It is commonly suggested that when using a large format CCD, it may not be worth using a 3.3 field reducer as this creates a vignetted image. I.e instead of using the full ccd it creates a circular image in the middle of the ccd.  However while experimenting with my QHY8 camera and some meade field reducers, I disovered that there may still be an advantage to using a 3.3 field reducer with a large format CCD even though this can produce a vignette. This holds true especially if you are using a high mega pixel sensor as the cropped image would still be of a reasonable resolution. 

What I basically did was take a photo without a field reducer, then with a 6.3 and finally with the 3.3 reducer. I resized, aligned and overlayed these images within photoshop to give an idea of the field of view. For the heavilly vignetted 3.3 reduced image, I cropped the maximum square image I could get within the vignette circle. Even Cropped this gave a substantially wider field of view compaired to the 6.3 field reducer.

What I then did is simply create a image that shows the three fields overlayed as different colours to give an idea of the field of view proportions.

Important notes:

Even though the green 3.3 reduced image is cropped, it still offers a much wider field of view but the resolution of the image would be reduced. Still with high mp sensors this may not prove to be too much of an issue.

Due to bad weather I could not conduct this experiment with an actual astro image. I was actually taking photos of a power pole down thw street. This means that I am not yet able to determin whether the 3.3 field reducer would introduce too much coma to be of any use. I will update this when I get a chance to conduct the experiment with an actual astro image.