Tutorial - Standard diaphragm apertures.
Photographers seem to have a funny way of counting: they put 5.6 after 4 and go straight from 8 to 11. These numbers characterize as you will have understood the standard diaphragm openings. This scale is the same in silver and digital photography.
What are these apertures ?
Cameras (SLRs anyway) are equipped with a diaphragm. That is to say a device made up of articulated slats which make it possible to adjust the size of the orifice while maintaining as circular a shape as possible. The diaphragm thus makes it possible to adjust the quantity of light which enters by the lens.
Apertures are indicated by F/4, F/5.6, etc. The larger the number, the smaller the orifice.
We can also speak in Stops or in EV, which is exactly the same: 1 EV = 1 stop = 1 diaphragm.
 F/4 aperture |
 F/8 aperture |
 F/16 aperture |
Opening up one aperture (going from 5.6 to 4, for example) corresponds to doubling the quantity of light which reaches the sensor.
How to find or remember standard openings ?
Here's a trick: we start from 1 and double each time :
| 1 | 2 | 4 | 8 | 16 |
Then we insert values on the same principle starting from 1.4 :
| 1,4 | 2,8 | 5,6 | 11,2 | 22,4 |
To make it simpler, from 11 we neglect the decimals. Here is the series of standard openings:
| 1 | 1,4 | 2 | 2,8 | 4 | 5,6 | 8 | 11 | 16 | 22 |
The mathematicians will have noticed that there is a ratio of 
between each standard opening, i.e. 1.414.
Example : 4 x 1.414 = 5.656 i.e. 5.6 after rounding.
We could extend the series but few objectives allow us to go beyond or below these values. On the other hand, we have access to intermediate openings, generally by 1/3 of a diaphragm. For a given lens and SLR, we can therefore have the following aperture settings, for example:
| 2.8 | 3.2 | 3.5 | 4 | 4.5 | 5 | 5.6 | 6.3 | 7.1 | 8 | 9 | 10 | 11 | 13 | 14 | 16 |
We divide the quantity of light by two by going from 8 to 11, but also by going from 9 to 13,
or from 10 to 14.
A bit of mental arithmetic.
I'm at F/5.6 and I want to overexpose the photo by 1 stop, i.e. open it by 1 stop. What aperture should I use?
Answer : F/4 (Tap or mouse over).
A little more complicated: I'm at F/5 and I want to underexpose by 1.3 EV. What aperture does this correspond to?
Answer : F/8
At ISO 100, I have a correct exposure at F/8. What aperture will I need for an identical exposure at ISO 200?
Answer : F/11
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