Diag = SQRT(Power([image X dimension],2)+Power([image Y dimension],2))
Then calculate the diagonal of the cropped image, using the X and Y pixel dimensions of the crop. Divide the original number by the smaller number to get the crop factor.
The effective focal length you used was 400 * crop factor.
Calculate the diagonal of the uncropped image:
Diag = SQRT(Power([image X dimension],2)+Power([image Y dimension],2))
Then calculate the diagonal of the cropped image, using the X and Y pixel dimensions of the crop. Divide the original number by the smaller number to get the crop factor.
The effective focal length you used was 400 * crop factor.