Aperture and a Crop Camera

This is my first thread, so please forgive me if I have committed any heinous faux pas'.
I did a search but couldn't find an answer to my query, and I am not sure if this is the right forum for the question, but here goes anyway. Oh, and forgive me if it is a very stupid question.

I often read that most lenses perform better when stopped down, and I have also read that cameras with a crop sensor only use the sweet centre spot of the lens. I will ask this question specifically related to my 50mm f1.8. When fully open it must surely be using the entire lens, rather than just the 'sweet spot', otherwise there would be no benefit to stopping down. And on a full frame camera, if that uses more of the lens so to speak, won't that mean it is letting more light in and so to get the right exposure on a full frame camera using the same lens you would have to use a faster shutter speed.
Forgive me if I have confused anyone, I know what I mean but I may not have worded it properly.

I think you have confused yourself. A lens that can be mounted on a crop or FF camera will always project the same sized image circle, regardless of the sensor format. So, a crop camera like your 550d has a smaller sensor and doesnt use the entire image circle of the lens - just the center. A full frame camera, on the other hand, has a larger sensor and takes advantage of the entire image circle which may not have the same optical quality near the edges.

The size of the image circle does not change when you change your aperture.

Aha, right I understand now, so basically a crop camera will project an image bigger than the sensor?

I am still slightly confused as to the effect of stopping down on a crop as compared to a ff.

Wide Boy wrote in post #13614916
I often read that most lenses perform better when stopped down, and I have also read that cameras with a crop sensor only use the sweet centre spot of the lens. I will ask this question specifically related to my 50mm f1.8. When fully open it must surely be using the entire lens, rather than just the 'sweet spot', otherwise there would be no benefit to stopping down.

No. The effect of the aperture size is distributed across the entire frame, not just the edges.

The lens is always letting in the same amount of light no matter what size sensor is used. If you pick any area of the sensor, lets say a 1/4" by 1/4" area in the top left corner, the light hitting that area is going to be the same regardless of whether or not any of the light outside that square is being recorded.

Wide Boy wrote in post #13614976
Aha, right I understand now, so basically a crop camera will project an image bigger than the sensor?

Close, but not exactly. The camera does not project an image, at all; that is sthe job of the lens. A 'normal', EF lens is designed to project an image circle for a 35mm sensor/film; so, those lenses project an image circle that is a fair bit larger than an APS-C sized sensor (what is in the 'crop' bodies).

EF-S lenses are designed for use on APS-C sized sensors and actually project a smaller image circle, that matches up to the smaller sensor. Doing so allows the lenses to be smaller, lighter and cheaper (usually).

Wide Boy wrote in post #13614976
I am still slightly confused as to the effect of stopping down on a crop as compared to a ff.

There isn't, really, any difference in stopping down between the two. As mentioned earlier, aperture doesn't have anything at all to do with the size of the image circle, etc. The lens is still letting in the same amount of light on both bodies; so, your exposure would be the same, also.

Regarding the sweet spot of lenses and why stopping down 1-3 stops helps with IQ, this article might help explain some of your questions.

http://www.peachpit.co​m …es/article.aspx​?p=1795816

Thank you TeamSpeed, a very useful article.