What is Godox ALT/MASK mode and how is it used?

Question

Some Godox strobes mention an ALT or MASK custom function (for example on the AD200, AD300, AD400, AD600, AD1200, and QT II/QT III series). What does this mode actually do, how are the groups assigned, and what kinds of photography is it useful for?

user27440 · Accepted Answer

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 n1 or n2 for alternate firing.

On the AD400, AD600, and AD1200, there are two custom function settings: UNIT and ALT.

The 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.

Uses

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.