How to mix strobe with sunlight
There are two basic concepts to keep in mind---
* The flash/strobe exposure is controlled by the aperture
* The ambient light exposure is controlled by the shutter speed
You are creating a double-exposure in one frame from two light sources.
The first think you need to know is the highest flash sync shutter speed for your camera. This is typically between 1/125 and 1/250 of a second, but varies by camera and could be higher or lower. This information is in your camera manual.
Start by metering the scene using the shutter speed about two stops lower than the max sync speed. For example., if your camera can sync at up to 1/250 of a second start the process by metering at 1/60 of a second (two stops lower than 1/250). Note the aperture required at that shutter speed. For example, let's say the meter said F/8 at 1/60.
Meter the flash/strobe with a hand-held flash meter. If the meter has a shutter speed setting set it to the max sync speed for your camera. For this example, let's say that the flash meter reads F/11.
Adjust the output of the flash to get a reading that matches the ambient light reading from the previous step. In this case we would lower the power of the flash 1 stop so that the meter reads F/8. If you cannot lower the flash power you will need to move the flash further away from the subject or add some diffusion material or neutral density filters over the flash to lower the output.
Set your camera (in Manual mode) to F/8 and 1/125. The scene should be balanced between the ambient and flash.
Now you can adjust your shutter speed for effect. A faster shutter speed will darken the background without making the flash-lit subject darker. But keep in mind that you can't set a shutter speed higher than your flash sync speed. A slower shutter speed will start to overexpose/wash out the background (make it brighter than the subject) But if you go too far you may also end up brightening the subject, too. And you also risk having some motion blur and ghosting around the subject (which is not necessarily bad, it can be an interesting effect).
But what happens if your flash exposure is F/22 but you want to photograph at F/8 or F/11? You have to lower the output of your flash unit. This can be done by lowering the power on the strobe. If you cannot lower it any more you can try moving the lights back (not good if you are trying to keep the light close and soft, moving them back may give too much contrast), or you can change the light quality by adding more diffusion, or you can use neutral density filters over the flash.
Another option is to use a Neutral Density (ND) filter on the camera. With a 3-stop ND filter your flash exposure will go from F/22 to F/8. Meter again for the ambient light with the ND filter in place to determine your balancing shutter speed. If you are using a hand-held meter for the ambient light remember to adjust it to take into account the ND filter. Again, you can adjust the shutter speed up/down to affect the look of the background (remember it has to be lower than the max sync speed of your camera).
Flash exposure f/22
Ambient exposure 1/60 @ F/22 (or 1/500 @ F/8)
Desired aperture F/8
Use a 3-stop neutral density filter:
Flash exposure F/8
Ambient Exposure 1/60 @ F/8