Understanding the workings and applications of the high pass filter is crucial for photographers and retouchers who want to improve the quality of their images.
How Does the High Pass Filter Work?
The high pass filter works by analysing the frequencies in an image. The process begins by identifying the different frequencies present in the image.
The frequencies in the context of the image refer to the rate of change in colour and brightness of the pixels. The filter then eliminates or reduces low-frequency elements, such as large, uniform areas.
At the same time, it enhances high-frequency elements, such as sharp edges and fine details.
As a result, the image after applying the high-pass filter contains mainly edge and detail information, while the larger, uniform areas become grey or neutral. It is this property that makes the high-pass filter such an effective tool for sharpening and highlighting details in photographs.
High Pass Filter in Retouching
The high pass filter is widely used in various digital retouching techniques. One of the most popular is photo sharpening.
This process usually involves creating a copy of the photo layer and applying a high-pass filter to it. The blending mode of the layer is then changed, usually to Overlay, Soft Light or Hard Light, and the opacity of the layer is adjusted for the desired effect.
Another important use is to improve local contrast. The High Pass filter can be used to increase contrast in specific areas of a photo, helping to highlight textures in elements such as skin, textiles or natural surfaces.
It also allows details in shadows and highlights to be highlighted without affecting the overall exposure, which can significantly improve the depth and ‘three-dimensionality’ of an image.
In portrait photography, the high pass filter is often used to subtly emphasise the details of the eyes, increasing their clarity and brilliance. It can also be used to sharpen the texture of the hair, giving it a more defined look, or to gently emphasise the texture of the skin while maintaining a natural look.
High Pass Filter Parameters
When working with a high-pass filter, the most important parameter is the radius (radius). It determines how strongly the high pass filter will affect the image.
A small radius, in the range of 0.5 to 2 pixels, is ideal for subtle sharpening and highlighting fine detail. This is often used in portrait photography, where we want to preserve the natural appearance of the skin.
Medium radius, 3 to 5 pixels, works well for general sharpening and improving local contrast. This is a frequently used setting for landscape or architectural photography, where we want to emphasise textures and shapes.
A large radius, above 5 pixels, can be used to create special effects or to emphasise textures clearly. However, large radius values should be used with caution, as they can lead to an unnatural looking image.
Choosing the right radius depends on the resolution of the image, the type of detail you want to emphasise and the desired end result. It is worth experimenting with different values to find the optimum setting for a particular image.
The High Pass Filter in Practice
The high pass filter is available in many professional graphics editing applications, but the way it is implemented can vary.
In Adobe Photoshop, which offers the most advanced options, the high pass filter can be found in the Filter > Other > High Pass Filter menu. Photoshop allows full control over the radius and also allows combining with masks and blend modes.
An interesting feature of Photoshop is the ability to create smart objects for non-destructive editing, which is particularly useful when working with the high-pass filter.
GIMP, as a free alternative to Photoshop, also offers the ability to work with the high-pass filter. Although there is no direct equivalent, a similar effect can be achieved using the Unsharp Mask tool from the Filters > Enhancement menu.
GIMP allows you to control the radius and amount, and also allows you to combine with blending modes.
Affinity Photo has a high pass filter available from the Filters > Sharpen > High-Pass menu. Affinity Photo is notable for its intuitive interface and quick real-time previews of effects, making it easy to fine-tune high-pass filter settings.
Capture One, while lacking a high pass filter, offers tools to achieve similar effects. The Clarity and Structure tools in the Detail panel allow for relatively advanced control over local contrast and sharpness.
Capture One is notable for its ability to selectively apply effects using layer masks, giving great flexibility in editing.
Frequently Asked Questions
What is a high-pass filter used for?
The high-pass filter in photography is used to sharpen the edges and increase the contrast of image details. It lets the high frequencies through (sharp edges) and attenuates the low frequencies (smooth areas), allowing details to be highlighted.
What is a high-pass filter image?
A high pass filter image is the result of applying the filter, characterised by emphasised edges and increased contrast of detail. It usually looks like a sketch or outline of the original image, with clearly defined edges.
What are the filters in photography?
Various types of filters are used in photography, including polarising (to reduce glare), neutral grey (ND, to control the amount of light), UV (to block ultraviolet radiation), colour (to change colours), and digital filters in post-production, such as high-pass and low-pass.
How do I set up a high-pass filter?
Setting the high pass filter depends on the image editing software. The usual process involves: duplicating the image layer, applying a sharpening filter (e.g. ‘Sharpen’ or ‘Sharpen Mask’), changing the layer’s blending mode (e.g. to ‘Mild Light’) and adjusting the layer’s opacity as required.
