Photoshop Filter and Tool Tutorials
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Digital Noise Levels in Photography
This Photoshop tutorial is designed to instruct on the use of Photoshop's "Noise" filter which can be used to remove pixel noise, or to add back the noise which is lost after retouching. The term "noise" refers to pixel graininess that is uniformly spread across the image, but the noise levels are typically more pronounced in the shadow areas of the image. A minimum level of noise in the photo also helps to create smooth gradations when the extremes of the gradation are similar in color and tone. Without the noise, the gradation would have a pronounced stair-step effect called "banding" which is especially noticeable in JPEGs which have a compressed amount of colors to begin with.
Noise is inherent in both digital and film photography, and each camera, camera lens, and ISO setting within a given digital camera has its own unique noise signature. When a photograph is shot at a higher ISO setting, or is shot in a low-light situation which automatically forces the camera to a higher ISO setting (in the auto mode), it can increase the noise levels dramatically. Much of this is a function of your digital camera's sensor capabilities, and is therefor unavoidable.
When you are retouching a digital photo, any new element that you add into it must be fully integrated to look realistic. Part of this process involves matching the exact noise characteristics found in your photo when highly magnified (beyond 200%). If the pixel noise does not match the retouched area will be significantly more noticeable.
Using Photoshop's Add Noise Filter
To match your existing photo, you will use a variety of filters to achieve a close match in noise level and quality. This technique is also useful when you are cloning in a part of another photo that has a higher resolution. The process involves a series of steps which must be performed in the proper sequence.
This tutorial assumes that you have already retouched an area of a photograph using the paintbrush or clone tool, and that the retouched area is on its own layer - above the original layer. The retouched layer should be duplicated, with a backup copy or two being turned off and stored under the original layer for safe keeping. First you will zoom in on the retouched area with 50 percent of the screen being occupied by the original layer (below, B) and 50 percent with the new area to be filtered (below, A). The zoom view should be 300% or more, and the larger, the better.
Now that you have a side-by-side comparison it should be easy to see that the new area has no noise while the original area has significant noise. The next step will be to go to the noise filter (Filter>Noise>Add Noise) and the Add Noise dialogue box will appear. Using the "Amount" slider bar you will start at a low percentage of noise - around 5% - and begin to experiment with the level in each direction. You will also want to try both the "Uniform" and the "Gaussian" settings under "Distribution." The Gaussian setting has a slightly more randomized pattern.
You may also want to check the "Monochromatic" setting in the Add Noise dialogue box because this sometimes works better than the unchecked version which can add too mush color to the noise. I prefer to do one layer with each setting and use the layer palette's "Opacity" setting to dial each back a bit - then merge the two together when they match the photo noise. All of this will require some experimentation to get the best match possible but there is one more critical step before the match is perfected - blurring.
Once you have added the noise level that comes the closest to the size and randomized patter of the original you will notice that it is not really possible to get an ideal match at this stage. This is because to Add Noise filter produces a very sharp noise patter which does not match the inherent camera noise which is slightly blurry. This is not a problem because we are going to add some blurriness to the noise.
At this stage you will once-again make a duplicate copy of your retouched (noisy) layer, only this time we are going to use both layers - the copy and the original noise layer. We are going to fade back the top noise layer to an opacity of around 50% and then we are going to go to Filter>Blur>Gaussian Blur. Once the Gaussian Blur dialogue box appears you will use the slider bar to adjust the Radius of the blur until it visually matches the original photo. Then you can adjust the opacity of the blurred layer until a match has been achieved. It is the combination of adding noise and blurring that noise which will give you a very close approximation of the original photo.
Photoshop's Noise Despeckle Filter
I don't recommend using the Despeckle filter to remove noise in your photo as it blurs too much, loosing important sharp edges. If you want to eliminate or reduce noise the best solution is to double or triple the image's resolution before applying this filter, then reduce it back to the original resolution. You can also create a duplicate layer of the original and after applying the Despeckle filter you can dial it back to an acceptable level.
Note: The following hardware and software was used in this tutorial: An Apple Mac Pro desktop computer, a Wacom Intuos 6x8 drawing tablet, Adobe Photoshop CS-CS5 photo editing software and a properly calibrated monitor.
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