In studio shoots, you use a flash meter.

But if you do not have one, can you do it? Sure you can. Here’s a trick:

1. Set up your lights. Guess the light’s power setting.
2. Get a grey card, and hold it in the exact spot where your subject will be, aimed half way between the light and the camera, as your model may be.
3. Set focus to manual (we are worried here about exposure, not focus!)
4. Fill the viewfinder entirely with the gray card (be sure not to block the light)
5. Click.

Now review the pictures. Press INFO or DISP, or hit UP/Down, until you see the view that includes the histogram.

Now here’s the trick. A good picture has the histogram peak (or peaks) in the centre. So if you see this, you are ok:

What if you see this, a histogram on the left side:

That means you are underexposing. You need to turn up the flash power and try again:

And if you see this, the histogram on the right side:

The histogram is on the right; you are overexposing: turn down the flash power, wait a few seconds so it can dump its excess charge, and try again.

As soon as you are in the centre, take a real shot and check – you should be OK. And you metered it – and all without a light meter!