I am pretty new to DSLR exposure setting with Flash, when I do not use flash, I know how to set Aperture ans Shutter speed to get correct exposure, but when I turn on flash, the metering system can not take that flash light source into calculation to give me current metering value(it is still based on current light condition).
I wonder how to set them to get correct exposure? In A or S mode, the camera may help a lil bit by adjusting the other parameter(mine is D7200, its buildin flash has TTL, so in A, the s changed, and vice versa), and does that mean I need to give couple of shots to manually figure out the exposure in M mode by reading the meta info of picture taken?
my scenario is night portrait with limited street light
There's two things that you may want to do here:
1. Light the scene with 100% flash using TTL
This question has pretty much been answered here. The gist is, set you exposure to just about anything you want (underexposed) in M and the camera will fire the flash to get a proper exposure. The more under-exposed your initial setting, the more powerful the flash - up until the point that you max out your flash's power.
2. Use the flash to add a bit of extra light while relying on the ambient
Let's say that you set your settings above to be 5 stops under-exposed and then took the shot. The photo would be 100% lit by flash. As you adjust your settings toward proper exposure (4 stops under, 3, 2, 1) the camera will use less and less flash to compensate, thus "mixing" the ambient light with the flash. This is often referred to as mixed lighting.
Using a flash to soften shadows that might be caused by a harsh sun or streetlight, for example, is called Fill Flash.
You can find the setting (aperture) for flash using an old fashion method. We are talking about “guide number”. This is a value associated with your flash that can be used to compute the aperture setting. Consult the manual that came with your flash for the guide number ,or you can run a simple test to discover it.
How it works: To use your flash in manual mode, set the shutter to a designated speed required from flash synchronization. Compose and focus your shot and note the subject distance as expressed by the camera’s focusing scale. Actually you are after the flash to subject distance; likely your ability to estimate distance will suffice.
Suppose your guide number is known to be 120 for a setting of 100 ISO, and after composing your shot, the distance flash-to-subject is estimated to be 15 feet. Now you simply divide --- 120 ÷ 15 = 8. This tells us to set the aperture to f/8. Voilà! Your exposure will be spot on.
Seems too simple to be true? You need to know, a guide number takes into account the ISO, flash intensity, and the division works the flash-to-subject distance. The guide number method is not perfect. Variables like the color and height of the ceiling can frustrate however this method works.
Your flash manual will likely publish the guide numbers for different ISO’s and different numbers for those folks that work with metric measurement. Suppose you can’t’ find the published value for your unit? Now we run a simple test. Select a room with a light colored ceiling that is moderate as to height 8 – 10 feet. Set your test subject 10 feet distant and shoot a series at 100 ISO using all the middle aperture settings. It won’t hurt to place a placard to be sure of the test aperture setting. Examine each image for best exposure. Suppose the f/11 image is best. Now multiply distance by f-stop --- thus 11 X 10 = 110. That’s’ the guide number to use for 100 ISO.
Once the guide number for a typical shot is known, you can modify, best to work in 1 f-stop increments. The modifiers are 1.4 and 0.7. Suppose your guide number is 110 for 100 ISO. What will it be for 200 ISO? Multiply guide number by 1.4. Thus for 200 ISO, the revised guide number is 110 X 1.4 = 150. For 400 ISO, it is 150 X 1.4 = 210. For 50 ISO, it is 110 X 0.7 = 75. If outdoors or in an auditorium with high ceiling, multiply guide number by 0.7.
If you think this is too much work, you can simply wet your finger and hold it and make an estimate (gobbledygook).
If you will be using your flash to fill shadows in sunlit vista, this method wills work provided you know the guide number. Again, suppose guide number is 120. Compose your group shot. Set the shutter to a speed that synchronizes. Aperture setting is per camera’s metering, suppose f/8.
Now divide guide number by f-number -- 120 ÷ 15 = 8. This is the distance in feet flash-to-subject whereby flash intensity and sunlight intensity are identical. The fill flash wants to be subordinate. Multiply distance by 1.4 --- 8 x 1.4 = 11. This tells us to place the fill flash 11 feet from subject to fill with flash 1 f-stop subordinate.
Do I need to take a couple of shots to manually figure out the exposure in M mode by reading the meta info of the pictures taken?
Well, you may need to take more than one shot, so you can see how the flash behaves, adjust settings, and reshoot. But with TTL, you may not need to.
Basically, unlike ambient-only exposure, where there's a single exposure level that works as a "good" exposure with flash, there is any combination of ambient exposure and flash exposure that can work from having a black background and all-flash lit scene to a fill-flash scenario where most of the exposure comes from the ambient and only a little from the flash.
Your meter, in M, as you've noted, is only good for setting the ambient exposure, not the flash. But TTL is going to do what you want: put the flash into the scene to meter the flash, and adjust the flash power to what the auto-exposure system thinks is good.
So setting your aperture and shutter speed is mostly to set your ambient level. If you want to kill the ambient, you'd set it to something like -4EV or -5EV. If you want fill flash, you'd set it to something very close to where you would if you were only using ambient light (the meter needle near 0). And if you want to slightly underexpose the ambient so the flash-lit bits of the scene "pop", you set to -1EV or -2EV (or however much you'd like).
You then take the shot. Hopefully, TTL got you what you want. But if it didn't, then you can use flash exposure compensation to adjust the flash power, or exposure compensation to adjust both the ambient and flash exposure together. You can judge the exposure from the shot taken by looking at the histogram and the thumbnail image together (no need to go EXIF diving). If you are using M on the flash instead of TTL, you basically have to make a judgement call on where to start (or you can just try 1/8 or 1/16 power, which are the middle settings of the 1 to 1/128 power range), and adjust from there.
Keep in mind that while ambient exposure is controlled by iso, aperture, and shutter speed; flash exposure is controlled by iso, aperture, flash power setting, and flash-to-subject distance. If you need a brighter ambient but the flash level is fine, use a slower shutter speed or increase the iso/aperture, but adjust the flash power (or hope TTL does it for you). If you need a brighter flash level, but you want the ambient to stay the same, you can increase the flash power, move the flash closer to the subject, or increase the iso/aperture and compensate with a faster shutter speed (assuming you're still within your sync speed; being above your sync speed and using HSS can require more flash power).