Sigma FP User Manual page 60

AF DURING IMAGE MAGNIFICATION
Press the button to magnify the image, adjust the subject to the "+"
mark, and half-press the shutter button. When the subject is focused, the
"+" mark changes to green. Then, release the shutter.
TIP
The shutter is released with the image magnified, so determine the

composition and focus frame position before shooting.
PRIORITIZE FOCUS / RELEASE
(STILL)
You can change the operation that starts when the shutter button is held
down in each AF mode.
To set this option, select [
Release] → [
Release
Focus
(Default)
In, when Drive mode is set to [Continuous], continuous shooting

is carried out at the first frame's focus position even if either setting is
selected.
60
AF shooting is possible during image
magnification. You can focus on a fine part.
[ /
Image Magnification] → [On]
Setting it to [On] displays the "+" mark in
the center of the magnified image.
SHOOT] → [Focus] → [Prioritize Focus /
Priority] or [
Releases the shutter even if the subject is out of
focus.
Does not release the shutter until the subject is
focused.
SHOOT] → [Focus] → [AF during
Priority].