PDAF

[Djehuty]

My rudimentry understanding is PDAF performance is affected by the amount of light coming in so glass plays a part but my question is, is PDAF performance affected by ISO value? Does it perform better if you bump up the ISO high up or is it just the same at 100 as it is at 12000?

 

[bobn2]

My rudimentry understanding is PDAF performance is affected by the amount of light coming in so glass plays a part but my question is, is PDAF performance affected by ISO value? Does it perform better if you bump up the ISO high up or is it just the same at 100 as it is at 12000?

By 'ISO' I presume that you mean the ISO setting. If so, there is no standard for what the ISO setting does to a camera's read chain, so I wouldn't think that there is any fixed rule. Assuming that some ISO settings change the read chain voltage gain, that would change the performance of the PDAF reads, if the taking settings were used for those. If I was engineering the camera I wouldn't do that, though, I'd be choosing gain settings that were optimum for the PDAF system when doing the AF reads, whatever the user's selected ISO. The same would apply to AF reads. Exactly how the designers of individual cameras have done it is usually the manufacturers' proprietary information, and not usually disclosed.

If you felt like doing some experiments and posting the results, I'm sure that people would be very interested.

PDAF, BTW, is relatively unaffected by the light levels, which isn't to say that it isn't affected at all. It needs some detectable contrast in the image at the AF point, but so long as there is that, it is OK.

 

[SrMi]

Thom Hogan has observed that the appropriate "lightness" under the focus cursor influences the focus accuracy/speed. It seems that the focusing operation occurs on the EVF stream. If the EVF shows the subject as dark, focus suffers.

Link to Thom's post.

 

[John Sheehy]

My rudimentry understanding is PDAF performance is affected by the amount of light coming in so glass plays a part but my question is, is PDAF performance affected by ISO value? Does it perform better if you bump up the ISO high up or is it just the same at 100 as it is at 12000?

On a DSLR, the PDAF sensors are under the mirror, and have nothing to do with anything happening on your image sensor including your exposure. The open f-ratio of the lens is the only real camera variable in what they see. These types of PDAF sensors don't just need light quantity, per se, so they can operate above the noise floor; they need light coming from outer bundles with low f-ratios to work at all, regardless of the amount of light. The same system that could focus in very low light at f/8 could completely fail pointed at a high-contrast target soaking in the mid-day sun at f/11, that is how sharp the cut-off seems to be.

In live view on DSLRs and mirrorless, if the imaging sensor has PD pixels (some pixels, or all, like recent Canons), then the lens may operate stopped down and that can vary with your settings, IME, and manufacturers have probably become diverse in how they handle this. One thing that a lot of former DSLR users have noted when using newer mirrorless cameras is that the AF system seems to see less DOF than a DSLR with similar optics, and is more likely to fail to see something thin, far closer to the camera than the current plane of focus. This happens most often when the aperture is large, and the background is already in focus, and the cameras will just park the AF as if they are already focused.