I'm trying to learn manual flash and I was wondering if say I have the perfect exposure at f4 1/200th iso 200 and the flash at 1/2 power. If I want to drop the flash to quarter power can I just bump up the iso to 400 or the aperature to 5.6 and still keep everything perfectly lit? Or will the iso/aperature affect the ambient and the exposure will be thrown off some? Thanks
Your premise is partially wrong.
If exposure is correct at 1/2 power with aperture f/4, reducing power to 1/4 requires OPENING the aperture by one stop to f/2.8 to compensate... (closing to f/5.6 would lead to 2 stops of underexposure)... but yes, you could increase ISO instead.
As long as shutter speed = 'X' sync or slower, and ambient isn't bright enough to contribute, you can disregard shutter speed.
When ambient IS significantly bright, it works like this.
* Ambient light is independently controlled by shutter speed. Do not exceed 'X' unless HSS available.
* Flash light is independently controlled by flash power setting .. also by distance if in manual flash.
* Lens aperture controls BOTH Ambient and Flash, TOGETHER ... (not independently, but in step).
For this reason the aperture is used to control the overall lightness/darkness of the shot, when you have got the ambient and flash in the balance you want.
(Some people put this on an index card and keep it with their kit until it becomes second nature.)
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