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I read following in a web site (http://realphototips.wordpress.com/tag/ai-servo/)

Lenses with maximum apertures of f2.8 allow the camera to use all high precision sensors. In low light or other situations that are hostile to autofocus, that’s a big deal. Lenses with a maximum aperture of f4.0 use only the center focus sensor in its “high precision” mode, and use the other sensors in their “horizontal line” only mode. Lenses with a maximum aperture of f5.6 use all sensors in their “horizontal-only” mode, and lenses with a maximum aperture of f8 use only the center sensor point, and that with horizontal sensitivity only.

I want to understand if this is completely true? I am raising this because recently I have purchased a Nikon 70-200 f/4 lens and would like to understand if I have made a wise investment, or, should I collect some more money and buy a 70-200 f/2.8 VR-II lens.

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This is very camera dependent, obviously you are shooting Nikon, but different cameras have different AF sensors. Some are capable of having all cross type work at f4. –  Robin Jan 22 at 18:28
    
Do you find yourself shooting a lot in low-light situations and having trouble with autofocus? –  fennec Jan 23 at 4:01
    
Hi Fennec I didn't understand the intention of your question. However, with my Tamron 18-270 I face AF issue almost 40% of the time in low light situations. But with Nikon 70-200 f/4 I don't have that kind of issues. But with Nikon D90 photos with more than ISO-400 are too much noisy; this frustrates me!! –  Niranjan Jan 23 at 4:05

2 Answers 2

up vote 3 down vote accepted

When the camera is focussing the lens, the lens is wide open, the aperture is only closed down to the selected setting when you actually close the shutter. So, based on that, an F/2.8 lens will let in twice as much light for autofocusing as the F/4 when wide open.

Did you make a mistake? Possibly not. The Nikon F/4 variants of the F/2.8 lenses are very good and, on modern Nikon bodies, don't suffer from autofocus performance issues at F/4. If you have an older Nikon body, maybe, but if you upgrade your camera at some point, the issue goes away anyways and you still have a nice lens.

Net effect, don't worry too much and enjoy the lens. You might also want to mention what Nikon body you have. :)

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Hi John, thanks for the explanation. I am sorry I did not provide my camera details. I shoot with Nikon D90 and planning to upgrade; currently considering either D7100 or D610. With D90 my 70-200 f/4 is not taking very nice photos. Basically D90's ISO performance is not good; beyond ISO-400 photos come up with visible noise. I have my photos on 500px (500px.com/nnanda). –  Niranjan Jan 22 at 4:45
    
Well, the D90 only has one cross-type sensor anyways, so the faster lens isn't going to help with AF performance, but could give you lower ISO. Body upgrade makes more sense to me. –  John Cavan Jan 22 at 4:50
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It's not just about the quantity of light, the larger aperture allows for a different AF geometry. A lot of cameras have AF sensors that simply don't work with f/4 lenses because the light they rely on doesn't get through the lens (the AF virtual aperture is outside the real aperture). –  Matt Grum Jan 22 at 9:01

There are two factors determined by physics that favor autofocusing with wider apertures.

  • More light An f/2.8 lens lets in twice as much light as an f/4 lens. The more light an AF system has to work with the faster and more accurate it can be.
  • Wider baseline Phase detection AF works by comparing the differences between the light coming from the right and left sides of the lens. The wider the effective aperture of the lens (more properly called the entrance pupil), the further apart the light rays that are compared can be.

To take advantage of the wider aperture, the pairs of sensors for a particular focus point in the AF array must be further apart from one another. But that makes those lines not very useful when a lens with a narrower aperture is attached to the lens. So camera manufacturers hedge their bet a little. Some of the focus points are more sensitive/accurate but only function well with a large aperture lens. Other focus points are tuned to be able to use the light from lenses with narrower apertures. But those points can't take advantage of the wider light rays provided by a wide aperture lens.

This is because the two lines on the focus array for each focus point are in a fixed position. If they are close enough to each other to be able to use light that gets through each side of the lens with a narrow f/8 aperture, they are not far enough apart from each other to sense the light that gets through the edge of the lens with a wide f/2.8 or wider aperture. Even when a faster lens is on the camera they are only using light falling on each side of the lens that is close enough to the center to make it through the narrower aperture.

How well the 70-200 f/4 takes advantage of your camera's focus system depends on the specific design parameters of you camera's focus system. In general though, a constant aperture f/4 telephoto lens will perform well. The only place you might be concerned is if you plan to use a tele-converter, since a 1.4X will raise the lens' maximum f-number to 5.6 and a 2X tele-converter will raise it to f/8.

Incidentally, the same physics that favors AF with lenses with wider apertures also favors DSLR cameras with larger sensors. Because the mirror is larger (particularly because it is wider) in a full frame camera than in an APS-C camera, the baseline used for the most sensitive focus points can also be wider.

For a little deeper answer on how cross type points work and a visualization of how f/2.8 points require lines that are further apart, see this answer.

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Hi Michael, the answer explains about Canon and I have read quite a few documents over the internet on how cross type sensors work. I have noticed a pattern; while Canon puts more cross type sensors, Nikon engineers take a different argument for not putting that. They say if the light comes with certain angle, cross type sensors behave as a normal sensor. Anyways, what is your point of view on Nikon D610 with my 70-200 f/4 lens? Will the performance improve? I am also planning to use a teleconverter, but thats a distant plan as of now. –  Niranjan Jan 22 at 5:31
    
The physics is the same regardless of the manufacturer. All PDAF focus points use a pair of lines. Cross type focus points use two pairs of lines: one pair for the vertical and a separate pair for the horizontal. The reason you read more about this online in regard to Canon is that they are very open about which lines (horizontal vs. vertical) on which focus points are sensitive at what apertures for each of the focus systems of their cameras. At Nikon this is apparently top secret, at least for the D90. There's a reason most top sports photogs use Canon: superior focus speed/accuracy. –  Michael Clark Jan 22 at 5:47
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If you are ever planning on using a teleconverter, especially a 2X, with a 70-200 lens you should consider an f/2.8 lens. With regard to the D610 there are two factors at work. The FF sensor means the FoV of the 70-200 will be wider than on an APS-C body such as your D90, where the lens acts like a 105-300mm lens in terms of FoV. You can always crop the result (or use Dx mode to let the camera do it for you), but you give up megapixels when you do that. The focus system of the D610 is much more advanced than the D90. 39 points, including 9 cross, with 7 sensitive @f/8... –  Michael Clark Jan 22 at 6:03
    
This means they will work with more lenses, but also means they can't take advantage of the wider baseline provided by faster lenses. –  Michael Clark Jan 22 at 6:05
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Because wider aperture lenses have entrance pupils that are wider. PD AF works by comparing the rays from the same spot that enter each side of the lens. Have you read the answer linked at the end of the one above? Although it is in regard to the 7D the principle is the same: The wider the baseline the more sensitive and more accurate a focus point can be. And don't get caught up over cross type points. In reality they are just two points, one vertical and one horizontal, that cross each other. In the case of a diagonal cross point they are two diagonal focus points that are 90° apart. –  Michael Clark Jan 22 at 6:26

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