In every video I watched when they want to shoot the guy who has a blue sky or open background behind him, they say "expose for the background" but I don't understand what do they mean? I am using flash to have some fill on his face so I am on "Manual mode" so I pick the numbers, so what is exposing for the background then? Do they focus on the sky at the back? Like in "Aperture" mode and then switch to Manual mode and use flash to light up the guy ?
It means to set your camera to expose the sky correctly. If you are in Manual exposure mode you need to look in your viewfinder to see if the ISO/shutter speed/aperture you have selected exposes the sky properly by looking to see what the meter is telling you. Adjust the ISO/shutter speed/aperture until the exposure meter in your viewfinder says you are properly exposing when pointing towards the part of the sky that will be behind your subject in the photo. This will almost surely leave your subject underexposed. Meter your subject while the camera is still set to properly expose the sky and see how many stops your meter says your subject is underexposed. To get your subject properly exposed, leave the camera set to properly expose for the sky and then add enough fill flash to make up the difference between the bright sky and your less bright subject.
It is like you said: Expose on the background in order to get the background correct. However, the face will then often be too dark, so you could use a flash.
But you cannot do it reverse: If you expose on the face, then the background is too light, but you cannot correct this.
As an option, if you want to avoid a flash, you could make a series and make a HDR photo.
Anyway, the best is to avoid such photos at all and look for situations where the sun is shining in front of the persons (i.e. behind the photographer), then the pictures are getting much better.
Essentially, this is related to the two exposures that you get when using a flash:
- from the ambient light
- from the flash light, which is
- brighter than the ambient, thus can overpower it
- shorter (does not last the entire shutter speed), thus can freeze action
- not illuminating everything
The last one is the important thing here. A usual way to think about constructing the image is to first look at the ambient light, that is, no flash. Everything that is not reached by the flash, will look like that in the final image. This terminology can be confusing. If somebody says "expose for X" something, that doesn't necessarily mean to find the "correct" exposure with the camera for X. If you are using flashes, there isn't really a "correct" exposure anyway. (at least not what you camera thinks is "correct"). "Expose for X" means dial in the exposure that you want for X.
Again, this is because the flash will not reach the sky and thus the ambient/noflash exposure is the final result, so "dial it in now" and be done with it.
You later add the flash.
Take a look at David's article about balancing flash and ambient. All images below are from this article It is a very good read. It is what my answer is based on. Full credit for the things below to him.
A "correct" exposure
Dialed down to create some "drama"
Exactly what you'd expect, right? Everything is darker. But there is still legibility everywhere - no big black areas. This legibility is important in the final image. [..] We have created a "safety net" of darkened ambient exposure. When we add flash, no part of this image will get any darker. So we'll end up with drama PLUS legibility.
And with flash added
Wow, right? Same exact spot as the first shot above, which was properly exposed open shade. Then we dropped down that exposure to get the sort of "safety-net" ambient-only exposure. Then we lit Jessie (and the back wall.)
I am under the assumption that according to the faq my quotes and usage of his material is ok. If not, please somebody remove it.
[I] may quote small amounts of material (one or two paragraphs)