Zoom lens autofocus speed?

Lightsped · Jun 25, 2016

I have a 70-200mm f2.8 and a 200-500mm f5.6. In identical conditions at identical locations it seems as if the 70-200mm (at 200mm) acquires targets faster than the 200-500mm (at 200mm). Same body, same settings.

So is it the lens that influences autofocus speed or just the body?

PaulWog · Jun 25, 2016

Both. Your camera has an autofocus system. Your lenses also affect autofocus (how quickly you acquire focus, accuracy, consistency, speed).

It's a little alarming that you would purchase such expensive gear and not know this though.

480sparky · Jun 25, 2016

The 70-200 is an f2.8 lens. The 200-500 is f/5.6. That's a 2-stop difference, or 4x. So the 70-200 has 4 times the light to work with.

KmH · Jun 26, 2016

AF is done with the lens as wide open as it can open, which is why Ken noted the different max aperture of the 2 lenses being compared.

If the aperture is set to a smaller than wide open lens aperture the lens does not stop down until the shutter button is fully pressed.

Lightsped · Jun 26, 2016

So if I set the 70-200mm at 200mm and f5.6 and kept all camera settings equal would both lenses autofocus at the same speed? Or would the 70-200mm still feel quicker than the 200-500mm even though both lenses are set at 200mm at f5.6?

480sparky · Jun 26, 2016

Lightsped said:
So if I set the 70-200mm at 200mm and f5.6 and kept all camera settings equal would both lenses autofocus at the same speed? Or would the 70-200mm still feel quicker than the 200-500mm even though both lenses are set at 200mm at f5.6?

What aperture you set the lens to use when you take the photo isn't the issue. The issue is one lens allows 4 times as much light as the other to focus with.

You can set both lenses to shoot at f/22. But neither lens will actually BE at f/22 until the mirror raises up and the shutter opens. When you mount the 70-200 on the camera and look through the viewfinder, you are looking through a lens that physically f/2.8. Replace it with the 200-500 and you are looking through a lens that is physically f/5.6. In short, a hole ¼th the size of the 70-200.