Crop factors and aperture
In my previous post I reviewed the Micro Four Thirds System and why isn’t worth it if you want to achieve the full frame look, but it is easier to understand with some graphics.
So, the Micro Four Thirds System has a crop factor of 2x and if you want to have the same look of a 50mm ƒ/1.8 you’ll need to divide that focal length and aperture, so, a 25mm ƒ/0.9 lense. Let’s see how theoretically that looks:
As you can see, you get the very same theoretical lens size but the lens is closer to the sensor, in other words, almost the same front element size as a full frame lens, and that is why it gather the same light as a full frame lense, but the option most manufacturers provide are a ƒ/1.4 or ƒ/2.8 which would look like an ƒ/2.8 or ƒ/5.6
Now, lets compare the same aperture values between focal lengths and even sensors size, again, this is only theory based on the math to mesure lenses in a simplistic way:
You can see that the same focus angle appears between different focal length while using the same aperture, so, if you want to have the same result of a full frame camera in a 2x crop camera, you’ll need to have half the focal length but the very same front element size of the desired lens to keep the same aperture, and that is why those lenses are as expensive as full frame lenses.