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Wondering about some shots that went wrong, which were shot with focus peaking and manual focus, and where there were metallic/mirror-reflective highlights (think scenes with crumpled car wrecks or broken glass in clear midday sun. Or reflections on water.)

Will a focus peaking system easily detect virtual images in reflective objects and suggest a focus that is off what would be right for the reflective object itself? Is this a concern to be aware of with CDAF autofocus too?

  • This might be somewhat camera-specific, since focus peaking is implemented as a software algorithm, and probably varies from model to model (or possibly even with firmware upgrades to the same model). – junkyardsparkle Apr 9 '19 at 23:02
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Any time there are objects at various optical distances from the lens, an AF system (even when being used to confirm manual focus) is susceptible to measuring/focusing on things at an optical distance other than that intended by the photographer. Since all AF systems use contrast to measure how well a system is focused, things with the most contrast in the areas of sensitivity being used by the AF system will get the most weight.

  • +1 for "all AF systems use contrast", denying the popular belief that phase detection AF uses magic! ;-) (but perhaps it should be "neary all", as the laser ToF autofocus is a thing) – szulat May 10 '19 at 9:38
  • Has anyone ever put a laser ToF sensor in a camera intended for actually taking photos as an artistic or documentary endeavor? (As opposed to machine vision or ranging for other, non-photographic purposes) – Michael C May 10 '19 at 15:09
  • yes, some smartphones use lasers for AF - phonearena.com/news/… – szulat May 10 '19 at 15:30
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Focus peaking does not "suggest a focus". It just highlights contrast. Focus is up to you, the photographer.

Blown out highlights, such as may be seen on reflective surfaces, will be highlighted by focus peaking because they are areas of increased contrast. It is up to you to not use such areas while focusing. Similarly, you may misfocus if attempting to use naturally high-contrast areas, such as text on a page. Instead, look for textured, matte surfaces, such as fabric. Focus peaking will make such surfaces appear stippled when in focus.

AF can arguably be misled more easily than you. When focusing on a semi-reflective surface with particles resting on it, AF can sometimes switch back and forth between focusing on the reflection or focusing on the particles. Similarly, when photographing through a window covered with water droplets, AF may alternate between the window and the scene beyond.

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I think you're asking if an AF system will be prone to focus on what is being shown in a reflection (focus distance being total from camera to reflective surface to reflected object). In general I would say that depends on where the focus travel (ring) starts from. I.e. if focus starts from infinity then the far subject (reflected) would come into focus first and is more likely to lock/confirm at that point. In all cases, things that are easier to focus on (larger details/greater contrast/etc) will get preference unless the system is forced to ignore them (i.e. single point focus).

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