Does a lens with a bigger max. aperture focus faster than a lens with a smaller max. aperture?

Question

The assertion by a well known photographer is that the Nikon 400mm f/2.8 set to an aperture of f/5.6 will let in 4 times the light compared to a Nikon 500mm f/5.6 lens set to f/5.6, keeping the shutter speeds identical.
The ultimate conclusion from this was that the focus acquisition time was much faster for the 400mm lens at f/5.6 than the 500mm set at f/5.6 due to more light.

While there may be different transmission factors for the glass, especially given one lens is the Nikon 500mm PF, I don’t believe the difference would be 4 times. He seems to be asserting that the faster lens is faster at any aperture when compared to a slower lens.

I would expect that if both lenses are set to f/5.6 and all other settings were the same, there should be less than a 1/2 stop difference in light transmission.

Where does the large difference in the statement of the photographer come from?

Answer 1

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.

Answer 2

All lenses with the same aperture value (e.g. f/5.6) acquire essentially the same amount of light, because that number is relative to the focal length. The aperture of a 400mm/5.6 lens is about 71mm, while 500mm/5.6 is about 89mm.

The speed of focus, however, is more related to the speed of the focus motors on the lens, and how many (and how heavy) lens elements it needs to move to achieve focus. A lens with a physically smaller aperture can be faster, as the moving lens elements are typically smaller.

The amount of light reaching the (focus) sensor may affect focus speed in that the sensor might have a harder time to discern if the focus has been achieved, or how far from focus it is -- resulting in hunting. Not exactly focus speed, but still relevant to how fast you can achieve focus, and is typically only relevant in low-light situations.

Answer 3

In general, cameras acquire focus with the lens wide open, so it is certainly true that the f/2.8 lens will be letting in (approximately) four times more light. To see this, half press the shutter to get a focus lock and see how the lens is still wide open.

Answer 4

Since the focus is measured with the lens at full aperture, the f/2.8 lens gets more light on the AF sensors so this can improve things (accurracy, reliability...). But focusing speed is also a matter of focusing motor...

In addition, the phase detectors used for auto-focus in a SLR have an accuracy which is related to the minimum aperture they can work with. On most cameras the standard sensors require a lens that can open at f/5.6 but they may not be accurate enough for very fast lenses that have a shallow depth of field, so in many cameras one or more AF sensors are doubled by a higher-accuracy sensor that is used if the lens can open at f/2.8 or more (even if the lens is stopped down for the actual photo).

Answer 5

Two factors:

• The longer the focal length, the harder it is to obtain perfect focus.
• The focusing happens at the max aperture of the lens anyway, so using the f/2.8 lens at f/5.6 doesn't matter, as the other answers noted.

I'm pretty sure these two effects fully explain whatever differences you're seeing.

Answer 6

In general, it might or might not. My Canon EF-S 15-85mm lens at f/4 (say) focuses much faster, and more quietly, than my 50mm f/1.8 lens. Several times faster.

In addition, a telephoto zoom lens can end up hunting back and forth along its entire range to focus, which is very slow; a prime (fixed) lens won't do that, but if the motor is slow (as on the old f/1.8 50mm) then it can still be slow to focus; the Canon f/2.8 70-200mm IS lens, by contrast, is very fast to focus.

So, it's not primarily about the amount of light.

As a minor aside, the factor of 4 light difference mentioned in one of the answers is theoretically true, but reality tends to be slightly more complex than theory at times :) so it's more a guideline than a universally exact statement: there's exceptions.