Why can an f/2.8 lens focus faster than an f/5.6 lens even if both are set to f/5.6?
Asked 6/27/2019
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A photographer claimed that a Nikon 400mm f/2.8, when set to f/5.6, lets in about 4× as much light for autofocus as a Nikon 500mm f/5.6 set to f/5.6, leading to faster focus acquisition. If both lenses are set to f/5.6, I would expect exposure to be roughly the same aside from small transmission differences. Why would the wider-aperture lens provide so much more light for focusing, and is that what explains the autofocus speed difference?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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During focussing, the lens is left at full aperture. It's only when you take the picture that it closes down to the appropriate f-stop. That's so you can see what's going on and so that the camera has enough light to focus. So, yes, a lens with a maximum aperture that's two f-stops larger than another will let in four times as much light during focussing.
Your camera probably has a button you can push to close down the lens to see what it looks like stopped down (Depth of Field preview button). You can play with that to get a better sense of how much light different lenses provide at different apertures.
Originally by user32101. Source · Licensed CC BY-SA 4.0
user32101
7y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—during autofocus, most cameras keep the lens at its maximum aperture and only stop it down to the selected f-stop at the moment of exposure. So a 400mm f/2.8 gathers about 4× as much light for focusing as a lens whose maximum aperture is f/5.6, even if both are set to shoot at f/5.6.
For the actual photo, if both lenses are stopped to f/5.6, exposure should be essentially similar except for minor transmission differences. The big light difference applies during focusing, not during the exposure itself.
That said, autofocus speed is not determined by light alone. It also depends on the AF motor, the lens design, and how much glass has to be moved. More AF light can help the camera avoid hunting and improve focus acquisition in difficult conditions, but it doesn’t guarantee one lens will always focus faster than another.
So the photographer’s “4× more light” claim makes sense specifically because autofocus is normally done wide open.
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