How to reduce the hateful noise in starry sky photography?

In photography, noise reduction must be often heard, so what is noise?

The noise is disturbing. In other words, noise refers to the point that interferes with imaging. Mainly refers to the rough part of the image produced when CCD(CMOS) receives and outputs light as the received signal, and also refers to the foreign image that should not appear in the image, usually caused by electronic interference. It looks like the image has been dyed and covered with some tiny rough spots. The digital photos we usually take may not be noticed if we use a personal computer to reduce the high-quality images and then look at them. However, if the original image is enlarged, there will be a color (pseudo-color) that did not exist originally, and this pseudo-color is image noise.

The cause of noise originally came from long exposure. This phenomenon of noise caused by long-term exposure mainly appears when shooting night scenes, and some isolated bright spots appear in the dark night sky of the image. It can be said that the reason is that CCD can't handle the huge workload caused by the slow shutter speed, which leads to some specific pixels out of control. This kind of noise was originally absent, and it was caused by long exposure. Secondly, we know that in order to increase the brightness under low illumination, high sensitivity is often used. While improving the sensitivity value and gain range, the existing noise information is also enlarged accordingly, so the noise is more and more obvious in high ISO images.

There are two main types of noise. Next, I will introduce the types of noise.

Fixed noise: the bad point of the camera sensor, the inherent noise generated by the camera working at a specific temperature, etc. That is, each photo has noise with a fixed color brightness at a fixed position.

Random noise: noise with random brightness and color at random positions caused by high iso signal amplification and thermal noise.

If you want to see the noise, you need to adjust the sensitivity of the camera to above 3200, cover the camera, take a black photo, and then enlarge the photo to see if there are any bright spots.

Different cameras have different abilities to deal with noise, and the high sensitivity level of cameras has also developed greatly in recent years. If you use a portable digital camera, the noise of ISO 400 may be similar to that of ISO 3200 of other cameras. And if you use the latest digital SLR camera, it can even be bought over ISO 800. But it doesn't mean that a camera with high pixels has no noise. If you are exposed for a long time or have high sensitivity, there will still be a lot of noise.

There is a long exposure noise reduction function in the camera. Long exposure noise reduction usually does not need to be turned on. If it takes more than 30 seconds, the shutter will open, because long exposure will make the photosensitive element heat up and produce noise, such as shooting the star track.

If you pursue perfection and have enough patience, you can continue to look down. Take the star orbit as an example. Enlarge your starry sky photos to 100%. Did you find many dots of different colors in the dark part of the picture? These points are noise, which makes the picture look rough and dirty and affects the photo quality. Noise reduction is a method to reduce the influence of noise.

Noise diagram

Let's take a look at the steps of starry sky photography first, and then see how to improve the steps to reduce noise.

1. Choose a solid place and put your tripod firmly. In order to take clear pictures of the Milky Way and the stars, you must first make sure that your camera is still. Note that some ground is not as solid as you think, such as muddy ground or beach, which will cause the tripod to sink very slowly. If the equipment must be installed in these places, please insert the tripod into the ground.

2. Turn your camera to M gear (manual gear), turn off the lens anti-shake stabilizer (if any), and turn off all noise reduction functions of the camera (usually high ISO noise reduction and long exposure noise reduction, which can be found in your camera setting menu).

3. Switch the lens to manual focusing mode and rotate the focusing ring to infinity indicated by the scale line (i.e. this symbol ∞). Note that focusing on infinity does not mean turning your focus ring to the end. Usually, the infinite symbol is rotated back a little bit from the bottom position.

It is no problem to turn the aperture of your camera to the maximum (the aperture value is the minimum). If the weather is excellent and your aperture is large enough (for example, 1.2, 1.4), you can also take back one gear.

5. Select the shutter speed according to the focal length of the camera. For a 24mm lens, I suggest that the exposure time should be controlled within 15 seconds, so that the obvious trailing afterimage will not be caused by the movement of stars in the final photo. If you have a wide-angle lens above 16mm, you can try to extend the exposure time to 20 seconds or even longer, but generally it should not exceed 30 seconds.

6. Select ISO according to the required exposure. You can take several photos with different ISO values and preview the effect. If the starry sky is clearly visible in the preview, there is no problem. Usually the recommended voyage is from ISO 1600 to ISO6400. Too low ISO can't completely expose the starry sky, while too high ISO will cause too much noise.

7. Aim at the starry sky you want to shoot, lock the knobs on the tripod and pan/tilt, and press the shutter. It's over.

In order to reduce the noise, we need to slightly modify the last step of the above shooting process: for each shooting of the starry sky (including every part of the starry sky that you use for stitching, if you want to do panoramic stitching later), don't just shoot one, but shoot by the following methods: fix the camera position and parameters, and shoot 65,438+00 photos in the same position continuously (these photos are called bright fields). At this point, if there is a regular cable release, use automatic. After taking 10 photos, cover the lens immediately, and take 5 all-black photos (these photos are called dark fields) with all parameters such as ISO aperture shutter unchanged. Every subsequent starry sky photo was taken according to this process.

To sum up, noise is a point caused by electromagnetic interference during CCD imaging. There are two reasons for noise, one is caused by high sensitivity, and the higher the sensitivity, the higher the noise, and the other is caused by long exposure. The longer the exposure time, the more obvious the noise. I also explained the general steps of starry sky photography and the steps to reduce noise through bright and dark fields.

I hope you can learn something after reading it.