Bill Brown refers to elements of exposure as "4 Speeds"
1. Film Speed
2. Shutter Speed
3. Camera Speed
4. Lens Speed
Since we are not shooting film; I'm going to focus on things that are analogous to the above "speeds" in digital video.
So as Brown states, filmmakers and videographers like to talk about "stops" as increments of exposure. If you change something by a stop, you are either halving or doubling the amount of light exposing your chip.
1. Film Speed refers to the ISO of film stock.
Remember that film is a photo-chemical process and it consists of grain, dyes, and gelatin on a plastic base. You can't see the exposure until someone at a lab dumps it in a bunch of chemicals.
Film that is more sensitive to light has a higher ISO (like 500T) and can work better in low-lit scenes. Depending on the stock, you tend to see more film grain with faster speeds. Interestingly enough, viewers tend to be more comfortable with film grain rather than video noise or pixels. Maybe it's an association with old home movies, but grain is usually considered aesthetically pleasing.
The following is a scene from Paris Texas, with the late/grate Harry Dean Stanton. It was shot in Super 8 and blown up to 35mm, a process that also shows a lot of grain.
Conversely, film with a lower speed has tighter grain and tends to look super sharp. That tends to be what we see the most of when we go to the movies whether we are watching a motion picture or HD projection.
Christopher Nolan quite famously shoots on film, and his movies look incredibly sharp with no visible grain.
So, how does film grain relate to digital video?
I like to think of the ISO as a form of gain and increasing exposure using that way is not optimal because you get digital noise--not to be confused with the more aesthetically pleasing film grain.
A couple of benefits of DSLR's
Usually DLSR's have an option for increasing gain that is aligned with film ISO's. That means that you can increase or decrease the gain by a stop at a time. (doubling or halving) If you are trying to be precise with your exposure, being able to change the gain by stops can be helpful.
Another thing about DSLR's that they tend to have larger sensors and therefore larger pixels which decrease the look of noise. That's why many DSLR's tend to work well in low light.
Shutter Speed and Camera Speed
I'm going to blend these two, because shutter speed on digital video cameras works a more like camera speed.
In motion picture film, shutter speed refers to the shutter angle and how large the opening is on the shutter wheel. The larger the opening, the more light that is allowed to expose the film.
Camera speed refers to how many frames per second the camera is recording. A slower film speed (fewer frames) increases the exposure by one stop. Example: changing the camera speed from 24fps to 12 fps will increase the exposure by a stop. Note: film shot at 12 fps running at 24 fps looks like fast motion.
In still cameras shutter speed controls how long the film or chip is exposed to light. Again, each change is considered a stop and either halves or doubles the exposure.
Just like with apertures, these numbers represent fractions, 500 means that the film or chip will be exposed for 1/500th of a second and if you stop up to 250 you will double the amount of light exposing your shot.
Shooting at 1/500th a second can capture very fast motion and shooting 1/30th will sometimes cause motion blur in a still, so you will have to use a tripod to get a sharp image.
You can get cool shots like this at slower shutter speeds:
At a slower shutter speed like 1/60th, the figure standing still is sharp, and the train has a lot of motion blur.
In video, the shutter speed controls the length of time the sensor collects light while each frame is being captured.
The numbers are the same, but the effect is slightly different. When the shutter speed is low, motion blur is increased, and you increase exposure a stop at a time. When you have a high shutter speed, you decrease the exposure, but get a video with less motion blur. The effect is similar to changing the shutter angle on a film camera.
Watch the following film on creating the look of certain shutter angles by adjusting the shutter speed and fps on your digital video camera.
Lens Speed: we have gone over this before, but I'll sum it up again. If a camera has a "fast" lens that means the aperture opens up wide and will allow more light in. A "fast" lens may open up to 1.4, and "slow" lens may just open up to 4.0. Prime or fixed lenses are more likely to be fast.
So if you are in a low light situation what do you use first?
- Check and make sure you have your neutral density lenses turned off
- If you don't mind shallow depth of field, I will start by opening up the aperture
- If you want standard motion blur that looks like a 180-degree shutter angle, then double the frame rate with your shutter speed. If you don't mind a little more, then match the frame rate with the shutter speed.
- I always save using the ISO or gain as the last resort, to minimize noise. The default setting for me is "0" or a low ISO like 100