Field of View = ((57.3 / scope focal length) * frame size)

FOV = ((57.3 / FL) * FS)

Where

• FOV = Field of View in degrees
• 57.3 = Conversion factor for radians to degrees
• FL = Scope Focal Length in mm
• FS = Frame Size in mm

Example: A Canon T2i (550D) has a sensor that is 22.3 x 14.9 mm. What is the size of the field of view on the long side when used with a 1,040mm focal length telescope?

FOV = ((57.3 / 1040) * 22.3)

FOV = (0.055) * 22.3)

FOV = 1.23 degrees

The long side of the frame has a field of view of 1.23 degrees.

Example: A Canon T2i (550D) has a sensor that is 22.3 x 14.9 mm. What is the size of the field of view on the short side when used with a 1,040mm focal length telescope?

FOV = ((57.3 / 1,040) * 14.9)

FOV = (0.055) * 14.9)

FOV = 0.82 degrees

The short side of the frame has a field of view of 0.82 degrees.

So the field of view on a Canon T2i (550D) with a 1,040mm focal-length telescope is 1.23 degrees by 0.82 degrees.

Note that this is a simplified formula that is accurate for focal lengths larger than about 50mm and becomes increasingly inaccurate for shorter focal lengths.