I'm not entirely sure whether it's the same between D3200 and D3300, but on the D3200, auto flash is deployed as needed in Auto, macro, portrait, child portrait and night portrait modes. It is not automatically deployed in sports and landscape modes or in P, S, A and M modes. In P, S, A and M modes it defaults to fill when it is opened manually.
I do not know how it behaves if a dedicated, compatible TTL flash is used. If a non-dedicated manual or auto flash is used, it will open anyway, but those flashes are generally meant to be used in manual mode anyway.
I bought an external flash for my D3300 in order to save battery power on the internal battery. Unfortunately this didn't work as the internal flash partially pops up and operates even with the external flash attached to the hotshoe. Any ideas anyone?
@muzzabut, certain modes will automatically pop up the flash when it's called for. Auto mode will, for example, and so do some of the other scenic modes. If you want to use an external flash you must choose a mode which does not pop up the built in flash. Those include P, S, A and M, Sports and Landscape. All these will operate a hot shoe flash when it's installed. I do not have a TTL flash to try out, but with a standard manual flash, at least, the Auto no-flash mode (next door to Auto) does not fire any flash including the hot shoe.
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I do not know how it behaves if a dedicated, compatible TTL flash is used. If a non-dedicated manual or auto flash is used, it will open anyway, but those flashes are generally meant to be used in manual mode anyway.
It was somewhat easier in the old days of slr film cameras with a sync setting with no worry about an integral flash.