Corner Sharpening

By Uwe & Bettina Steinmueller Back to

Steinmuller5nd9

Lenses are not ideal. Most of us have ended up with images in which the corners of the frame look too soft. This is not important for all pictures, but matters when the main subject covers the full frame and focus falloff is obvious. In such cases, post-capture fixes present an opportunity to save the picture. Of course using more appropriate lenses in the first place is the best solution, but it’s too late once you have taken the photo and cannot go back.

I’ll show my technique for improving a photo with lens falloff in the context of a full workflow. I will be using my own commercial detail- extraction and sharpening tools (available at Digital Outback Photo) but the technique is very general and you can easily use your tools of choice, such as plug-ins already found in Photoshop. The initial capture was converted to a TIFF using Iridient Digital’s RAW Developer (www.iridientdigital.com/products/ rawdeveloper.html), which I use because it produces excellent detail.

Step 1: Improve overall contrast

As you can see, the image I’m using to illustrate my technique (Figure 1) covers the entire frame, and suffers from an obvious falloff of detail in the corners. To fix it, first slightly brighten and add some more midtone contrast using Curves or your contrast software of choice (Figure 2).

Step 2: Improve Detail

Improve detail using detail-extraction tools (like my DOP EasyD Plus) or some sharpening with another tool such as the Unsharp Mask filter. The middle section of the image shows good detail but the corners are too soft due to the lens being soft in the corners (Figure 3).

Step 3: Strong detail extraction and masking for the corners

I now create a copy of the image on a new layer and perform a much stronger detail extraction on the full image. The corners are improved but the center is now over-processed (although this kind of subject often needs a lot of detail extraction).The goal is to restrict this second pass of detail increase only to the corners. So I need to create a Layer Mask that limits the stronger detail extraction to the corners.

To do this, create an Elliptical Marquee around the portions of the image that don’t have falloff (Figure 4), and use a strong feather at the edges using Redef ine Edges (Figure 5).Now invert this selection and apply it as a Layer Mask (Figure 6). The corners now look nearly as sharp as the center. If it looks too sharp, you can lower the opacity of this layer so that it blends in a way you find more pleasing or natural.

Final tuning

After some final tuning of the exposure using Curves (Figure 7), we get the final result in Figure 8.

Of course, this technique cannot truly recover lost sharpness from the lens, but it can help make the image quite acceptable. The same technique also works very well for other types of selective sharpening where one can paint a Layer Mask as needed to limit the sharpening to specific areas of an image.




About the Author

Uwe & Bettina Steinmueller
Contributor
All images are copyright Uwe & Bettina Steinmueller. German photographer Uwe Steinmueller and his wife and partner Bettina came to live and work in the United States over a decade ago. They concentrate on taking photos for fine art prints, mainly nature and urban landscapes. Uwe has authored numerous books and articles about digital workflow. He is also the owner and editor of Digital Outback Photo www.outbackphoto.com.