F/Stops Explained
Written by: Here’s a quick (or not so quick) post about f/stops. This isn’t a post about apertures, although your f/stop relates to your aperture.
The f/stop is the focal length (f) of the lens divided (/) by the Stop of the aperture. This gives you the diameter of the aperture (which you don’t really need to know to understand f/stops)
What are Stops?
They are ratios that half/double the hole of the aperture. Wow, that sounds confusing! Technically it can be confusing, but don’t worry you don’t need to know the technical end to take good pictures. I’ll try to explain it in a simpler way.
Lets start with a sequence of numbers you may be more familiar with.
1 1.4 2 2.8 4 5.6 8 11 16 22 32
These are your f/stop numbers. So maybe you would shoot at f/2.8 or f/16. When you change from one stop to the next, you are either halving or doubling the size of the aperture (in square mm) which also means you’re either halving or doubling the amount of light passing through to the sensor/film.
When setting your aperture you may see other numbers like f/3.5, f/6.3, or f/7.1. These numbers fall in between the full stops listed above. Since the numbers above represent doubling/halving the amount of like, these numbers represent a 1/3 differences. So f/6.3 is a 1/3 less light than the full stop of f/5.6. F/7.1 lets 1/3 more light than the full stop of f/8.
NOTE: Sometimes when you hear the word “stop(s)” being used, they may just be talking about increasing/decreasing the amount of light but these ratios (in general). “I want a stop more light,” or “I want two less stops.” They are just talking about the number of times they are halving/doubling the amount of light.
OK, you NEED to understand this part
This part isn’t as intuitive as it should be. If I were to ask you which f/stop lets in more light, f/2.8 or f/11, which would you choose? Most people would probably say f/11 will let in more light than f/2.8 cause it is a higher number, but that would be wrong.
The higher the number, the smaller the opening and the less light will pass through. Again, to keep it simple but to explain, these numbers are a ratio and are related to the diameter of the aperture (in mm). The higher the number, the more you are dividing by which means your final number is less. That means less light.
Soooo… When you move from f/4 to f/5.6 you are now letting in half the amount of light. When you move from f/11 to f/8 you are doubling the amount of light passing through the aperture. From f/5.6 to f/2.8, you are doubling, then doubling again the amount of light passing through the aperture.
What f/stop is your lens?
Lenses are categorized by their maximum aperture. That is how wide the aperture opens. And as you know by now, that is the lowest number it can open too. Here’s two examples:
I have a 70-200mm f/2.8 lens and a 28-135 f/3.5-5.6 lens. The first set of numbers are the focal length of the lens. Both of these lenses are zoom lenses. The second numbers are the f/stop numbers. On the first lens, the maximum aperture is f/2.8. That means that if my focal length is anywhere between 70mm or 200mm, my aperture can open to f/2.8. On the second lens it’s a bit different. It shows a range of f/3.5 to f/5.6. This means that when I am focused at 28mm I can open my aperture to f/3.5, but when I zoom in to 135mm the most I can open the aperture is to f/5.6.
When you are talking about zoom lenses, it’s common to see a range listed for the aperture. This allows for lenses to be lighter and cheaper. The cost and weight of a lens goes up significantly as the aperture widens. They require more glass elements, larger glass elements, larger barrels, and cost more to manufacture.
What is a “Fast” lens?
Lens speed refers to the maximum aperture a lens can open to. The more you can open the aperture, the more light can pass through. If more light can pass through your aperture, then you can use a faster shutter to get the same exposure. This relationship is explained more in a post I did about the photographic triangle which talks about the relationship between the aperture, shutter speed, and the ISO (sensor/film sensitivity).
What does “Stop Down” mean?
Stopping down just means you’re decreasing the amount of light passing through the aperture by half. So you would stop down from f/4 to f/5.6, or from f/11 to f/16. The reverse would be opening up and doubling the amount of light. Now you go in the opposite direction from f/5.6 to f/4 or from f/16 or f/11.
Conclusion
Understanding f/stops aren’t hard once you get a bit of practice. Just remember that the lower the number, the bigger the opening and the higher the number, the smaller the opening. You also should know that each time you move from one stop to the next you double/half the amount of light passing through the aperture. The technically aspect may be interesting to read once, but isn’t really necessary when taking pictures.
