I need to take a photo for my University's ID card. Specific instructions say that face should fill 50-70% of the photo. How can I find out if I took the picture right?
Be sure your image has RGB color mode. This is useless for other modes.
Select the face area and fill it with pure white. Invert the selection and fill the rest of the image with black. Apply averaging filter, for ex. in Photoshop Filter > Blur > Average. Other programs have different place for it.
The brightness of the result shows the wanted percentage. It's 100% or 255 if everything was white. In scale 0...255 divide the result by 2,55 and there's your percentage.