When should a high-pass filter be used?
The high pass filter is used when you want to accentuate details and edges in an image. It is particularly useful for landscape, architectural or macro photography where you want clarity of detail. It should be used in moderation to avoid over-sharpening and artefacts.
High Pass and Product Photography
Product photography is an area where a high-pass filter can significantly improve the quality and appeal of images without distorting the appearance of products. One key application is to highlight the textures of materials.
In the case of materials, the high-pass filter can help to highlight the weave, which is particularly important in photos of clothing and textiles. For leather products, a gentle application of the high pass filter can highlight the natural patterns and textures of leather, adding depth and character to images.
For metallic surfaces, a high pass filter can help to accentuate the shine and subtle details, which is crucial for jewellery or luxury accessories photos.
Edge sharpening is another important aspect of using a high pass filter in product photography. In the case of electronics, sharp edges are key to highlighting the modern design of devices.
The high pass filter can help to sharpen these edges, making the product look more precise and technologically advanced. In jewellery photography, a gentle application of the filter can accentuate the edges and facets of gemstones, increasing their brilliance and visual appeal.
For product packaging, sharpening edges can improve the legibility of labels and lettering, which is crucial for effective product presentation in e-commerce.
The high-pass filter is also indispensable in enhancing the fine details that often make a product unique. In watch photography, for example, it can help to highlight the intricate components of the mechanisms, emphasising the precision and craftsmanship.
For products with fine patterns, such as ceramics or printed fabrics, a high pass filter can help to highlight them, making them more distinctive and eye-catching.
The same goes for product labels and lettering – gentle use of the filter can improve their readability, which is important in the context of online sales.
4 Tips to Use HP Professionally
- Work on a separate layer. Always use a high pass filter on a duplicate of the original layer to maintain the ability to edit and compare with the original.
- Experiment with blending modes. Different blending modes (e.g. Overlay, Soft Light, Hard Light) produce different effects. Try them out to find the best one for your photo.
- Control the intensity. Use the layer opacity adjustment to adjust the strength of the high pass filter effect.
- Apply selectively. Use layer masks to apply the highpass filter only to selected areas of the photo.
3 Most Common Mistakes When Using HP
- Over-sharpening. Applying the filter too heavily can lead to an unnatural look and artefacts.
- Ignoring noise. A high pass filter can amplify noise in a photo, especially at high ISO values.
- Lack of selectivity. Applying the filter to the entire photo can lead to unbalanced results.
4 Advanced Techniques
- Combination with other filters. Combining the high pass filter with other tools (e.g. sharpening mask) can produce interesting effects.
- Creating an HDR effect. Multiple applications of the high pass filter with different settings can simulate the HDR effect.
- Improving microcontrast. Gentle use of the highpass filter can significantly improve the perception of detail without apparent sharpening.
- Creating artistic effects. Extreme settings of the high-pass filter can be used to create unique graphic effects.
Alternatives to the High Pass Filter
- Unsharp Mask: A traditional method of sharpening that offers good control over the effect.
- Smart Sharpen: An advanced tool in Photoshop that allows for precise sharpening with noise reduction.
- Clarity: A tool available in many programs to improve mid-tone contrast.
- Photoshop Noise Filter: Can be used to simulate a high pass effect with the right settings.
Challenges of the High Pass Filter
Despite its effectiveness, the high-pass filter can pose some challenges for photographers and . One of the main problems is the risk of overdoing the effect, which can lead to unnatural-looking photos.
Another is the correct application of the high-pass filter depending on the type of photo and its intended use. Photos intended for print may require different settings than those that will only be displayed on screens.
The filter may also amplify unwanted elements of the photo, such as noise or compression artefacts. It is therefore important to take care of the overall quality of the photo before applying the filter.
Finally, effective use of the high-pass filter requires a good understanding of the principles of composition and aesthetics. Sharpening alone will not improve a poorly composed or underexposed photo.
For Beginners
- Start with small values. It’s better to gradually increase the effect than to apply too strong a setting straight away.
- Compare before and after. Frequently switch the view between the original and the filtered version to assess the effect.
- Experiment with masks. Use layer masks to apply the high pass filter selectively to different parts of the image.
- Ask others. It’s easy to judge your own photos by the work you’ve done, without looking at them from the perspective of a stranger viewing them. Show your photos to others to judge whether you have over-sharpened and the effect doesn’t start to look artificial.