I think the 20D has a 1.6 crop factor and for the forum archiving here is how that value is determined

The crop factor is the resultant value by comparing the sensors diagonal length with that of the know diagonal value of the 35mm film form factor.

35mm form factor is:

24mm high x 36mm length with a diagonal length of 43.3mm.

Height = 24mm x 24mm = 576

Length = 36mm x 36mm = 1296

576 + 1296 = 1872

Square root of 1872 is 43.2666153 or 43.3mm diagonal length

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D20 sensor is:

15mm high x 22.5mm length

Hight 15 x 15 = 225

Length 22.5 x 22.5 = 506.25

225 + 506.25 = 731.25

Square root of 731.25 = 27.04163456 or 27.04 diagonal length

Now compare both 35mm and D20's diagonal lengths to determine the crop factor by 43.3mm / 27.04 = 1.600976114 or 1.6

------------ Extended Fun facts ---------

Lets say you are taking pictures with your new Canon D20 using a Canon EF-S 10 - 22mm effectively that means the 35mm equivelant is 10 x 1.6 = 16mm to 22 x 1.6 = 35.2. So this lens is really a zoom lens that is 16 to 35 mm.

Since this is a zoom lens.. chances are that you probably zoomed in a bit or in between values. Ok, not to worry. The images usually have an associated EXIF data set available when you preview the images in the camera "PLAY". Thats where the image date and time, focal length, and other image specifications are located. Look for the focal length value. I think you have to use the 1.6 crop factor value to find the effective 35mm equivelant value. So an image taken at FL12mm really is 19.2mm (35mm).

/s/

Dave

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