Q Recently, a very generous friend lent me his gear for a small project that I was shooting. This setup was from a guy who really knows his stuff and who has taught me a lot, so I thought I was going to know everything I needed to know about his "kit." But when I started assembling things, I realized that I didn’t know much about some of the filters he had. Yeah, I know about grads and polarizers, but he had a filter that I really didn’t understand: an IR filter. I understand that the "IR" stands for infrared, but I don’t know much more than that. As far as I know, he isn’t shooting heat signatures like in some spy thriller. Since I’ve already bugged my friend about this and that, I’m reluctant to show my ignorance about this (my ego may be involved, too).
Via the Internet

A The IR that you’re referring to is infrared. Infrared relates to a particular spectrum of light that isn’t visible with normal human vision. Before I get on to what this filter is doing in a kit, I would like to explain a little about why this filter may be a somewhat new addition to your friend’s gear.

A great deal of this explanation deals with exposure. First, let’s take a look at what you would face when shooting film. If you’re shooting in different situations where lighting-level changes are significant, you might consider changing film stocks, depending on the scene, in order to maintain proper exposure. For low light, you would use a higher speed; for normal scenes, a medium-speed stock; and then for bright scenes, a low-speed film. But changing stock may require careful consideration if they have differences of look that you aren’t happy with.

Of course, you could consider just sticking with one stock and compensate by adjusting the lighting in a scene. That isn’t always practical. (Sorry to you gaffers out there for the rather weak pun!) And if you’re capturing with an image sensor rather than a film camera, you can’t change stock. Yes, you can change ISO, but that will add noise to the image that may not be acceptable.

But, believe it or not, your question really deals with adjusting exposure at the camera and lens, rather than adding or removing light in the scene, so I’ll bypass changing film stock and ISO speed, and deal with adjusting exposure. There are a couple of options for changing exposure.

In order to increase or reduce the amount of light reaching the film or image sensor, your options are to change the shutter speed of the camera, the shutter angle of the camera or the aperture of the lens. Let’s take these one at a time.

The shutter speed of the camera can be adjusted, but with some severe side effects. Namely, you’re changing the frame rate at which you’re shooting. If you’re typically shooting at 24 frames per second, it’s highly unlikely that you’ll want to change that to 48 for a couple of bright scenes. Why? Well, you would be changing the timing of the scene—motion shot at 48 fps would be slowed if kept at the traditional 24 fps playback. So, you’d need to do some motion processing when you got into post. This probably isn’t a good solution for controlling exposure.

A more reasonable solution would be to change shutter angle. For you video folks, who haven’t had to deal with shutter angle, it acts a bit like a shutter speed for each individual frame that’s captured.