Why Does Camera Change Focus Even in Manual Mode?

[Denniscasey]

I have learned to use manual focus to keep fast moving race cars in focus as I pan with them (auto focus was losing focus sometimes as it would "look" at intervening objects to focus on). Because of great DOF of small sensor camera HMC40 usually keeps cars in focus near (i.e. 20 feet away) and far (i.e. 150 feet). However, sometimes at high zoom settings (i.e. 88 or so) object will go in and out of focus--acts like camera is "hunting" for focus even though it is in manual focus mode.

Is there some setting I should change to stop camera hunting for focus in manual focus mode?? For instance, Barry mentions on page 226 of HMC Book that leaving camera's Optical Image Stabilization on can result in "noticeably jerky appearing pans," so in similar fashion could there be some setting in HMC40 that causes it to hunt for focus at high zoom settings?

Recent example was yesterday at aircraft exhibition when I manually focused on near and far objects using focus assists--camera "chose" MF95--bright sunny day with polarizing filter installed and iris opening around 5.6 to 8. As aircraft made high speed passes (WW II type aircraft) I would zoom to about 88 setting to pick them up as they started pass and then change zoom as they came closer. At zoom 88 aircraft would go in and out of focus--like camera was hunting for focus--but stayer in sharp focus as I changed zoom to lower amounts and aircraft passed in front of me and then roared away. So I wonder what is going on?

[Crafters of Light]

Just a guess.
A hot day, planes were close to the ground (within 500ft). Slight breeze, almost still. You had the runway, or some large cleared field between you and them as they flew by. If any of these sound familiar, you might have experienced atmospheric scintillation (heat waves). As you go wide their effects are greatly reduced.

[Felipe Henao]

Choose the definitive focus reference point ( The maximum close up you will do ), if you pan that might confuse you, because that reference point will change.
Manual focus for sure will not hunt because is manual.
Something I did was to add to one of the custom buttons the automatic focus which with one press give you an automatic when you are in manual focus.
That only for an emergency like a miss conception in manual focus.
Also assign to another custom button EVF DTL that emphasizes outlines of images for easier focus.

[Denniscasey]

Just a guess.
A hot day, planes were close to the ground (within 500ft). Slight breeze, almost still. You had the runway, or some large cleared field between you and them as they flew by. If any of these sound familiar, you might have experienced atmospheric scintillation (heat waves). As you go wide their effects are greatly reduced.

Good thought and very likely it was heat waves--very hot day, runway and tarmac partially between me and zoom. I was so intent on thinking about camera only I did not consider impact of environmental conditions. Dennis

[Denniscasey]

Felipe, As always thank you for good advice. Based on your (and others) past posts (and the HMC Book) I had previously implemented your thoughts which is why this apparent "hunting" for focus was baffling. I think Crafters of Light reply may have the answer with the heat wave thought--have seen that in movies with desert scene zooms "shimmering" in the heat. Dennis