Many of today’s cameras come with built-in filter effects that can be applied”¨ to shots as you take them. They are only applied to JPEGs because raw files are, by definition, created without adjustments. However, most (if not all) filter effects can be applied once images are in an editable format (JPEG or TIFF) and there’s plenty of software to help you.
A typical dropdown menu (this one is from GIMP) showing the wide range of filters available in a typical image editor. Each filter usually provides two or more variations, the ones for distortion are shown here.
Why use filters?
Most reasons for using filters fall into “¨two categories: useful filters and effects filters. Useful filters include sharpening, blurring, noise suppression, rendering for lighting effects and lens correction.
Typical effects filters include those that add mosaic or ripple patterns, simulate oil paintings, crayon drawings or pencil lines, introduce colour effects that boost or subdue colours or change colour balances. Other common filters include linear effects like bas-relief (which makes subjects appear as if they’ve been carved out of stone or hammered in metal), distortions like whirl and pinch and texture effects like craquelure, embossing, stylisation and granulation.
Arguably, all digital images require a little post-capture sharpening to correct the slight blurring that results from combining three sets of pixels (red, green and blue) to produce a full-colour image. Unfortunately, although some sharpening is usually applied to JPEG images in-camera as part of the normal image processing, and there are plenty of sharpening tools for post-capture sharpening, some techniques are better than others.
When done correctly, sharpening will make almost any image look better and improvements created can be equal to upgrading to a higher-specified camera lens. However, over-sharpening will produce unattractive artefacts, which show up as speckling or outlines along edges in the image. Under-sharpening will leave images looking slightly soft.
Locating the Unsharp Masking filter in Photoshop.
The best and most popular sharpening tool is the Unsharp Masking filter, which applies sharpening by increasing contrast along contrast gradients (edges). This filter provides a high degree of control over the sharpening process. The key to effective sharpening is creating a balance between emphasising edges while at the same time minimising the under- and over-shoots that create sharpening halos.
This illustration shows the effect of sharpening with the Unsharp Masking filter. The image on the left is the entire frame as it came from the camera. Part of this frame has been enlarged in the upper section to the right. Unsharp masking has been applied to this area and it is reproduced in the lower right section. Note the differences between the unsharpened and sharpened images.
Most editors provide three adjustments within the sharpening filter: Amount, Radius and Threshold. The amount slider controls the strength of the sharpening and is usually listed as a percentage. A good starting point can be at 100%.
The Radius slider controls the width of the edges over which the sharpening will be applied. The smaller the radius, the more detail is emphasised. Try to match the radius setting with the finest detail in the image.
The Threshold slider controls the minimum brightness that will be sharpened. Its main use is to avoid sharpening noise with a setting that”¨will sharpen more distinct edges, while leaving subtle edges unaffected. Watch the preview image while adjusting this slider to obtain the best results for the image.
Note: if noise reduction is required,”¨it should always be carried out before sharpening since sharpening will reduce its effectiveness. Consequently, most photographers apply sharpening at the end of an editing session.
The three adjustment sliders in Photoshop’s sharpening filter.
Sometimes you want to blur part or all of an image, either to make part of the scene stand out, to subdue blemishes or to create an artistic effect. If your image editor includes a sharpening filter, there’s a good chance it also includes a Blur filter. This filter works by decreasing contrast among adjacent pixels within the selected area. It is located in the same dropdown menu as the sharpening filter.
Locating the Blur filter in Photoshop.
Sophisticated editors like Photoshop provide a ‘Gallery’ of different blurring tools for different applications. Inside the Blur sub-menu you’ll find the following settings: Average, Blur, Blur More, Box Blur, Gaussian Blur, Lens Blur, Motion Blur, Radial Blur, Shape Blur, Smart Blur and Surface Blur.
The Average setting simply takes in everything that has been selected and averages the colours, providing one uniform colour to smooth things out.”¨It is mainly used for removing texture or noise from a photograph or selection and removing colour casts from images.
The Blur and Blur More settings find contrast gradients and apply blurring primarily to them. The Blur setting is quite subtle; the Blur More setting more emphatic.
Box lur averages pixel colours in a selected area. Users can select a radius to define the ‘box’ in which the blur will be applied, which determines the extent of the blurring. The effect isn’t realistic so it’s best reserved for special effects.
Gaussian Blur has many practical uses, largely because it applies a much smoother blur than Box Blur. It’s useful for smoothing skin tones and applying soft focus effects.
Using the Gaussian blur filter in GIMP to aply a slight softening smoothes the texture of the subject’s skin.
Lens Blur attempts to mimic the background blurring you get from shooting with a wide lens aperture. Photoshop lets you specify the size and shape of the iris diaphragm to simulate depth-of-field blurring.
