Today’s Question: If a photographer opts to purchase a camera that is not equipped with a Low Pass Filter, is there a way to remove moire effects in Photoshop if they should appear in some images?
Tim’s Quick Answer: Yes, there are several options available when it comes to removing moiré patterns in Photoshop. The first approach would be to avoid them in the first place, of course, by adjusting focal length or other capture settings. But you can also reduce the appearance of moiré patterns using Adobe Camera Raw, the Camera Raw filter, or a simple blur applied selectively to the image.
More Detail: To begin with, you can generally avoid moiré patterns in the first place if you are aware of their potential. Simply by changing the focal length of the lens (if you are using a zoom lens), or changing the lens aperture, you can eliminate the interference patterns that cause moiré patterns. If you’re using a camera without a low pass filter you will obviously be aware of the potential risk, but you also need to evaluate the scene to determine whether there is fine texture that may lead to moiré patterns. Reviewing the images at a relatively high magnification on the camera’s LCD display will allow you to determine if the current camera settings are causing moiré patterns, and you can take steps to adjust your capture settings to avoid those patterns.
If you are processing a RAW capture that contains moiré patterns, you can use Adobe Camera Raw (or by extension, Lightroom’s Develop module) to reduce the appearance of these patterns. Simply select the Adjustment Brush, making sure that all of the adjustments are at their default values. Then increase the value for Moire Reduction to the maximum value of 100, and paint on the image in the area where the moiré patterns appear. You can then reduce the value for Moire Reduction to the minimum level required to remove the interference patterns.
The same adjustments referenced above can also be found in the Camera Raw filter if you are using Photoshop CC for a non-RAW capture (or a RAW capture that had already been processed). Simply choose Filter > Camera Raw Filter from the menu, and use the same controls referenced above.
If you are using an older version of Photoshop without the benefit of the Adjustment Brush in Adobe Camera Raw (or the Camera Raw Filter), you can also apply a selective blur to the image. For example, you can create a copy of the Background image layer by dragging the thumbnail for that layer to the Create New Layer button (the blank sheet of paper icon) at the bottom of the Layers panel. Then choose Filter > Blur > Gaussian Blur, and apply just as much blur as is needed to remove the appearance of the moiré patterns.
Next, add an inverted layer mask to the Background Copy layer you created by holding the Alt key on Windows or the Option key on Macintosh while clicking the Add Layer Mask button (the circle-inside-a-square icon) at the bottom of the Layers panel. This will add a layer mask that is filled with black, so the blurred version of the image will disappear. Then use the Brush tool with a soft-edged brush to paint with white into the areas where the moiré patterns appear, so that the blur effect only applies to those areas of the image.