Question

You use your 8× binoculars to focus on a yellow-rumped warbler (length 16 cm ) in a tree 20 m away from you. What angle (in degrees) does the image of the warbler subtend on your retina?

Answer 1

When we do not use binoculars, the warble subtends an angle 0.16 / 20 radians at the retina.

Now, with 8x binoculars, the angle is 8x larger,

means the resulting angle = 8*(0.16 / 20) = 0.064 radians.

                                          = 0.064 * (360 / 2*pi) = 3.67 degrees.