Someone told me I should shoot all my video with the iris wide open, but I can’t remember why, and it doesn’t really make sense to me. Shed some light?
This person was probably assuming you were going for that much-hyped film look with your video.
Film looks the way it does for many reasons, but one of the main identifiers is depth of field, or lack of it, more accurately. That’s how much of the image is in acceptable focus from front to back. One giveaway that you’re shooting video is that most everything tends to be in focus, whereas film (because of the lenses beaming onto a relatively large negative compared to video’s tiny chips) tends to have a comparatively small depth of field. While some may see that as a disadvantage, in many circumstances it’s far sexier, as it draws your attention to what’s important and blurs the rest, like most portrait photography.
Iris affects depth of field. Wide open means that as little as possible will be in focus, while a stopped-down iris gives you deep focus, so that’s probably what s/he meant. Even more effective than iris, though, is lens length. For the smallest depth of field, combine a wide iris with the telephoto end of your zoom, physically moving the camera back until you get the desired framing. Best of all would be to use a 35mm lens adapter system, but those are pricey and cumbersome.
Speaking of exposure, a couple more things. A wide-open iris brightens the image, so this is the way to go if you need to maximize low light levels. But first check that any gain is disabled to avoid electronic noise. If you still don’t have enough sensitivity, check the shutter speed, and slow it down until you do. I wouldn’t go below 1/30, and preferably 1/60. Beyond that, you’ll need to augment with lights or change your position. In low-light situations, avoid the temptation to overexpose. Some parts of the frame are meant to go dark, so let them. A face doesn’t have to be at 75 IRE all the time (that’s Zone 6 for you Ansel Adams dorks). Digital cameras tend to be noisy in the blacks anyway, so let’s not accentuate them unnecessarily.