Flash compensation vs exposure compensation?

My studio test shots came out relatively dark, my settings:

1/125, f/8, ISO200 with Nikon D90, 60mm prime lens, SB-910 speed light.

How do I get brighter result without changing the shutter speed, aperture and ISO?

I wish I have a studio and experience all by myself.

Your professional advice is greatly appreciated.

Without changing the shutter speed, ISO and aperture, the other ooptions you have for increasing exposure are;

- switch the flash to manual (not TTL - it may be mis-metering the scene) and increase the power by stops until you get what you want. Eg if your images are too dark at 1/8 power, then increase it to 1/4 power - that's a doubling of the power output.
- move the flash closer to the subject (inverse square law applies - if you halve the distance, you'll increase the flash by x4)

If the flash flashed, and the flash-subject distances are not excessive, I would expect the flash to give you an exposure that is right or very close. That is if the flash is in default TTL mode. Is it? If it is in a manual mode with the power turned down, that would explain the underexposure.
On doing more experiments with my D90 and SB600, in TTL mode it can underexpose if the compensation is turned down.
This is all tweaked with the menu of the flash unit itself. I don't think there's an adjustment that needs to be made from the camera body's controls.
I'm of course doing my experiments in a living room which has a nearly white ceiling. If you shoot at longer distances or try to bounce flash from distant or dark surfaces, you would need to turn up ISO or open aperture.

Assuming you are using auto TTL exposure then the speedlite is giving all it has and that is insufficient. So, if you are fixated on those exposure settings:

1. Reduce the distance between speedlite and subject
2. Use a second (or more) speedlite(s)
3. Use a reflector to get more light on the subject
4. Increase ambient lighting

One thing you didn't say is what mode your SB-910 is in.

You also didn't say what mode your camera is in. I'm going to assume your camera is in MANUAL mode.

If you are shooting in studio, the distance between your model and the camera must be relatively short. Having ISO 200, and f/8 (shutter speed does NOT matter in this case), it should be more than enough, ASSUMING your flash is firing at full capacity.

My guess is, it is NOT firing at full capacity. You'll have to put the flash in M(anual) mode, then rotate the thumb wheel on flash and move it toward 1/1 setting.

If that doesn't do it, then you only have three choices. Open your aparture more, incrase your ISO, or get closer to the subject. Actually one more. Your flash has a zoom control. Make sure you aren't using wide angle setting. Set it to 60mm. SB910 will automatically adjust to crop ratio of your camera.

IF your camera is in Aperture priority mode and flash in iTTL, use your flash compensation and increase exposure.