Nyctinasty (flower response to light: opening with or closing without light) in hibiscus plants is a mechanism to protect against adverse conditions such as cool temperatures that can be damaging. Through a lack of light stimulus and circadian rhythms the plant is able to trigger the molecular movement of ions to allow for the closing of the flower.
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Here are three macro photographs of this bloom, all from the Canon 100 “macro” lens. I start with the shutter diaphragm open at 4.0 (“F-stop”), a little narrower at 4.5 and a bit more at 9.0. For this lens the maximum opening is at 2.8, the narrowest is 32. As the opening narrows (F-stop increases) the exposure time needed to capture enough light lengthens and the range of the image in focus increases.
Wikipedia – “hibiscus.”