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I read that with some cameras (but which?), using an f/2.8 lens or faster enables a more sensitive phase AF point, so there is quantum jump in AF accuracy.

But that's not really my question :) My question is whether I can expect some AF improvement between an f/5.6 lens and an f/4 one, since the standard AF points will have more aperture and light to work with?

xenoid
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    I would guess that it depends on the amount of ambient light and the specs of the autofocus sensors, at least, that's how my autofocus (aka manual focus) seems to work. I've also noticed difficulties associated with increased depth of field. The smaller the aperture, the more equally focused/blurry everything starts to look. – xiota Dec 29 '18 at 02:12
  • A problem with some lenses with big apertures is everything can be blurry because of various aberrations, so it's impossible to really focus on anything. Also, depth of field can be too narrow, making it hard to lock focus on anything. The sweet spot where the amount of light, contrast, and depth of field is optimal probably depends on the specific lens and sensors involved. – xiota Dec 29 '18 at 09:27
  • The answers to the duplicate are also applicable to the differences between f/5.6 and f/4. – Michael C Dec 29 '18 at 21:35

2 Answers2

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Since most cameras focus with the lens wide open, a wider aperture allows the possibility of better AF performance. This is because the baseline between the light entering on opposite sides of the lens is wider.

But this potential can only be realized if the camera's AF system is designed to take advantage of it. Some cameras' AF systems are and some are not. So ultimately, it depends on the particular camera.

The following is primarily with regard to SLRs that have a dedicated PDAF sensor that is used when the camera's reflex mirror is down. Mirrorless cameras' main imaging sensor based AF systems are also affected by a lens' maximum aperture, but in different ways.

In general, the lines of demarcation are between f/5.6 and f/8 with respect to the minimum wide open aperture a lens can have and still allow a specific camera body to AF at all, and between f/4 and f/2.8 on the other end with regard to the minimum wide open aperture a lens needs to take advantage of the more sensitive sensors in a camera's AF system.

Many cameras require an f/5.6 or faster lens in order to AF at all, but there are some that can AF with an f/8 lens or combination of lens + teleconverter/extender. Most cameras that have some AF focus points that are more sensitive require f/2.8 or wider lenses in order for those points to function, but there are a few cameras that have PDAF sensors with a combination of f/2.8, f/4, and f/5.6 or f/8 lines on the AF sensor.

For a more complete discussion of how PDAF sensors work with lenses of various maximum apertures, including illustrations, please see this answer to: Does autofocus work better with f/2.8 lenses vs f/4 or slower?

Michael C
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  • "In general, the lines of demarcation are between f/5.6 and f/8 with respect to the minimum wide open aperture a lens can have and still allow a specific camera body to AF at all, and between f/4 and f/2.8 on the other end with regard to the minimum wide open aperture a lens needs to take advantage of the more sensitive sensors in a camera's AF system." My question is about that f/4-F5.6 range where you are only using "standard" AF sensors. For a given type of AF sensors, it there any advantage (AF-wise) to using a faster lens? – xenoid Dec 29 '18 at 12:54
  • @xenoid As the answer clearly states: It all depends on the design of a specific AF system. Without knowing what camera you are asking about, it is impossible to answer that question. There's no such thing as a "standard" AF point. Every camera design can be, and often is, different. – Michael C Dec 29 '18 at 19:52
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Aperture in relation to PDAF relates to whether the required image (objective lens) area can be seen by the PDAF module and not the amount of light the module receives.

The AF module receives light equivalent to approximately f/7 due to its' own aperture restrictions, so having a wider lens aperture does not transmit more light to the module. Having a lens of f/2.8 or wider is only of a benefit if the camera is designed to use the f/2.8 objective areas... i.e. some Canon models like the 1Dx have f/2.8 "helper" focus points (initial focus is with f/5.6 points and then refined using the f/2.8 points).

If this is in relation to on-image-sensor focus, then yes... every bit of extra light/contrast helps.

Steven Kersting
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  • The equivalent f-number an AF module receives varies greatly from one camera to the next. The "approximately f/7" quote is from a DPR discussion in which Marianne Oelund is referring to a specific camera and in the same discussion she also acknowledges that other cameras have different effective f-numbers! – Michael C Dec 29 '18 at 22:49
  • Please see the answers and comments to Does autofocus work better with f/2.8 lenses vs f/4 or slower? for a fuller discussion of this errant claim. – Michael C Dec 29 '18 at 22:51
  • The simple fact is that the PDAF module uses small images taken from small areas of the objective lens which creates a "virtual aperture" as discussed in the DPR article. It simply cannot be equivalent to the lens' max aperture.

