Question: I’ve looked as many of your nightscape photos as I could and it seems that all of them are so noise free. Like you, I use a Canon 6D or the but I always seem to get that magenta colored noise in the foreground when I shoot at 25-30 seconds. That’s why I do composites for most of my nightscapes. I see that you are using a Nikon 14-24 mm lens. Is that your secret weapon? Or do you some other secret? How do you keep down noise?
Answer: I use the Canon 6D or the Nikon 810A cameras. The magenta color comes from lightening the underexposed dark areas. It is a little worse on the Canon as compared to the Nikon, but can occur with any camera. There is no secret weapon unless it is the cameras themselves. Both do incredibly well at hight ISO.
The key is to pay attention to detail at every strep of the process. Remember that anything you do early in the processing can snowball and make noise worse at each subsequent step in the processing. Remember also that contrast and sharpening can make noise appear worse so you want to minimize or simply eliminate any sharpening and contrast increase EARLY in the processing. Also remember that the sky and foreground have to be treated separately. The sky is a very high contrast subject (light stars and dark sky), and the foreground is an extremely flat or low contrast subject with little dynamic range. Also remember that the appearance of sharpness is created by 2 things, lens/sensor resolution and contrast. Since the sky is hight contrast you do not need to do much sharpening. You will only increase the appearance of noise there. The foreground in low contrast and may need more sharpening.
This is what I do:
(1) Image acquisition - try to shoot to the right (of the histogram). There is a lot more info in the pixels to the right side of the histogram, and less on the far left dark side. A lot of respected Astrophotographers are going to ISO 12,800. The D6 or 810A can handle this ISO. When you push the histogram to the right then you are moving the dark pixels more to towards the middle of the histogram, and there is a lot more info in the pixels. The thought is that the same photo taken at ISO 12,800 will push the curve to the right and has more info in the dark pixels that the same photo taken at 6400, or 3200. You can deal with the noise in Lightroom of Adobe RAW. When you push the histogram more to the right, there is much less of the magenta color in the dark areas when you lighten that area. You get less noise and less magenta in the dark areas, and you can see much more foreground detail.
Many experts argue that you lose dynamic range when you push up the ISO, and technically they are correct, BUT there is little dynamic range in night photos to start with. There is extremely limited dynamic range to deal with in the foreground, and low dynamic range in the sky as well. Landscape Astrophotography is completely different than daytime photography where you are trying to expose bright accents properly and see into shadow areas at the same time. Having perfect dynamic range in your photo is not going to help you when there is no dynamic range in your scene. It is best to expose the dark areas properly from the beginning and then deal with the high ISO noise in post processing.
(2) Open the image in Lightroom or Adobe RAW. In Lightroom or camera RAW I am NOT trying to make the image look good, I am only prepping it for work in Photoshop. Always acquire RAW images when shooting. Use the lens profile correction function. The vignetting will pop up at 100%. Reduce it to about 50-60% or so. Too much vignetting removal will bring out that magenta color in the lower corners. I then use the temp and tint sliders to make the DARKEST part of the sky as neutral as possible (grey). The lower sky does not matter at this stage because it is altered by light pollution and air glow. If I cannot make the darkest sky a neutral grey then I err to the slightly blue side. Concentrate on the upper and darkest sky, not near the horizon. Do not make the sky blue at this early step if possible. I then go to the curves function in LR or Adobe RAW and I create a steep "S" curve (to increase contrast) and then re-examine the sky. I make fine adjustments to the Temp and Tint sliders to make the darkest sky a neutral blue (as above). THEN RETURN THE CURVE IN THE "CURVES" FUNCTION BACK TO A FLAT LINE. REMOVE THE "S" CURVE.
DO NOT use Clarity or Saturation or Vibrance at this time. DO NOT do any real sharpening at this level. Leave the shahrpening at the default of 25%, and increase the mask to 75%. DO NOT do any contrast adjustments at this level. If you do it will only get worse later. I use a Luminance Noise factor of about 50 for the 6D, and detail of 50% and the Contract at 0. I use a Color Noise Factor of 17-20, and set the Smoothness slider to 100%. Use the Shadow Slider to lighten up the dark areas to your personal taste, but do not dog above 80%. Again, DO NOT use the Clarity Slider. The image will look very flat. Click Edit or Export to Photoshop or click the “Open Image” in Adobe RAW.
(3) In PS go to the Select Menu and choose "Select and Mask". You Select or Mask the Sky or Foreground and place them in separate layers. Alternatively you can select or mask the foreground or sky by whatever means you want. You need to process them differently. I place the sky and foreground in separate layers and process then separately.
(4) For the Foreground Selection: I may use the Shadows/Highlights Function the lighten up the dark areas (a little) using a setting of 3 to 5 (very low). Then increase the contrast a Little, by using the Curves function and place a Minimal"S" shaped curve on the foreground. Alternatively you could open the Levels function and move the shadows slider to the right. I may use the Topaz Denoise plugin to further decrease noise in the foreground. It may make the foreground soft. I then do some limited sharpening. Remember, sharpening increases the appearance of the noise, so I go about it differently, and use the Unsharp Mask or Smart Sharpening to INCREASE LOCAL CONTRAST. This is a lot like the Clarity function that you find in Lightroom. Do this only after you have done any color correction on the foreground, and you are happy with the foreground otherwise. It is the opposite of normal sharpening. Use use an amount of 10-20 and a radius of 40-60. I usually use 15 and 60. It increases local contrast and the Perception of sharpness without increasing the perception of noise. If there is magenta in the corners I select that area and go to Hue/Saturation function, choose Magenta, and decrease saturation. Alternatively you can select the magenta area and go to the Color Balance function and add green, or a third way to get rid of the magenta is to paint or brush over it with green. You will not need to do that much usually.
(5) For the sky selection, I first use the Levels Command and take the Shadows slider and move it to the right, making the darks darker. This does not do much to the lighter shades. I AVOID the Curves function as much as possible. You do not really need it (except for maybe slight adjustments at the end), and adjusting “Curves” changes the colors, making them much more saturated, and they get out of hand very quickly. You can adjust Contrast and Brightness with the Levels Command and the Contrast Command, and it is MUCH easier to keep the colors and noise under control. Curves can also make the noise look worse, I may occasionally increase the vibrance about 10 or so. It helps to darken the sky without increasing the Contrast so much. I then go to the Color Balance Command, and increase Blue slightly. It does not take much. I then go back to the Levels or Contrast Command and adjust the Milky Way and Sky to the brightness or darkness I desire. I primarily darken the sky with the "Levels" command, by moving the dark slider on the left of the histogram. I may use a little Dodging or Burning. I Do Not sharpen the sky. I prefer the sky to look a little softer rather than crunchy or crispy. Remember the sky is already a high contrast subject, so you do not really need to sharpen it to have the perception of sharpness. Over sharpening the sky can look very un-natural. Remember, perceived sharpness is a function of contrast and resolution, you already have one component of sharpness (hight contrast) in abundance.
So managing noise requires you to be diligent throughout the process, from acquisition to the end of processing. The most important thing is to not make noise worse in the processing. If you magnify noise at the start of processing, it will only get worse at each step along the way.