In this third instalment of understanding your lens series, we will be concentrating on f-stops.
First, we need to clarify the word f-stop which a lot of photographers (including myself) refer to as aperture. Although f-stop and aperture are related, they are different. Aperture refers to the actual opening of the lens which is usually measured by the diameter of the opening. F-stop, on the other hand, is the ratio of the focal length and the diameter of the aperture of the lens. This is a very important distinction between the two and we will look into that in detail later on.
Every photographer should be familiar with the common f-stop numbers. They are as follows:
f22, f16, f11, f8, f5.6, f4, f2.8, f2, f1.4
Memorize those numbers because they are very important especially when you start using manual exposure mode. You should also know by now that the smaller the f-stop number, the bigger the lens opening. That is, f16 is a smaller opening compared to f8 on the same lens. The photo below shows the huge difference between f1.4 (left) and f16 (right).
(Photo taken from Wikipedia)
Notice that I emphasized on the same lens. Different lenses have different opening sizes. On a telephoto lens, f8 will have a larger opening compared to a normal lens at f2.8. Let’s see why …
Consider a 300mm telephoto lens and a 50mm normal lens. Recall that f-stop is the ratio of focal length and aperture diameter or mathematically:
f-stop = ( focal length / aperture diameter)
aperture diameter = (focal length / f-stop)
So for the 300mm lens at f8 the opening is:
300mm / 8 = 37.50mm
and for the 50mm at f2.8:
50mm / 2.8 = 17.85mm
As you can see in the above example, there is a huge difference in the diameter of the lens openings. You can actually see this for yourself if you have a zoom lens. Look through the front element of your zoom lens and then zoom in and out without changing the aperture. You will notice how the opening increases in size as you zoom to a longer focal length.
Understanding this very basic concept is important because f-stops control exposure. F-stops regulate the amount of light that hits the camera’s sensor. It is quite obvious that the larger the opening the more light comes in. The problem is that it is not enough that you know the size of your lens opening. Back to the 300mm vs 50mm example above, 37.50mm is obviously larger than 17.85mm but we also mentioned that f8 is smaller than f2.8 so what gives?!
Well there is another factor that affects the amount of light hitting the sensor and that is distance from the light source. The farther the light source, the lesser the amount of light and that’s why stars look much fainter than our sun. In photography, this distance is the focal length of your lens. The longer the focal length, the farther the rear end of the lens is from the sensor. Another thing that you should understand is that the intensity of light varies as the square of the distance. What this means is that given the same lens opening, light coming from a 50mm lens has four times the amount of light coming from a 100mm lens. Mathematically,
light intensity = (100mm / 50mm)2 = 4
It follows that for the 100mm lens to get the same amount of light as the 50mm lens, it needs to have a larger lens opening. The immediate question is, by how much larger of an opening? This is why knowing your lens opening is not enough to control light. You should also keep track of your focal length. This makes photography so much more complicated than it should be.
And so they “invented” the f-stop. Again, recall that f-stop is a ratio of the focal length and the lens opening diameter. It is very clever because it calculates the light intensity for you automatically no matter what your focal length and your lens opening are. So given this information let’s calculate the lens opening diameter for both 100mm and 50mm at the same f-stop, say, f8:
100mm / 8 = 12.5mm
50mm / 8 = 6.25mm
So double the focal length requires double the lens opening diameter for the same amount of light hitting the sensor. Imagine having to change your lens opening every time you zoom in and out. Instead, you just set your f-stop to, say, f5.6 and let the lens handle the opening according to your focal length of choice. And that is exactly what you see when you keep a constant f-stop and look through the front element as you zoom in and out. Easy!
How does f-stop relate to the amount of light hitting the sensor? Each stop of difference is double the amount of light. For example, going from f5.6 to f4 is twice the light intensity and going from f4 to f2.8 is also double the intensity. So going from f5.6 to f2.8 is four times the amount of light and so on.
Let’s summarize what we have discussed so far:
1. The amount of light hitting the sensor is affected by the lens opening (aperture).
2. The amount of light is also affected by the distance of the light source to the sensor (lens focal length).
3. A f-stop is the ratio of #2 and #1. This allows us to easily calculate light intensity because the lens automatically adjusts the aperture as focal length changes. We only have to worry about one parameter instead of two.
From this, it is easy to see why fast lenses, those with wide apertures such as f1.4, are much larger than slower lenses of f4. For the same focal length, the faster lens needs to have a wider opening diameter. This also explains why some zoom lenses have varying apertures and others have constant apertures. Lenses with varying apertures, say, f4 on their widest to f5.6 on maximum zoom, are cheaper because the aperture does not change much going from wide to telephoto and are therefore smaller in terms of diameter and only need smaller glasses. Constant aperture zooms are not only more expensive but also bigger and heavier because they have to open up much wider as the focal length increases. Wider, bigger and more glass. Finally, this also explains why m43 lenses are much smaller than their full frame counterparts. The smaller m43 sensors require shorter focal lengths to cover the entire sensor area and therefore have smaller lens opening diameters.
Before I end this post, let me address a very common misconception. A lot of photographers think that full frame sensors are better than m43 sensors at capturing light because they have larger surface areas. This is not true simply because a sensor without a lens in front of it is useless. Now with a lens in front, we know that an f-stop is the same for any sensor size. A full frame camera requires a longer focal length and therefore a lesser amount of light hits the sensor compared to a m43 camera that requires a shorter focal length because of the smaller sensor. A f-stop of f5.6 in a full frame camera allows exactly the same amount of light per unit area as f5.6 in a m43 camera.
I hope you learned something in this post. There will be more next time.