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i have canon slr 1200d.
i have problem with night portrait photography . with flash it focus only the subject and background become very dark. and without flash all are same lightning but subject not in focus.
i want subject as focus and also background has some light and blur effect.
i face this problem at outside photography. when background as nature and not able to gain flash.
also suggest exposer iso sutter speed and apature value for that.
The technique you are looking for is called "Slow Sync Flash". On your Canon camera, you enable it by turning you mode dial to "Av" (Aperture priority). In this mode, the flash will fire to expose the foreground, but the shutter will still remain open for a long exposure of the background, just like when you shoot without flash.
Beware, that in this mode, you have to expect subject movement during exposure. The flash freezes the motion, but depending on lighting conditions, you may still get "ghost" images.
If you want the backgound to be visible, but darker than your foreground, you have to dial in some negative exposure compensation (by turning you wheel while holding down the "Av" button).
ISO, speed, and aperture really depend on your specific situation, there can be no general rule. Your ISO should be as low as you can get away with, aperture has to be set to achive the depth of field you want, shutter speed results from the exposure you want to achive.
This depends on if your problem is focus or camera shake. If it is actually focus, then you can simply configure the flash to fire in focus assist mode. In this case, it will flash before taking the photo so that the camera can focus, but the photo itself will be taken without the flash.
If, however, your images are out of focus because the camera is moving during the time that it takes to expose the background, you will instead need to use a higher ISO or wider aperture (smaller f/ number) to allow for a faster shutter speed. This is because the actual problem is that exposing the background takes too long and it gets exposed on multiple parts of the sensor due to the camera moving.
Either the camera moves too much while getting an exposure of the background or it doesn't. You don't have enough flash power to cover the background, so you need to fix the focus or the shake. The slow sync flash will help get the background a little bit brighter, but the brighter you make the background, the more fuzzy the subject will become (even though the flash did the majority of the exposure on the subject), and the background will always be darker than the subject using that technique.
