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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?

marked as duplicate by Michael C lens Dec 29 '18 at 21:34

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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 2:12
  • 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 9:27
  • 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
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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.

  • 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 2 at 16:38
  • Only on-sensor AF uses the full objective lens area (aperture) cumulatively for autofocus... – Steven Kersting Jan 2 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 2 at 22:53
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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?

  • "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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