otolock

How Aperture Affects Auto-Focus Speed

I’ve always heard lenses being categorized as either fast or slow, with the contributing factor being maximum aperture which is how much the lens can open. The more light the lens can let in, the better.

I always associated the relationship between aperture and speed with shutter speed. The more light the lens lets in, the greater the shutter speed, which in low-light situations, means you have a greater opportunity to take photos without motion blur.

I’d never realized how aperture contributed to auto-focus speed until I got my first slow lens, the RF 100-400 f/5.6-8. Since I started taking pictures of birds, the thing I’ve struggled with the most has been getting them in focus. Before the RF 100-400, my main lens had a constant f/4 throughout the focal length, and I’ve always had f/1.4, f/1.8 or f/2.8 lenses before that. Auto-focus had never been an issue before and I couldn’t figure out what I’d been doing wrong.

That was until I was watching a bird photographer on YouTube talk about gear, and he defined what a slow lens really meant. Slow auto-focus. After that, it all started to make sense. When I was photographing within dense forests, auto-focus was a struggle, so much so that I’d started doing manual focus. But when I had great lighting, I had zero issues with auto-focus.

So for anyone getting into bird photography after years of landscape and portrait photography like me, you’re going to struggle with auto-focus, and its not you, and its not your camera. Its your new slow telephoto lens.