This excellent answer by @MarkAdler to the question Opportunity's last tau was 10.8; what does that mean and how is tau defined and measured? explains that Curiosity uses its PanCam to image the Sun's disk to perform photometry (measuring the Suns apparent brightness) in order to analyze the opacity of Mars' atmosphere due to dust, and includes the first GIF below from Emily Lakdawalla High tau for Spirit and Opportunity.
I stumbled upon the following second GIF below in the Wikipedia article Syzygy_(astronomy)
Do these cameras have adjustable apertures (irises) in order to look directly at the Sun safely and yet still work on Mars near dawn and dusk?
If so, what range of f/no does each camera have?