Flash with P mode, kinda like slow HSS?

The way I understand ettl, in AV TV and manual modes, the camera takes in to account ambient light and just tries to use your flash to fill in up to a proper exposure, and in P mode the camera doesn't take in to account the ambient light and uses your flash to get the entire exposure from only it.

So when using P mode, you can get a darker background because the camera uses only the flash to light the subject, causing the background to be darkened, so then isn't P mode with flash indoors kind of like HSS mode in bright outdoors?

If I raise my shutter speed in P mode the camera will fire the flash at higher power to compensate thereby darkening the ambient light. So as long as you are in a normal indoor environment and not in brighter daylight, then the 1/200 shutter speed should be able to lower the ambient light while the camera gets a good exposure on the subject.

So P mode with flash can be considered as kind of a low-light HSS function, is that right?

This is a good read for you and will be much more eloquant than I:

http://photonotes.org/​articles/eos-flash/

Most of your first paragraph is false. Regardless of camera mode, E-TTL does not "take into account" the ambient light level. Ambient exposure and flash exposure are metered separately and independently.

Also, the camera will render the same ambient exposure in Av, Tv or P modes. The only exception to this is that in P mode, it won't use a shutter speed slower than 1/60. So when your ISO setting and the lighting conditions are insufficient to get proper ambient exposure at 1/60, the camera will choose underexposed ambient rather than the potential motion blur from a slow shutter speed.

Now, on to your second incorrect assumption, that HSS mode renders underexposed ambient in a bright environment. HSS simply allows fast shutter speeds (and therefore wide aperture settings) so you can use flash while blurring the background in a bright environment.

In your third paragraph you mention, "If I raise my shutter speed in P mode..." P mode doesn't allow you to set the shutter speed per se. It does allow you to shift the exposure settings, to speed up the shutter while opening the aperture, or vice-versa.

In summary, the premise of your thread title has no merit.

If you want to control the ambient exposure in a flash photograph, switch to M mode and use a shutter speed + aperture + ISO combination that will render the ambient exposure you desire.

Which means that if you do shorten the shutter speed in P mode, the aperture will become larger and the flash will actually use a lower power, not a higher one. But you can't shift the shutter speed in P mode when you use a flash, so it's a moot point.

Thanks for the great explanations guys, just what I was hoping for.

Yeah, in P, AV, or TV mode the camera will first figure out settings that will allow for an "even" exposure regardless of whether or not the flash is there. I almost always use my camera in M mode and usually use manual flash over ETTL, too, so I'm not the best to ask. But if you're in P, A, or TV and using the flash in ETTL the flash is going to be low power and basically just fill. That might yield a good result and the flash will help freeze the subject if you have a low shutter speed. You can of course use exposure compensation in the flash or camera to give yourself more control -- but seems like utilizing manual modes might be best if seeking control.

It's actually so that cameras with E-TTL II (all newer digital cameras) measure the difference between ambient and flash light, when the preflash is fired. The different zones in the exposure meter are used for this, so cameras with 63 zone metering (7D) does this better than the previous 35 zone metering (5D Mark II).
If a zone peaks heavily when the flash is lit, then it's disregarded as probably being a mirror or something similar.
On the other hand, if there's no different with and without flash, the zone is disregarded as obviously being outside the range of the flash.
The the zones that are in between, i.e. made brighter by the flash, are evaluated, with emphasis on zone(s) that coincide with AF points that are in focus.
Ambient metering is also a bit adjusted, since if the camera's setting is such that the camera can achieve a standard exposure without the flash, then that standard exposure is reduced by about one stop, to allow for adding flash light without overexposing things that otherwise would be at standard exposure already before the flash fired. If you use P-mode indoors in the evening, it will not allow you to get up to standard exposure of the background (lest you have sky-high ISO), so this reduction isn't necessary. The flash will provide the brunt of the light anyway.

apersson850 wrote in post #15029131
It's actually so that cameras with E-TTL II (all newer digital cameras) measure the difference between ambient and flash light, when the preflash is fired. The different zones in the exposure meter are used for this, so cameras with 63 zone metering (7D) does this better than the previous 35 zone metering (5D Mark II).
If a zone peaks heavily when the flash is lit, then it's disregarded as probably being a mirror or something similar.
On the other hand, if there's no different with and without flash, the zone is disregarded as obviously being outside the range of the flash.
The the zones that are in between, i.e. made brighter by the flash, are evaluated, with emphasis on zone(s) that coincide with AF points that are in focus.
Ambient metering is also a bit adjusted, since if the camera's setting is such that the camera can achieve a standard exposure without the flash, then that standard exposure is reduced by about one stop, to allow for adding flash light without overexposing things that otherwise would be at standard exposure already before the flash fired. If you use P-mode indoors in the evening, it will not allow you to get up to standard exposure of the background (lest you have sky-high ISO), so this reduction isn't necessary. The flash will provide the brunt of the light anyway.

An eloquent description of flash logic. Canon flash does have programming called 'NEVEC' which does alter exposure settings a bit, but NEVEC does not always hold true. Most of the time (except for academic discussion) I ignore the effects of NEVEC. Unfortunately the text in blue cannot be proven to be true, as this series shows...

• Photos 1, 3, and 5 have no flash, as baseline exposures; 2, 4, 6 all have direct ETTL flash, to judge the effect of addition of flash to baseline.
  
• The camera was in evaluative ambient metering, with the focus point on top of the Colorchecker target, so ambient metering would be biased to that focus location.
  
• ETTL was set so that evaluative flash metering (factory standard) was set; CFn for Av shutter speed was set to force 1/250 for flash shots.
  
• Post processing settings were identical for all shots, with the exception of White Balance, due to some shots being made with incandescent overhead downlights; WB was post processed with eyedropper to render 4th grey sample as neutral.

• Eyedropper values for photos 1,3 (no flash) RGB=52%
  Eydropper values for photos 2,4 (flash) RGB=42%
  Eyedropper value for photo 5 (no flash) RGB=45%
  Eyedropper value for photo 6 (flash) RGB=61%
  
• Photos 5 and 6 only differ in ambient settings by 1/3EV (f/3.5 vs. f/4), and that difference can be easily explained that P mode selects 1/60 f/4 as default.
  We also see that photos 4 and 5 differ in ambient settings by -4EV (1/8 vs. 1/60) because P mode selects 1/60 as default flash shutter speed.
  
• Photos 5 and 6 differ in target brightness by about 0.78EV of exposure (flash shot 6 is 0.78EV brighter than no-flash shot 5)