What is noise and how to handle it? (in 5 minutes ;)
I think noise (or “grain” as called in the analog photography) is the most common problem of most of us, especially for photographers who work outdoor without artificial lighting. There are many softwares, plugins and filters who try to deal with noise issue and most of us use them as “subtle” as possible, just because whenever you try to clean noise, you clean some of the valuable details off of your photos too...
Whatever software you use to clean noise, you should first know what are you trying to clean and how you should fight against noise?!
What is noise and why it happens?
Before I give a couple of tricks about handling noise, I think we should know a little about our ennemy!
%99 of stock photographers use digital cameras and digital cameras use a digital sensor. Digital sensors are made of thousands of photodiodes. Small tiny parts which are sensitive to light.
When we boost ISO value, photodiodes become more sensitive to light and heat, as a result they warm more than normal and spread heat to their neighbours. Thus they all go crazy and send wrong values to the image processor of the camera. (That’s why bigger sensors have less noise, they have more area for photodiodes, which makes it easier to isolate them from each other)
Another cause of noise is the base noise of sensor itself. We boost the base noise, when we use higher ISO values. Unfortunately base noise is increased with long exposures too, because sensor have much more time to collect it’s own noise.
Generally, underexposed images or dark parts of images have much more noise. I think, it’s because the left side of the histogram (dark side) is able to contain much less color information then the right side (highlight side). So the difference between mistaken values are bigger and more visible! On the white side, there are same amount of mistakes, but there is much more color information, making the steps smaller and less visible. (I wish I was a native English speaker, I don’t think I can explain myself perfectly here)
So...How to handle noise?
Noise can be handled in two basic ways, during photo shoot and during post process.
During a photoshoot; if you have to boost your ISO value during a shoot. Use +0,7 exposure value, which means +0,7 step more ISO. You will get a brighter shot, which will create less “visible” noise.
During the postprocess; (assuming you have a RAW file) use the raw converters noise reduction very subtly (maybe, until you clean the noise from the brightest parts of the image). Leave the rest to image editor program.
As I already mentioned and most of us already know; dark areas have much more noise. This means we are going to use separate layers during noise reduction. Dark parts of the image don’t grab attention and a viewer doesn’t expect nice details in the dark parts and that gives us the opportunity to boost the noise reduction value.
So we will use 2 different noise reduction values; one for darks and one for highlights (if needed).
Here is how to do it step by step :)
-Duplicate the background layer (ctrl+J)
-Menu>Image>Adjustments>Threshold... this filter will create a totally black and white image. Use the single lever until you get your shadows black and the rest of the image white. (Staying around 60 – 80 will be nice)
-Menu>Filters>Blur>Gaussian Blur... to add some graduation to the treshold image you just created. Do not apply too low or too much, you should have a nice graduation, so that viewers won’t understand the different noise reduction process. Sorry I don’t have any ratio or exact value for this part.
-Press ctrl+A select all...
-Now that we created our mask, go to Channels window and create a new channel using the “Create New Channel” button at the bottom of the window.
-Press ctrl+V to paste the treshold image in the new channel window.
Now that we have the dark parts black (which means “unselected” in a mask), we have to invert our channel to make them white, thus “selected”.
-Press ctrl+I to invert the channel (this shortcut doesn’t work on keyboards with different languages. Menu>Image>Adjustments>Invert is the “longcut” of this shortcut :)
-Press ctrl+D to deselect all
-Now go back to the Layers panel and delete the treshold layer.
-Duplicate the background layer again (ctrl+J)
-Go back to the Channels panel and ctrl+Left click on the new channel. This should create a selection.
-Press ctrl+2 or simply click on the RGB channel on top of the Channels panel list.
-Go back to the layers panel and select the duplicated layer.
-Click on the “Add Layer Mask” button, at the bottom of the Layers panel. (small circle in a rectangle)
If you click on the EYE icon next to the background layer, you will see that we only have dark parts of our image visible. You can modify your mask using brush tool, use black paint to mask out unwanted parts. Remember that you should click on the mask, before painting (I always always always forget it LOL)
Now use noise reduction on that layer. Then re-duplicate the background layer, invert the channels mask and apply it on the new duplicated layer to have a “highlights” layer. Use your noise reduction on that layer too...
Please remember that we cannot save every noisy photo. You better keep an eye on the exposure, because even you are on a low ISO value like 100-200, you can get noise just because you pic is underexposed.
Please correct me if there are any mistakes in this artictle. I'll be happy to learn from you...
Here is a nice "noisy" picture :)) (added post-process noise)
Photo credits: Parkinsonsniper.