The work-in-progress mod will be accessible through both the pause menu and a hotkey, and it'll come with all the options you'd want in a photo mode: field of view, view roll, depth of field tweaks, saturation, contrast, colour tints, and more.
You'll be able to change everything in an easy-to-navigate pop up menu as you line up your shot, as seen in the image at the top of this post. You can see more images on the mod's Nexus page (opens in new tab), but you'll need to sign in to your Nexus account first.
Fallout 4 never had a photo mode, but you can fudge one by using various console commands—this will make it much easier. Modder registrator2000 does great work, and has 24 Fallout 4 mods (opens in new tab), so I have high hopes for this.
Here's a video of an older build of the mod in action:
If you're interested, here's Chris's list of the best mods for Fallout 4 (opens in new tab).