    AFAIK, all PDAF systems are designed primarily around an f/5.6 lens aperture so that there will be no shading/vignetting/light loss at f/5.6 or wider... i.e. f/5.6 is the maximum light potential to the module. Some have additional points that can use the f/8 or f/2.8 objective areas, but that has no effect on the amount of light/size of the virtual apertures.

     – Steven Kersting Jan 02 '19 at 16:38
  • Only on-sensor AF uses the full objective lens area (aperture) cumulatively for autofocus... – Steven Kersting Jan 02 '19 at 16:38
  • That all depends on the lens' maximum aperture compared to the effective aperture of the PDAF system. In the case of an f/4 lens on a camera with some AF points sensitive to f/2.8, the lens is more restrictive than the microlenses and length of the lines on the PDAF sensor. Thus the lens is the more restrictive bottleneck. Why do you keep insisting on taking Marianne Oelund's comment regard one specific camera and universally applying it to all cameras, even when she herself stated later in the same discussion that other cameras' PDAF system can be, and sometimes are, wider? – Michael C Jan 02 '19 at 22:53
  • There's no need to use the "full objective lens (aperture)..." in order to give a PDAF sensor (or some segments of it) an effective aperture wider than f/7 or f/5.6. One only need use two small areas from opposite sides of the lens' entrance pupil. That's how PDAF systems work. It's why some AF points tuned for larger apertures (i.e f/2.8) don't work with lens with narrower apertures (i.e. f/4 or f/5.6 or f/8). The f/2.8 lines on the sensor only measure the light at the extreme edges of the lens. – Michael C Jan 02 '19 at 23:03
  • Microlenses and phase lines for each AF "point" on a PDAF sensor module are independent of the other microlenses and sets of lines for other AF "points" in the same system. The length of each set of lines and the distance they are from one another, along with the design of the specific microlenses for each of those two lines collecting light from opposite sides of the lens determine the "effective aperture" of the PDAF system for that specific AF "point". Just because Nikon once (but no longer does) tuned every AF point on their PDAF sensors to the same width does not mean everyone does. – Michael C Jan 02 '19 at 23:11
  • This is the set of microlenses for a simple 9-point AF system. https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcRpLacQmeTf3f4OcsRYduEKyulCxwBQl6u4cOvsedQToeg6_P1ivg – Michael C Jan 02 '19 at 23:15
  • The ten sets of diagonal lines widely spaced from one another are for the five diagonal "cross type" f/2.8 sensitive portions of the Canon EOS 1D X and 5D Mark III AF system. – Michael C Jan 02 '19 at 23:26
  • This chart further explains which AF points are tuned for which apertures. The microlenses do not restrict the "effective" aperture narrower for any of them. – Michael C Jan 03 '19 at 00:23
  • It sounds to me like you are confusing aperture location with the virtual aperture which is what limits the amount of light available. The lens' aperture simply dictates wether the location of the virtual aperture (source image location) is visible or not.

    This image is a simplified drawing of how the PDAF system works: https://photos.app.goo.gl/LHCL183cCT8Zk9tD7

     – Steven Kersting Jan 06 '19 at 17:58
  • Sorry, new link to annotated image: https://photos.app.goo.gl/w4pCTQwSiKbhD5rP8 – Steven Kersting Jan 06 '19 at 18:06
  • I'm not confusing anything. I'm merely pointing out that your answer breeds confusion by appearing to assert that PDAF systems can not see any 'locations of the virtual aperture' that are outside of what is allowed through by a lens aperture of approximately f/7. That is not true. After all, the OP is asking what is the difference between an f/4 and an f/2.8 lens, not the difference between a PDAF sensor with microlenses that are effectively as bright as f/2.8 or f/4. – Michael C Jan 07 '19 at 02:07
  • We just disagree... the aperture brightness is completely unrelated to the aperture location for a PDAF focus point. The aperture related to a focus point is simply whether or not that area of the objective element can be seen beyond the aperture blade restriction. You seem to understand that f/2.8 points are using separate images taken from the periphery of an f/2.8 objective element, and the same is true for f/5.6 points and objective elements. And I'm sure you know that the f-ratio of a lens is limited by the objective element diameter... – Steven Kersting Jan 08 '19 at 20:59
  • The effective aperture (light) for a PDAF focus point is limited by the area of objective element where each image is taken from (and then compared for position/phase). It has nothing to do with a lens' max aperture. I can concede that it may not be f/7 for all designs (I did say "approximately), but it is simply impossible for it equivalent the the diameter of the objective lens... – Steven Kersting Jan 08 '19 at 21:00