cameras.

Shutter speed- the shutter is like a window shade that covers the sensor so no light gets to it, until you press the shutter release. Shutter speeds control how long the window shade stays open. Shutter speed also controls how you view objects that are in motion relative to the camera. A fast shutter speed will "freeze" motion, so if you shoot a spinning airplane propeller with a fast shutter speed it might look funny because it looks like it is standing still. and conversely a slow shutter speed is good to convey motion, I'm sure you have seen waterfall or stream photos where the water looks creamy and flowing...that is due to a slow shutter speed, leaving the shutter open for a longer time to capture the movement of the water.
Aperture- aperture is basically the size of window you are allowing the light to flow through, inaperture it is important to remember that the smaller numbers F1.4 are a larger window than the bigger numbers like F22 being a small window. Aperture controls how much depth of the picture is in crisp focus. (depth of focus) A large aperture opening will have a small area that is in focus and most of the foreground and background will be out of focus (callled Bokeh). A large aperture has the advantage of allowing more light to fall on the sensor in a shorter time so in low light situations, you can get faster shutter speeds allowing you to get better photos handheld without blur. Conversely if you are shooting a portrait of someone with a distant mountain in the background and you want the subject and the mountain to be in crisp focus then you will want to set a smaller aperture such as F22, but will get a better photograph if you are on a tripod because the shutter will have to stay open longer to allow the same amount of light to fall on the sensor.
4. added light- self explanitory . auxillary light can work with the other three settings to change ISO needed, Shutter speed needed or aperture needed for any particular exposure.

If you really want to get a handle on exposure I would reccomend reading as much as you can on the subject. Many folks on here reccomend Bryan Peterson's understanding exposure, and I know John Hedgecoe has a few good beginners books.

I assume you are wondering why the 50mm 1.4 lens is more expensive than the 50 mm 1.8. I can't say without a doubt but I would imagine build quality has a lot to do with it.

Try this book, Understanding Exposure by Bryan Peterson. It's a great book to explain aperture an other things.

Engineering high quality optics and keeping tolerances in order gets harder the closer you come to a 1:1 ratio between max aperture and focal length (which the 50mm f/1.2L very nearly is.)

Have a read of this site and this thread.

The aperture of the lens matters because your camera can't select and use something that the lens is not capable of.

If you need to use f/1.4 to get a correct exposure, there is no point in putting a f/1.8 lens on your camera, as the picture will be underexposed (dark).

Generally speaking, lenses with larger maximum apertures are more expensive to make.

Hard12find - actually, ISO speed does not control how much light reaches the sensor. It just dictates how much the sensor amplifies it.

xs5875,
Well like others said the camera can only offer the apertures your lenses are capable of, try to select an aperture on your 18-55 lens that it doesn't have

Aperture isn't the only difference between the 50mm f/1.8 and 50mm f/1.4
There is a difference in the blades that make the aperture which controls the shapes of out of focus areas (bokeh).
Here is a great review of both lenses which explains and shows the differences between the two lenses Canon EF 50mm – F1.4 vs F1.8 MK II <-- click to see

One of the reasons for the price difference is build quality:
1) the f1.4 has better autofocus (usm IIRC) whereas the f1.8 does not.
2) higher quality glass is used in the f1.4 (as well as more pieces)
3) the F1.8 has a plastic rear whereas the f1.4 has metal

Roy,
What I actually said was that ISO controls the sensors sensitivity to light, I was trying to keep it extremely simple as this person mentioned they were a beginner and obviously hadn't grasped aperture yet.

hard12find wrote in post #9045219
Roy,
What I actually said was that ISO controls the sensors sensitivity to light, I was trying to keep it extremely simple as this person mentioned they were a beginner and obviously hadn't grasped aperture yet.

I don't wish to be argumentative but you didn't say that "ISO controls the sensors sensitivity to light". What you said is that it "is one of four things used in combination with one another to control how much light reaches the sensor". This just isn't true.

Really trying hard to get into this Canon Forum but do not know how to get started on a post please please please will someone send me an idiots guide how to do this please. Ive been using computers for years and actually teach basic IT but for the life of me cannot get started on this.
Many many thanks in anticipation of answer
Daisychain

I don't understand daisychain - you've already done it haven't you? Or did you mean that you wanted to start a new thread?

Yey! Thats exactly what I mean thank you Roy can you help please?
Cheers
Daisychain

You have to go to the root of the section you want (ie Competiions, Small Flash and Studio Lighting etc). Near the top of the page, you will see a drop down list entitled Forum Tools. One of the options here is 'Start a New Thread'. Click on it and away you go!