Samsung NX1000 User Manual page 87

When you cannot achieve an app@priate exposure because of a strong
color contrast, lock the exposure and then capture a photo.
After locking _ne exl)osure_ aim the lens where you desire, and press
[Shutter].
To lock the exposure, adjust the photo's composition on which you
want to calculate the exposure, and then press the custom button.
(p. 127)