## Monday, September 13, 2010

### Can a Picture be Too Sharp?

In a word, "Yes."  All in all, there are more important aspects to a photograph than its sharpness - like the subject possibly.  And, depending on the subject, too much sharpness can be bad.

"There is nothing worse than a sharp image of a fuzzy concept."

 Contact Print from 4x5 Negative
First of all, sharpness can be a slippery concept.  Mostly it's a matter of appearance rather than something concrete.  Many factors contribute to the apparent sharpness of a image. Those would include the quality of the lens, use of the optimum aperture, the stillness of the camera during exposure (shutter speed, use of image stabilization, use of tripod), the film (if using a film camera), and the format (bigger generally = more apparent sharpness).  If printing film in the darkroom, additional considerations come into play - the sharpness of the enlarging lens, using the best aperture on that lens, the choice of developer for the film, how absolutely parallel the negative and lens and printing paper are with each other, the inherent contrast of the paper used (or if using a multicontrast paper, which contrast filter is used), etc.  Obviously many of the same variables apply in digital, including lens quality, optimum aperture, avoiding camera shake, the size and quality of the sensor.

Print size is also important - especially in film.  Due to light scatter in the film itself and in the air between the enlarger lens and the paper, the more one enlarges film to make a print, the less sharp it will appear.  All things being equal (which of course they never truly are), an 8x10 made from contact printing an 8x10 sheet of film will appear sharper than an 8x10 from a 35mm camera.

 Olympus e-420 Picture with lots of Sharpening via Clarity in ACR.
When we talk about sharpness, what most of us have in mind generally is "acutance."  A good working definition of the word is micro-contrast of the edges in a picture.  Think of a knife laid on a piece of photo paper and exposed to light.  When developed, it should consist of just too colors - the paper should be perfectly white where the knife was, and perfectly black everywhere else, with a very distinct line between the black and the white.  That line is acutance.  The bolder it is in a picture, the sharper it appears. Now this is all a general discussion of acutance.  It in fact has a very specific definition: $A = \frac{1}{N}\sum_{n=1}^{N} (G_n)^2 \times (D_1 - D_2)$.

 Sigma DP2s with Foveon Sensor Picture.
Right.  So anyhow, that is sharpness in a nutshell.  In the digital world, we are blessed with several different methods of sharpening, though most involve, primarily, increasing the acutance of edges.  That is why when we go too far the edges get halos in the software's attempt to increase the micro-contrast of the edges.  Because most digital cameras use a Bayer filter so that different pixels capture different colors of light that then have to be combined and reconstructed, their output needs at least a little sharpening.  In the picture above I pushed the Clarity slider pretty far to the right when converting from the RAW file to bring out the texture in the snow.  One of the reasons I like the Foveon sensor so much is that each pixel captures all the colors and the images start out sharper than other digital cameras produce.  They don't need much sharpening to have excellent acutance.

 Picture from Contax G1 on HP5+ Film.
In shooting 35mm film, I ultimately found a system that I like a lot.  The Zeiss lenses on my Contax G1 produced very sharp images.  I used Ilford's HP5+ film, an old school, thick emulsion film, but one known for good acutance.  I developed this in Ilford's DD-X developer, which produced negatives with good overall contrast at a film's rated ISO speed.  It didn't really enhance the acutance, but with inherent sharpness from the lens and film, this combination produced acceptable acutance.  I then printed the negatives a little softer than most would (and softer than I used to do) when I came to understand that "softer and darker" results in more actual details being visible.  No one would probably say the resulting pictures were extraordinary for their sharpness - but to my eye they have enough sharpness while also having a glowing tonality that I find pleasing.  As I said, there are more qualities to a good picture than sharpness. Like everything else, a picture needs the sharpness it needs and no more.