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Old July 3rd, 2011, 06:18 PM
Maris Rusis Maris Rusis is offline
Join Date: Feb 2007
Location: Noosa, Queensland, Australia.
Posts: 382

Mountain Image

Gelatin-silver photograph on Fomabrom Variant III exposed in contact contact with a Fomapan 200 8x10 negative developed in Xtol. The camera was a Tachihara 810HD triple extension field view camera with a Fujinon W 300/5.6 lens. Exposure was 1 second at f90.

This isn't a digital image but it illustrates the uncomfortable compromises needed to balance lens resolution, image sharpness, image detail, and diffraction. The lens, a Fujinon-W 300/5.6 is maximum sharp in the centre of the field half way between f5.6 and f8. When used as an astronomical objective it gives me good views of Saturn and its rings; it's that sharp. But for the deep focus Mountain Image all that is of no use.

The mountain-shaped rock is only two metres in front of the camera and the most distant mountain (Mt Townsend, Australia's second highest) is 10 kilometres away. How to bridge the focus gap?

Tilting the front and back of the camera puts the Scheimpflug plane half-way up the near rock ( lichen patch) and through the top of the distant mountain. The tip of the rock and the herbs along the bottom edge are way outside the depth of field and rather blurry. Under the dark focussing cloth and with a 8x magnifier on the ground-glass I stop the lens down slowly and watch what happens.

The lichen patch in the plane of focus gradually loses sharpness as I stop down. The bottom edge herbs and rock tip gradually get sharper as the increased depth of field swallows them. Eventually at f90 the tiny leaves are as sharp as they can get; f128 is definitely worse.

It is a dreadful compromise. A lens that can image the moons of Jupiter is delivering an image infected with diffraction at only about 5 lines to the millimetre. This still looks sharp to the eye, just. One more stop down, f128, and the whole picture would look soft. I've destroyed resolution, I've damaged sharpness, I've let diffraction run riot, but I have maximised the total amount of detail in the photograph.

I reckon out-of-focus is a bigger killer of image sharpness than diffraction and sometimes it's worth taking things to the limit.
"Photography or the application of the chemical rays of light to the purpose of pictorial representation...". Photography, the word, coined and first uttered by Sir John Herschel at the Royal Society, Somerset House, London; 14 March, 1839.
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