The scale used to measure the size of the opening of the iris on a lens. Opening the iris wider lets in more light, and closing it down, smaller, lets in less light. F-stops can be a little confusing, because the larger the number, the smaller the opening of the iris, and conversely the smaller the number, the larger the opening. The typical f-stop scale is 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22. When the reading is between stops, this should be accounted for it setting the lens, however, it is much more clear, even if it sounds grammatically incorrect to the mathematically inclined, to say “One third above 5.6” rather than “5.8” because it is very hard to judge the distance in decimals between numbers like 5.6 and 8, whereas 1/3rd above 5.6 is perfectly clear.