使用对焦峰值来评估相机噪点

当涉及到压缩图像时, 拥有尽可能干净的视频会给你带来好处. But what setting on your camera really provides the cleanest video? ISO什么? 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.

我的测试相机是去年购买的松下GH4, 以及我最近为相机套件购买的液晶显示器. 但实际上，你几乎可以使用任何相机. 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. 它能帮我找到摄像头最清晰的噪音分布图.

专注达到顶峰

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. 但是硬件知道这一点, 当某物聚焦时, 那件物品的边缘有强烈的对比, 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. 肩膀上有摄像头, this was used with the little tube monitors that provided the image in the viewfinder. 每个摄像头里都有电路. 达到峰值其实是一回事, only hyper-overdone by pushing the voltage changes between light and dark in the tube monitor.

现在, 带液晶屏, this has to be done with graphics processing software and hardware, 需要一个芯片来采集图像, 评价它, 然后在图像上画出焦点峰值. 这就是为什么不是所有的lcd都有焦点峰值. 它必须是有意添加的.

But one advantage of adding it later is that you can change how it works. 在我的相机里, 我可以设置一个低或高灵敏度的峰值, 改变峰值效果的颜色.

Software is much faster to react than the analog tweaking in video camera viewfinders (下面的图1). 该软件实际上可以看到并突出显示相机噪声. 这是一个缺点, unless you want to utilize this “feature” to get a good look at your camera noise across multiple camera settings, 尤其是ISO.

图1. 一个索尼肩挂相机取景器与峰值控制

iso与现实

ISO，或ASA，来自于在相机中使用胶片. 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. 所以ISO 100的胶卷可以用在任何相机上, and exposed at a certain shutter/aperture to achieve a properly exposed photograph. 更高的感光度是ISO 400. 那么专业电影的评分会高得多，比如B&夜间摄影用w3200胶卷(下面的图2).

图2. Iso 3200 b&W胶卷用于夜间摄影

膜速度越快，膜的颗粒越粗. Lower ISO films had a finer grain and were better at capturing detail. 感光度较高的影片看起来比较粗糙(噪音)(下面的图3), but they were good for capturing images in low light, or when high speeds were needed. 在杂志上，比如 美国电影摄影师协会, 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 (图4，在图3下面).

图3. 多么粒状的胶片啊.

图4. 一系列薄膜颗粒.

当我们进入数字化时代, 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. 通常，较高的iso比较低的iso有更多的噪声. 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.

一般来说，据说相机的“基础”ISO是最干净的. 这是传感器本身提供图像的地方. 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.

用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. 你可能会认为ISO越低, 图像就会越好, 我也是这么想的——直到我测试了它.