resolution of LF lens

[Michael S. Briggs]

I don't know who "Sb" is, but they are only qualitatively correct. The numeric values are wrong. LF lenses have modestly lower resolution in line pairs per mm, but the greatly decreased enlargement required for a given size print more than compensates for the modestly decreased resolution on a negative. As an example, suppose that a 35 mm lens can do 80 lppm and a 4x5 lens 50 lppm. On an 11x14 print these become about 7 and 18 lppm, predicting the LF resolution advantage that we know is true from experience.

In practice, few 35 mm photos are taken with a tripod, so the theoretical resolution of 35 mm is not achieved and the resolution advantage is even more on the side of LF. Of course, there are some photos that can't be made with LF or with a tripod.

[Pete Andrews]

That's not the way diffraction works Thilo. The smallest point of light that can be theoretically focussed by a lens is called the Airy disc. The radius of the Airy disc, to its first bright fringe, is given by the formula: r = 1.22lambda*Nf. Where lambda is the wavelength of light, and Nf is the numerical aperture of the lens. So for yellow light this is approximately 0.72Nf microns in diameter, or in terms of resolution 1392/Nf lppm.This is true for any diffraction limited lens, regardless of focal length, and regardless of image circle.Any theoretically perfect lens with an aperture of f/5.6 will therefore have a resolution limit of 248 lppm, whether it covers 35mm or 20"x16".If you care to think of diffraction as an 'information density' limit, then it works against smaller formats.

What limits resolution in practise is mainly chromatic abberation, which is inextricably linked to focal length for refractive lenses. Even so, at apertures of f/16 and above (numerically), large format lenses can achieve resolutions close to the theoretical maximum.Anyway, this obsession with pure resolution is still a complete red herring when it comes to assessing the quality of an image, and the ease with which a given quality can be acheived.

[Andre_1200]

Mr.Jnorman, 1)i just want tell Mr.Simon that 'line pairs per mm' is _not_ so important in final print, compare #4 and #6 photos, may be #6 is little bit less sharp than #4 and so what? The _total_ quality of photo from 4x5 negs is much better. 2) i switch myself from 35mm to 6x6 and 4x5 due to quality, but I _understand_ that each piece of equipment has it's own place. It's like a screwdriver sets: if you good mechanics, you have in your vest Philips #1,#2,#3 screwdriver. It's simply impossible to made all you work with one size screwdriver because of different screws sizes.

[Struan Gray]

The photodo article is talking about sharpness, not tonality or grain. A less contentious example than the format comparison is the way that film with hard-edged grain can be very sharp but have dreadful tonality: lith film is a limiting case. Similarly, in the photodo examples the edges of the test patterns are well defined in the 35 mm shots but the areas of solid colour have bad grain. It's sharp, but ugly.

Roughly, the tonality of film is given by the grain density, so it scales with the area of the format. Sharpness is related to the differential of the grain density - how fast it varies from place to place - so it only scales linearly with the format size. For the same angle of view, the required focal length grows linearly with the film size, but the difficulty of making that lens grows at least quadratically so there is the possibility that the overall system sharpness will actually get worse. In real life there is so much variation between lenses, focussing systems and the way the film is held that all bets are off.

The sharpest civilian lens Kodak ever made was a plastic moulded singlet used in their disc cameras. That should give some clue as to how well sharpness alone relates to image quality :-)