f/8 refers to the size of the aperture. It is typically expressed as a number known as “f-number” or “f-stop”, with the letter “f” appearing before the number, like f/8.

Most likely, you have noticed this on your camera before. On your LCD screen or viewfinder, your aperture is indicated by numbers like f/2, f/3.5, f/8, and so on. Simply put, the smaller the f-number, the bigger the aperture and vice versa.

While looking to attain a 600mm focal length with the newer DSLR EOS cameras, I bought a 2x teleconvertor (TC in short on many posts), only to find that it couldn't fit the older Canon lens that came with the EOS40D that I had bought cheaply off Carousel. This started a prolonged search which culminated in having to locate an autofocus lens which had a minimum aperture of f/4 (which is a relatively 'bright' lens, meaning it has an aperture that is on the bigger side), which could still focus automatically once I attached a 2x teleconvertor onto it (attaching a 2X TC reduces the aperture by 2 stops to f/8; attaching a 1.4X TC would reduce it by just one stop to f/5.6). The ability to do autofocus at such a small aperture size is unfortunately limited to the higher end cameras, which ran contrary to my tight budget.

If you are a Canon user and facing the same problem, you can get more details on the models at the URL below:

https://shuttermuse.com/canon-cameras-autofocus-extenders-f8-aperture/