Known Issue ID: 15925
The Sigma 18-35mm f/1.8 DC HSM Art lens may show in the camera as a f1.7 lens, and may lock up the aperture. This occurs with Sigma lens firmware v2.00. RED currently supports firmware v1.01, v1.02, and v2.01 for this lens.
Found in Firmware Version:
Upgrade the Sigma lens to firmware v2.01. You can download the firmware from Sigma here: https://www.sigma-global.com/en/download/lenses/firmware/.