We’re clearing up another bit of confusion that comes up with lenses and crop factor!

Tony tFuntek was wondering why the actual aperture diameter of his 25mm f1.8 micro 4/3 lens is different than the aperture diameter of his 50mm f1.8 full frame lens.

Here’s Tony’s question:

Here is the question that I have not found an answer…I shoot with a full frame camera and I also use a M4/3’s camera when portability is an issue. Why is is the aperture physically larger on my full frame 50mm f1.8 than my 25mm F1.8 M4/3’s lens ? The same is true on my 45mm f1.8 M4/3’s lens. How can they both be f1.8 with 2 different size openings? It would make sense the M4/3’s lens is passing less light through. Your thoughts and thank you for all your videos. Tony

There’s two issues that I discuss in the video here that clear this up.

The first is where the f number comes from, and the second is false equivalencies being drawn between lenses that are inherently different.

Micro 4/3 cameras have a crop factor of 2, which means if you put a 25mm lens on a micro 4/3 camera, and put it right next to a full frame camera with a 50mm lens, and take the same picture, compositionally those pictures will be the same.

That leads to some people thinking that a 25mm micro 4/3 lens is the same as a 50mm full frame lens, which is not true. It is in fact a 25mm lens.

Once we realize that we can deal with where the f-number comes from, which involves a little math.

Watch the video to learn where f-numbers come from and why f1.8 is different on different lenses.