Sunday, January 20, 2013

The Airy Disk and the Real World

The Airy Disk is used as a basis for many explanations, such as those re: DOF or resolution for example. However such explanations, although they are helpful in context, often miss - or fail to emphasize - an important connection to the Real World where real photographers ply their trade!

It all stems from the use of the phrase "The Airy Disk", along with it's various formulae, as if the said disk is a uniquely visible item. You see, any formula for the disk, radius, diameter or even the Rayleigh criterion contains the value of the light wavelength, usually called lambda. And there we have it - by using a unique value for lambda (usually 555nm, a.k.a. green) the explanation is calculating an Airy Disk property for monochromatic light which, by and large, does not exist in the Real World - with the possible exceptions of laser-lit nightclubs or futuristic battlefields.

So, even with a 1um pixel pitch and a lens set to f/45, you are most unlikely to capture one of these:

"What's he on about?" I hear y'all say . . .

Well, a real image might, for example, contain a yellow flower. A yellow flower does have some reflection of our perfect monochromatic green at 555nm, but it also has reflection up in the reds and infra-reds, as shown by this Real World image (alpine meadow, Tibetan Plateau) from this paper:

So, in my view, the diffraction effect causes round blobs at the cameras image plane - not fancy-looking disks with rings. You could think of the blob as being made up of many, many Airy Disks, one for each wavelength, all averaged together in your image, i.e. integrated. It would definitely be a blob for the flower above, which spans the wavelengths from c. 500nm to 1000nm (thank heaven for IR blocking filters). Blob range: at f/8 the Airy Disk diameter for 500nm is about 10um; for 1000nm, about 20um. Blobby, blurry, blecchh . . how do we even manage to take good pictures?

Disclaimer: Maybe I've just discovered axial chromatic abberation! And, for sure, I'll never see an Airy Disk on my Sigma SD10 with its nice big fat pixels :-).

But, if you're waiting for a 36MP 4/3" camera, . . . beware.

Best regards, xpatUSA