Photographic techniques
During a recent photo trip to Ireland, our accompanying photographer asked a basic question that we struggled to answer: What do the values on the aperture rings (or displayed on the camera screen) mean?
While the aperture does depend on the diameter of the diaphragm, this concept isn’t so simple to explain.
The higher the value, the more the diaphragm is closed and the less light enters.
However, the sequence of numbers (Also called f-number or also known as the focal ratio, f-ratio, or f-stop) 1, 1.4, 2, 2.8, 5.6, etc., is not equal to the diameter of the lens minus the “radius” of the diaphragm’s closing angle.
This would therefore vary for each lens.
But the values are the same for all lenses!
In reality, what we call aperture is the relative aperture of a lens and actually refers to the ratio between the focal length and the diameter of its front element (N=f/D).
When we change the aperture N, we vary, as we have seen:
• the regulation of the sensor’s illumination.
• the modification of the image’s depth of field.
• the limitation of the lens’s optical aberrations.
Therefore, consistent values have been calculated for aperture indices, so that they can be used regardless of the lens. Furthermore, by convention, it has been established to use a series of indices where the illumination is multiplied by 2 when moving from one aperture value to the next.
Thanks to complex calculations, a geometric sequence with a common ratio of “square root of 2” has been found, which gives the values of :
Opening index values (F-number)
| Index(i) | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | … |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Value of Ni | 1 | 1,41 | 2 | 2,83 | 4 | 5,66 | 8 | 11,31 | 16 | 22,63 | 32 | 45,25 | 64 | 90,51 | 128 | … |
| Relative Aperture N | 1 | 1,4 | 2 | 2,8 | 4 | 5,6 | 8 | 11 | 16 | 22 | 32 | 45 | 64 | 90 | 128 | … |
Each value is therefore a multiple of the square root of 2, which is roughly 1.4!
The values engraved on the lenses are approximations, but perfectly applicable in everyday use. They correspond to the click stops on lenses that have them!
For those mathematically inclined and those who want to learn more, you can follow the Wikipedia link below: http://en.wikipedia.org/wiki/F-number