Diffraction In Digital Imaging

In the past, it has often been said that the rules of photography are the same whether one shoots digitally or using film. Certainly this is true when it comes to the art of composition, but when it comes to capturing the image there are certain technical aspects that are not quite the same.

The first one that comes to mind is the so called ‘expose to the right’ issue. Most people doing serious digital capture and that are shooting in RAW format are familiar with this concept. In short, most of the data and, therefore, detail that the digital sensor can capture is ‘located’ in the ‘brightest’ stop or two of the image. Therefore, the image should optimally be exposed such that the histogram is pushed as far to the right as possible, without actually clipping. While the image is now overexposed by conventional standards (and therefore this method will not work acceptably well in jpeg format), by shifting the exposure downward in the RAW processor the correct degree of brightness can be achieved and more detail captured than if the exposure had been ‘correct’ by conventional standards.

But you already knew that. What is more interesting is material that I recently read in the magazine Photo Techniques (which is a small magazine in terms of page numbers, but one that is well worth subscribing to). In the Jan/Feb 2009 issue, I learned that there may be significant differences with current digital capture when it comes to diffraction effects and depth of field.

Without going into detail (for that I strongly recommend getting the issue), I would like to provide a quote to whet your interest. In the conclusion of the article by Lloyd Chambers entitled “Diffraction: Resolution Taxed To Its Limits” Mr. Chambers states:

“As megapixels increase, diffraction will become the dominant factor limiting image sharpness, unless and until improved optical designs allow near-diffraction-limited imaging at apertures such as f2, f2.8, and f4. Such lenses are feasible, but will be larger, heavier, and much more expensive than today’s optics. When depth of field is a priority, ‘tilt’ lenses should be used in order to evade the diffraction/depth of field conflict.

To paraphrase an old maxim: f8 and stop there. That simple rule will maintain optimal or near-optimal lens performance and image contrast resolution with today’s DSLRs, while offering reasonable depth of field for many subjects. Stopping down to f11 or f16 is warranted with some subjects, but the contrast compromise should be kept in mind.”

Interesting stuff….the idea that with today’s high quality sensors and optics, diffraction plays a role much sooner (in terms of stopping down to achieve greater depth of field) than when using film.

I put some of this information to use with results that surprised me. I have previously mentioned and shown extreme macro-images I have made from slabs of stone. Though these slabs are flat in the general sense, they are cut with a saw and thus in the ‘macro’ sense are not truly flat. In order to maximize depth of field at high magnification (>1:1) I had been shooting at f16 and f 22. I recently tried shooting at f8 and f11 instead and found that the results were, indeed, sharper. And in this case the depth of field was still adequate with enhanced apparent sharpness. I now shoot all these types of images at f8.

Obviously, when shooting landscapes or other scenes where there are near/far objects that one desires to be in focus, there will be trade offs between sharpness and depth of field. However, it is worth experimenting with one’s equipment, or at least taking shots at a variety of apertures since, in many instances, the improved sharpness will be more desirable than increased depth of field with sub-optimal sharpness throughout.

There are definitely some interesting things to ponder here!