I've read about an "alt/mask" feature on some of Godox's studio lights (AD200, AD600, QT600II). It looks like it's some method for creating a firing sequence among multiple lights. What exactly is this feature, and what is it good for?
The MASK / ALT Feature
The MASK or ALT custom function on some of Godox's higher-end strobes defines groups and a firing sequence for those groups for multiple lights. Successive presses of the shutter will fire a different group of lights each time. All lights must be in M mode. It's called
MASK with only two groups because it's often used to create silhouette shots/masks for product photography with one group for the backdrop and one for the image. It's called
ALT by Godox when 2-4 groups can be assigned, not just two.
On the AD200, AD300, and the QT II/QT III series of AC monolights, one of the custom functions is
MASK, and you can set it either to group
n2 for alternate firing.
On the AD400, AD600, and AD1200, there are two custom function settings:
UNIT setting defines how many groups of lights you want the firing sequence to cycle through (2-4). The
ALT value sets which group the light is in (1-4).
Once everything is set, firing rotates through the defined groups of lights in sequence. So, group 1's lights fire first, then on the next shot, the second group of lights, and so on.
There are two main uses this function is good for.
The first is for high-volume product photography, where you often want to mask out the background. If you define your key lights as one group, and your background lights as the other, you'll get a shot of the subject, and then get a white-background/black silhouette shot, perfect for defining a mask, as well. This is why the two-group setting is also called MASK on the AD600.
The second usage is to cut down the amount of time required to recycle a light. You can alternate firing among multiple units, effectively halving/thirding/quartering the recycle time between bursts, depending on how many flashes you have set up in this manner. Burst shooting with sports can be possible.
What is this mask?
(I will prepare a couple of diagrams for this)
A mask is an image of the silhouette of what you need to be masked to be used later in post to cut the object and make a transparent background.
If you take a shot of something with a white background and no illumination in the front, you can use that as a mask.
So the idea is taking two shots, one illuminating your subject and other illuminating only the background.
If you want to use this for a person you need to take those two images in a rapid succession.
How can I do this?
This is not an exclusive feature, but something you can create with some strobes that can, in optical slave mode, sync to the second strobe. (S2)
You fire a shot with some flashes sync to the master shot, either optically, by cable or radio.
And immediately you shoot the second photo hoping that only the master flash is firing, and this second shoot triggers one flash that is only lighting the background. This flash thinks that the first shoot was only the "pre shoot" and did not fired, now it does fire when receives the second burst of light.
This means a couple of things.
This master flash needs not to be fired at full power si it can shoot immediately the second burst.
All the other flashes should be at full power so they do not have time to shoot again immediately.
This could potentially work too playing, for example, using a different channel and firing one group of flashes first and then the second group.
If the object is not moving at all, you have time to shoot this flashes in different ways, simply taking a shot first to illuminate the object turning on and off different flashes.