In the photography it is important to know how high the intensity is on a point on the focal plane. The light intensity is dependent on the diameter of the objective, as we saw earlier, and on the focal length. If you double the focal length, the surface of the focal plane will fourfold. The same amount of light will spread over a fourfold surface, so the light intensity will be four times lower. Now you can see, if you double the diameter and the focal length of the objective, the light intensity will stay the same. The focal ratio: f/d, where f = focal length and d = diameter, is often used as a measure of light intensity. The same measure is used with cameras, to adjust the diaphragm. A low value means a high light intensity and vice versa. My telescope has a diameter of 80 mm and a focal length of 1200 mm, so it has a focal ratio of 1200 / 80 = 15. Often notated as F/15.
| f (mm) | d (mm) | f. ratio |
| 80 | 80 | 1 |
| 400 | 80 | 5 |
| 1200 | 80 | 15 |
| 4000 | 80 | 50 |
| 4000 | 160 | 25 |
| 4000 | 320 | 12.5 |