Exposure "Speeds"

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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? 

Photography is built on the three pillars of exposure: shutter speed, aperture and sensitivity. Shutter and aperture are controls for adjusting how much light comes into the camera. How much light is needed is determined by the sensitivity of the medium used. That was as true for glass plates as it is for film and now digital sensors. Over the years that sensitivity has been expressed in various ways, most recently as ASA and now ISO.

The “normal” range of ISO is about 200 to 1600. With today’s digital cameras you can sometimes go as low as 50 or as high as 204,800. The number chosen has two important qualities associated with it. First, it sets the amount of light needed for a good exposure. The lower the number, the more light required. The more light that’s required, the more likely a slow shutter speed will have to be used. That means low ISOs, like 100 or 200, are most often used in bright situations (like sunlight) or when the camera is mounted on a tripod. If you don’t have a lot of light, or need a fast shutter speed, you would probably raise the ISO.

Each time you double the ISO (for example, from 200 to 400), the camera needs only half as much light for the same exposure. So if you had a shutter speed of 1/250 at 200 ISO, going to 400 ISO would let you get the same exposure at 1/500 second (providing the aperture remains unchanged). This is why high ISOs are so often used indoors, especially at sporting events. Needing a fast shutter speed to stop action, photographers regularly choose ISO 1600 or above. The other important quality tied to ISO is the amount of noise in the image. In the days of film, as you used film with higher ISO values (often referred to as ASA then), your images had more visible grain. Film grain is what made up the image, and higher numbers resulted in larger grain, which was more obvious. Most people found visible grain objectionable and so photographers worked to avoid it when possible.

In digital cameras, raising the ISO means a similar decrease in quality, with an increase in what’s called “noise.” It’s the digital equivalent of grain and results in a sort of “chunky” look to the image. Very early digital cameras had objectionable levels of noise at ISOs as low as 800. Today most digital SLRs can make good quality images at ISOs up to 1600 and above. However, several variables affect this.

One important factor affecting the amount of digital noise in an image is the size of the pixels used on the sensor. Large pixels result in less noise than small ones. That’s why digital SLRs perform much better at high ISOs than compact cameras. The SLRs have larger sensors and larger pixels.

Another factor is the amount and type of noise reduction being applied in the camera. Because all pixels collect some noise, every digital camera runs processing on every image (although with a NEF, or RAW, file that can be changed later) to minimize that noise. Newer cameras use newer technology to reduce that noise, with the result being less noise at similar ISOs than what earlier cameras could achieve.

All of this means photographers are constantly doing a balancing act. They want to keep their ISO low for high quality images (low noise), but also they need a fast enough shutter speed to get a sharp picture. That’s why there’s such high value placed on groundbreaking cameras such as Nikon’s D3, D700 and D3S that allow photographers to shoot at higher ISOs with less noise than ever before.

Auto ISO was introduced into digital cameras several years ago to help photographers manage that balance. Turning on that feature allows the camera to push the ISO up when it decides the shutter speed is getting too low for a good picture. Even better, newer Nikon cameras have added “ISO Sensitivity Auto Control” to the menu choices. This takes Auto ISO and lets you have some say about what happens. Using it, you set the limit for how high it can go (800? 3200?) and at what shutter speed it should start raising the ISO (1/125? 1/30?). The amount of control this feature allows means more photographers will start taking advantage of it.

A solid understanding of ISO will help you make smart decisions about how to set your camera. And that, in turn, will lead to better pictures.
— Nikon: understanding ISO sensitivity http://www.nikonusa.com/en/learn-and-explore/a/tips-and-techniques/understanding-iso-sensitivity.html

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

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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.

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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