To convert this formula to diameter (d) instead of radius ®: OK, moving on, that gives us this formula, using 555 nanometers for all future calculations: It happens to be a nice yellow-green, not far from William's choice anyway. In his book, Williams selects 0.0005 mm as an average wavelength of light, but I prefer 0.000555 mm, or 555 nanometers as being the wavelength that is dead center in the spectrum of sensitivity. OK, f/a can be renamed N where N is the familiar f-number that describes the ratio of focal length to aperture diameter. "f" is for focal length and "a" is for diameter of the aperture. Airy is the astronmer who discovered diffraction in 1890 and diffraction's disks are named after him, Airy, not airy.) I can't type a lamda, so I have substitued a "w" in the formula below, for wavelength. He gives the following formula that calculates the radius ® of an Airy disk. Williams authored a book called *Image Clarity, High-Resolution Photography* that has a discussion of diffraction. The good news is that at the diffraction-limited apertures for each of the formats, the exact same depth of field can be achieved, with the larger formats having a disadvantage of longer exposures. They are diffraction-limited to using wider apertures than those which can be used by the larger formats.ĭepth of Field is squelched by diffraction and the point at which this happens moves to smaller f-number values (wider apertures) as the format diagonal decreases. The smaller the aperture through which light must pass, the greater the effect and the more an image at the film plane must be magnified to create a print of a given size, the more visible the degradation in that print.Small formats are proportionately more vulnerable to the effects of diffraction than larger formats, so much so, that the smallest formats, APS and to a lesser degree, 35mm, can not exploit the Depth of Field advantage they have over large formats at their smallest available apertures. It is an image degrading phenomenon for which there is no means of correction. At least the results here will give you a rough idea.ĭiffraction is the only aberration suffered by pinhole cameras. If you are like me and more critical about sharpness, you can modify the equations. The assumption is that the viewing distance is equal to the enlargement diagonal. In short, the minimum aperture at which you can shoot at depends on the format, enlargement size/dimensions, desired CoC in the enlargement, viewing distance, and wavelength of light. This is the most detailed explanation I've 's long, but I think most MFD readers will find it relevant though the discussion was really for LF.
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