Using Focus Peaking to Assess Your Camera Noise

When it comes to compressing an image, having the cleanest possible video pays dividends down the line. But what setting on your camera really provides the cleanest video? What ISO? What in-camera noise reduction can ensure that you’re not wasting bits preserving noise instead of important details in the low-bandwidth video stream? I found an easy way to use a camera’s or external monitor’s focus peaking feature to assess the camera’s noise profile as I change various camera settings.

My test camera is the Panasonic GH4 I purchased last year, and an LCD monitor I recently purchased for my camera kit. But really, you could use almost any camera. The external monitor has focus peaking, and the camera itself has focus peaking as well. Truth be told, the in-camera focus peaking is way better than that in the external monitor. But when I was trying to show someone how bad the external monitor's focus peaking was--and how it was highlighting every bit of noise in the image--I realized that this could be useful for assessing camera noise as I change camera settings. It could help me find the camera’s cleanest noise profile.

Focus Peaking

Focus peaking is a process that looks at the contrast between elements within an image. It doesn’t actually know what is in or out of focus; we determine what part of the image we actually want to be in focus. But the hardware does know that, when something is in focus, the edges of that item have sharp contrast, a change between bright and dark in a very small area of the frame. Think of a person’s eyelashes--very thin lines that pop when you have critical focus.

Focus peaking tries to help you see it by accentuating the contrast. With on-shoulder cameras, this was used with the little tube monitors that provided the image in the viewfinder. There’s a circuit in every camera for detail. Peaking is simply the same thing, only hyper-overdone by pushing the voltage changes between light and dark in the tube monitor.

Now, with LCD screens, this has to be done with graphics processing software and hardware, requiring a chip to collect the image, assess it, and then draw the focus peaking on the image. This is why not all LCDs have focus peaking. It has to be deliberately added.

But one advantage of adding it later is that you can change how it works. In my camera, I can set a low or high sensitivity for peaking, and change the color of the peaking effect.

Software is much faster to react than the analog tweaking in video camera viewfinders (Figure 1, below). The software can actually see and highlight camera noise. This is a downside, unless you want to utilize this “feature” to get a good look at your camera noise across multiple camera settings, particularly ISO.

Figure 1. A Sony shouldermount camera viewfinder with Peaking control

ISOs and Reality

ISO, or ASA, comes from using film in cameras. It was the way they standardized how the sensitivity of different films were rated. This informed the user how they could adjust any camera for proper exposure on different film stocks. So a film rated at ISO 100 could be used in any camera, and exposed at a certain shutter/aperture to achieve a properly exposed photograph. Higher speed would be an ISO 400. Then specialty films would be much higher, like a B&W 3200 film for night-time photography (Figure 2, below).

Figure 2. ISO 3200 B&W film for night photography

Films would have coarser grain with the higher film speeds. Lower ISO films had a finer grain and were better at capturing detail. Higher ISO films had a more gritty (noisy) look (Figure 3, below), but they were good for capturing images in low light, or when high speeds were needed. In magazines, like American Cinematographer, you could read about why certain cinematographers chose certain film stocks and film speeds for different parts of their movies. The film’s grain was part of the character of a film and was often deliberately picked (Figure 4, below Figure 3).

Figure 3. What grainy film looks like.

Figure 4. A range of film grains.

As we moved to digital, the easiest way to manage the transition was to make the sensor behave like film and allow the user of the camera to change “ISO speed” like changing films. Generally, higher ISOs have more noise than lower ISOs. But digital chips (first CCDs and now CMOS imagers) are not exactly like film. The size of their pixels doesn't change like the size of the crystals in film. So how sensors interpret light is handled electronically instead.

Generally, it’s said that a camera's “base” ISO is the cleanest. This is where the sensor natively provides the image. Settings above this require amplification of the image, and that creates noise. Settings below this base ISO often lose detail and latitude by the processes that are used to deliver a “lower” ISO than the sensor’s base. Sometimes camera manufacturers don't even reveal what a camera’s base ISO is.

With the GH4, several base ISO numbers have been mentioned by different people, but there is no official “Base ISO” specification from Panasonic that accompanies this camera. You may think that the lower you go with ISO, the better the image will be, and that’s what I thought too--until I tested it.