What’s That NOISE? Part 2
In the last post I covered the most widely recognized cause of digital noise in your photographs and solutions for removing and minimizing this problem. Now I want to cover two other causes and how to combat the different noise that results
Cause #2: Exposure
Didn’t see that one coming did you?
We love digital photography because we can fix our mistakes on the computer after the fact. e all underexpose our images from time to time and lighten them later but this isn’t really any different than what’s going on when you’re shooting at high ISO’s. By underexposing your images you are dropping the signal closer to the level of the noise and by lightening it in Lightroom or Photoshop you are boosting both the signal and noise similar to the way your camera would have done by shooting at a high ISO.
In fig 5 I overlaid two pictures of the same subject. On the left is a photo taken at the proper exposure and on the right is a photo that was underexposed by 2 stops and then lightened. You’ll clearly see a crosshatch pattern much like the effect of shooting through a screen door. This is really unpleasant and easily avoidable.
Um…don’t underexpose your images! Okay, that one is obvious and I know that it’s consistent with your goals anyway. The crazy thing is that many photographers, when first starting out, have been told to intentionally underexpose their images.
Try to get the best exposure possible and beware of what will happen when you shoot at high ISO’s and underexpose…screen door city!
Cause #3: Long Exposure
The process of creating long exposures produces a whole different type of noise and requires another approach to eliminating it.
Every time you take a picture, your camera charges your sensor while the exposure is being made. The longer your exposure the longer the sensor receives the charge. As you may have guessed, the sensor heats up when it’s being charged so longer exposures result in the sensor getting hotter. By using really long exposures (let’s say anything longer than 8 seconds for older cameras and 15 seconds for newer ones) your camera’s sensor starts exhibiting noise due to this heat. This is often called thermal noise and, as you might expect, more heat = more noise.
As your sensor heats up, different pixels on your sensor start to “fail.” This looks like specks of false color that are most apparent in the mid tone and dark areas within your photo (see Fig 6).
Newer cameras do better at long exposures than old ones but every camera has its limit. The fortunate thing about this type of noise is that it’s predictable and repeatable and that makes it easy to remove.
Solution #3 Let your camera do the work!
There’s a setting in your camera, called Long Exposure Noise Reduction (Long exposure NR) that you want to turn ON.
Here’s what it does. Say you take a 10 second exposure. Your camera will operate normally during that 10 seconds but then it will take a second exposure for 10 seconds with the shutter closed creating a Dark Slide. For both exposures the sensor was charged for 10 seconds and in both cases it produced the same thermal noise at exactly the same pixels; in the photo you took and the Dark Slide that your camera took. Then your camera goes through a process called dark slide subtraction in which it identifies the pixels that failed in the dark slide and fixes those exact pixels in your photo. Some cameras differ in the way they do this but the process works like magic and there is no equivalent in computer post processing that comes close so be sure to use this awesome camera feature.
Turn it on and leave it on, it only goes through the process on long exposures.
BE WARNED! You’ve got to remember that this feature is on so that you don’t think your camera is broken the first few times you use it. Remember that your camera is taking a second “picture” after it took yours. If your exposure was 30 seconds long then the dark slide is also 30 seconds long. That means your camera will prevent you from doing anything (like hitting “play” to see your photo) for those 30 seconds and then a few more while it performs dark slide subtraction. You’ll be standing there in the dark thinking your camera is broken but it’s just doing its job. In fact it will tell you so on the top of the camera. It will say something like JOB or NR on the LCD. Let the camera do its thing and don’t turn it off during this process. When it’s finished, prepare to be amazed with your gorgeous image, free from thermal noise.
BE WARNED #2. Long exposures eat up batteries and you’re taking two of ‘em for every photo. Be prepared to go through batteries quickly!