Motion Blur is used to simulate”¨the blur obtained through panning. You can set the angle and direction of the movement and the strength of the blurring effect.
Radial Blur replicates the effects of zooming during an exposure or rotating the camera while the shutter remains open. Simply apply the filter and select Zoom or Blur then drag the plus sign”¨in the Blur Centre box to set the centre of the blur. Three quality options are available: draft, good or best.
Shape Blur applies a blur by blurring pixels within a selected shape (called a kernel). We can’t think of any practical applications for this setting. Smart Blur is also rather limited and mainly useful for reducing grain in an image, without affecting image detail.
Surface Blur applies blurring to areas where pixels are slightly dissimilar, leaving areas with dramatic differences intact. It’s handy for retouching skin blemishes and removing wrinkles and can also be used for reducing noise.
In addition to these options, there’s also a Blur Gallery with five settings: Field blur, Iris blur, Tilt-shift, Path blur and Spin blur. Each blur tool provides on-image controls for applying and controlling the blur effect plus Bokeh controls to apply additional adjustments.
The top screen grab shows how you can create a layer to isolate the area you want to keen sharp while blurring the background. The lower picture shows how the Iris blur tool in the Blur Gallery can be used for background blurring. The red circles indicate the adjustable controls in each filter.
Another useful filter is the Noise filter, which can be used to reduce the visibility of noise in images captured with high ISO settings. This filter also contains a setting that lets you add noise, in case you want a speckled, grainy looking picture.
The Noise filter options in Photoshop include an Add Noise setting as well as Despeckle, Dust and Scratches, Median and Reduce Noise.
With all noise-reduction software, the key word is ‘reduction’. You can’t actually remove the noise without blurring out most of the detail in the image as well. Selecting the Reduce Noise setting opens a dialog box with sliders for adjusting the Strength of the processing, how much emphasis to place on preserving details, controlling colour noise and sharpening details. There’s also a checkbox for removing JPEG artefacts (which can adversely affect noise reduction).
An enlarged section of an image taken at ISO 12800 showing quite substantial image noise. The dialog boxes to the left show the Basic and Advanced dialog boxes for applying noise-reduction adjustments.
Photoshop’s noise reduction filter opens with a Basic dialog box but also includes an Advanced dialog box for applying noise reduction separately to each colour channel. This can be handy when images contain specks of colour and when false colours are introduced as a result of noise-reduction processing.
The other settings in the Noise dialog box operate in slightly different ways. The Despeckle filter detects edges where colour changes occur and blurs all of the selection except those edges, which are retained to preserve details. This filter can be used to remove banding and other artefacts that can appear in scans of printed materials.
The Dust and Scratches filter reduces visual noise by changing dissimilar pixels.
The Median filter seeks out pixels of similar brightness, discarding pixels that differ significantly from adjacent pixels.It replaces the central pixel in each group with the median brightness value of the searched pixels. It’s mainly used for eliminating or reducing undesirable noise patterns.
Another collection of filters than may come in handy at times is grouped in the category of ‘lens effects’ and includes lens corrections and filters that can add distortions and flare effects. Photoshop includes a Lens Correction filter that”¨can access stored ‘profiles’ for different cameras and lenses and provides a quick way to correct aberrations like distortions and vignetting. Unfortunately, the list of profiles is fairly limited and mainly covers traditional camera brands.However, clicking on the Custom tab lets you access a full suite of adjustments for the most common aberrations as well as a grid overlay that makes it easy to correct perspective.
Photoshop’s Lens Correction filter provides manual adjustments for distortion, chromatic aberration, vignetting and perspective.
Lens flare, which results from the scattering of light within a lens, can produce starbursts, concentric rings or hexagonal patterns that spread across the scene. Most serious photographers prefer to avoid them by seeking out high-quality lenses. However movie producers often utilise flare effects to invoke a sense of drama or create an impression of a ‘real life’ scene.
Lens flare was one of the first special effects developed and one of the easiest to replicate. Consequently, filters that simulate lens flare are quite common in even basic editing software.
The Lens Flare filter in GIMP is located in the Light and Shadow sub-menu. It lets the user control the position and amount of the flare effect.
The Filter Gallery setting in both Photoshop and Photoshop Elements contains lots of ‘artistic’ effects that can be applied to images by simply clicking on a thumbnail. Large preview images make it easy to select the effects that work best with different types of images. Be cautious whenever you use filters and never apply them to original image files. Always keep an unedited copy of each image to go back to in case you don’t like the edited versions.
The Filter Gallery makes it easy to apply different artistic renderings to digital images.
Excerpt from Photo Editing Pocket Guide