Digital photographers who migrate from film often complain about the lack of sharpness in digital images when compared to film. There is some validity to that claim. The truth is that digital sensors can produce very sharp images. However, that sharpness is compromised slightly by the use of anti-alias filters in most digital cameras. Most DSLR cameras have an anti-alias filter (with the exception of some full-frame and medium format digital cameras and backs). This filter adds a slight blur to images, to minimize the stairstep effect seen on diagonals and the moire effect that sometimes appears on fine lines in digital photos. That is why all digital images need to be resharpened in camera or with post-processing software.
The typical method used to sharpen digital images is Unsharp Masking. Unsharp Masking enhances edge contrast to create the illusion of sharpness. It is really a relic from the days of film scanning and does not directly address the problem of blurring caused by anti-aliasing filters. There are other methods of sharpening digital images that can produce better results.
I have been experimenting with another form of sharpening that is quite unlike Unsharp Mask. Rather than enhancing edge contrast (which creates a slight halo on edges in the image), RL (Richardson-Lucy) Deconvolution is designed to undo the slight blurring that anti-alias filters add to a digital image. In effect, it reverse-engineers blurring added to images by anti-alias filters. The result is a more natural-looking sharpness in digital images. Deconvolution is a sharpening technique used by the Hubble telescope (and spy telescopes) to extract the greatest amount of detail from deep space images. There are several programs on the market that offer raw image deconvolution sharpening, including Photoshop CC ( it’s the Smart Sharpen filter), DxO, and a few others as well. However, there is an excellent free raw conversion and editing program that implements RL Deconvolution sharpening as well: Raw Therapee. Raw Therapee offers users the choice of standard Unsharp Masking or RL Deconvolution. You may come to the same conclusion I have: In most cases, RL Deconvolution produces greater image detail and sharpness while avoiding the artifacts and halos caused by Unsharp Masking.
I find that RL Deconvolution controls in Raw Therapee give digital images a more natural sharpness that reminds me of a good film image. It extracts details that can only be seen with extreme levels of Unsharp Masking that lead to oversharpening. For viewing images online, it produces a much smaller file sizes than a similarly Unsharp Mask sharpened image (that means faster loading web pages). And, did I mention how natural the sharpened image looks? Raw Therapee is a great tool for the frugal photographer who likes to extract every last ounce of quality from his digital images. In fact, I consider the results it produces equal to the best (and most expensive) RAW image editors on the market. The RL Deconvolution sharpening tool is a wonderful option that may work wonders on your digital images. If you are willing to master this program it can produce amazing results. You will need to read the manual to get the best from this program.
If you are using an older 32-bit Windows computer you may find that use of the Smart Sharpen filter in Photoshop CC often leads to “Out of Memory” problems because its algorithm demands alot of processing power. Raw Therapee with RL Deconvolution is not as demanding overall and will run faster on older computers.
If you like getting the very best detail from your digital images, try RL Deconvolution for your image sharpening. -Don Peterson