Amateur Photographer & What Digital Camera: What Camera

You have it mostly right.

One feature of image quality - noise - is governed by the size of the individual photosites - the smaller these are, the noisier the image will be. Beyond a certain point, this is forced by physics, there is little the manufacturers can do about it. A combination of a high pixel count and a small sensor inevitably means a certain amount of image noise, especially at higher ISO ratings.

If you want a high pixel count and relatively noiseless images at high ISO, the bigger the sensor, the better. This is the main way in which full frame scores over APS-C/DX cameras - with the same pixel count, you can go about twice the ISO speed for the same image noise.

Another advantage of full frame cameras when used with wide angle lenses is in the geometry of the lens. For reasons of compatibility, APS-C/DX cameras have the same mount-to-sensor dimension as full frame cameras of the same marque. This means that, since wide angle lenses need to have shorter focal lengths in order to cover the same angle of view, the retrofocus ratio has to increase, making the lens design more complex to design and manufacture, with a possible detriment to image quality.

However, for long focus work, the smaller sensor camera has a big advantage - a telephoto lens required to cover the same angle of view will be much smaller, lighter and less expensive when it only has to cover an APS-C/DX sensor.

Cropping a large sensor is certainly possible, but has its drawbacks. The crop factor being approximately 1.55, if an APS-C/DX size image is cropped from a full frame sensor, the number of pixels in the image will be reduced by a factor of 1.55^2 = 2.4 i.e. a 12 MP full frame camera will be effectively only 5 MP when used in this way. If that's good enough for you, then fine; and, bearing in mind that there is a tradeoff in poor light between a noisy 10 MP image and less noisy 5 MP one, sometimes it might even work better. But, by and large, the smaller sensor camera is a more sensible solution when the object is to make images of distant creatures like birds or wild animals.

I'm lucky enough to have both a 5D and a 40D; I love them both for their own good qualities. The 5D has undoubtedly better image quality when used at high ISO in poor light, but the 40D's integrated sensor cleaning makes it easier to use in many circumstances. The difference between 12 MP and 10 MP is neither here nor there. I'm sure you'd find either a useful upgrade from your 300D.

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