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Old 03-02-2019   #32
willie_901
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Quote:
Originally Posted by ptpdprinter View Post
...You never heard any of these concerns about comparative diffraction between FF and MF film shooters. ...
I don't have any images. But, the reason for less concern is because film shooters evaluated perceived image sharpness from prints while digital image sharpness is typically evaluated by viewing high-resolution monitors.

The system spatial resolution – MTF50 – depends on the lens, film resolution, development technique, enlarger optics and paper resolution. For small to medium sized prints, diffraction broadening could be less obvious going from small to large film surface areas. Before digital, film work that required large prints was usually done using large film formats. If 135 format film was printed to the same large size, and the negatives had a high system MTF50, diffraction broadening would be a concern.

Details

Digital imaging system spatial resolution (MTF50) is often superior than film because image comparisons are typically made by visual inspection using high MTF50 monitors.[1] Viewing prints alone, the system MTF50 will be lower and the diffraction broadening will become less obvious. Another difference from film involves pixel pitch. As pixel pitch increases the effective diffraction broadening deceases. Sensors with both large pixel pitches and large surface areas are susceptible to diffraction broadening.

For all images, when the airy disk diameter becomes larger than the circle of confusion (CoC), diffraction broadening becomes relevant.

Diffraction broadening is caused interference between light rays passing through an aperture. Circular apertures in 2D imaging produce a diffraction pattern – the airy disk. The airy disk primary peak diameter quantifies how much broadening occurs.

Diffraction depends on the film area because the CoC is a function of film area. For 135, 645 and 4X5 formats the CoCs are 0.029, 0.047 and 0.11 mm respectively.

The CoC depends the ratio of the viewing distance divided by the desired final-image resolution in lp/mm for a 25 cm viewing distance. This value is then divided by the enlargement factor (for a direct contact print the enlargement factor = 1). Larger media surface areas require less enlargement and this means the CoC is larger.

1. Neither film nor digital prints are have an inherent MTF50 advantage. The system MTF50s for film and digital prints both depend on more than just the sensors' and films' resolution limits.
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