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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?

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    All of the answers so far address the misconceptions the op had and how the camera/lens works but i do not see how they address the question of does the amount of light as a result of aperture size during focusing result in appreciably faster focusing times ? – Alaska Man Jun 27 at 19:24
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    The extra light is invisible to Phase Detect AF. It'll only be visible to contrast-detect AF. Phase Detect AF looks at a particular ring on the exit pupil of the lens, typically at f/5.6 with few exceptions. If your lens is slower than this then the PDAF sensors are literally in the dark and you get no AF at all. If your lens is faster than this then you'll get a slight benefit from less vignetting, but that's it. – Adam Jun 28 at 3:31
  • Possible duplicate of Does autofocus work better with f/2.8 lenses vs f/4 or slower? – Hueco Jun 28 at 16:20
  • @Adam the idea that most PDAF sensors are limited to f/5.6 is woefully out of date (unless you're talking about Nikon, in which case it is only slightly out of date). – Michael C Jun 28 at 23:19
  • @MichaelC that doesn't change the fact that a lens faster than the PDAF point won't provide any advantage. – Adam Jun 29 at 22:11
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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.

  • Yes, and i should have realized this! – Michael Davis Jun 27 at 17:29
  • Actually, on phase detection autofocus system (present in most DSLR's, if not all of them), it's not the amount of light but the optimal depth of field, what makes a difference. I haven't tested it to be able to say it's true, but is usually said that f2.8 lenses are the faster focusing ones on phase detection, as wider apertures make DoF too shallow and phase detection gets "confused". – Lisan Jun 27 at 20:18
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    In addition to my previous comment, DoF depends not only on diaphragm aperture but also on focal lenght and sensor size, and undoubtely there's difference between phase detectors in DSLR models, so knowing which exactly would be the fastest focusing lens surely needs of much more math than just saying 2.8f is always the fastest. – Lisan Jun 27 at 20:34
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    @Lisan my f/8 lens focuses faster than my f/2.8 lens because it's forced to have more DoF by design said no-one ever. – Hueco Jun 27 at 23:41
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    Jokingly, the smallest aperture pinhole focuses the fastest, because it always is in focus everywhere. – Agent_L Jun 29 at 10:37
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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.

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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.

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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).

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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.

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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.

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