RESOLUTION ISSUES IN SCIENTIFIC PHOTOGRAPHY

ON THE OPTIMAL RESOLUTION REQUIRED FOR 3D PRESENTATION

Prof.  John Hart

Program in Atmospheric and Oceanic Sciences

University of Colorado

Boulder, CO 80302

November, 2001

hart@tack.colorado.edu

nimbus.colorado.edu/hart/science.htm

www.crystalcanyons.net

How much resolution is required for the best possible presentation of 3D images?   The answer to this question is of importance when deciding strategies and purchasing equipment.  For example, it influences the choice of cameras, film, projection methodology, etc.  This paper outlines a theory that provides a guideline.

1)  Stereoscopic presentation, in the form of prints, slides (or presentation on a screen), or in a viewer, is different from presentation of standard 2D material.  The difference is that there is a preferred viewing location for stereo material called the ORTHOSTEREO POSITION.  This position is located on the perpendicular axis running out from the center of the image at a distance

D =  ImageWidth(meters) x { FocalLength of Taking Lens(mm) / Width of Film Used(mm) }  *      (1)

At this position, the image will appear with approximately the same depth perspective as it did when it was taken. 

Although taking-lens focal lengths may vary from normal, to weak telephoto, to strong wide angle, let us assume that a typical ratio for the bracketed { } in (1) term is unity.  This implies that a typical lens for 35mm film is 35mm (weak wide angle), or about 60mm for medium format film.  Fair enough. Then

D = Image (screen or print) Width.     (2)

Relation (2) gives a preferred viewing angle (from the viewer, across the print) of  53 degrees.  In a real slide show people may sit slightly closer (wider viewing angle), but most will sit at, or slightly behind, the so-called "orthoseat" at distance D from the screen. Most people will have a  viewing angle equal to or smaller than 53 degrees.   Note that this situation IS DIFFERENT FROM TYPICAL 2D GALLERIES, where patrons often walk up close to a print, so that the viewing angle is much larger!

2)   The human eye can resolve about 2000 line pairs in an image that is as wide as it is far away.  Thus for a viewing angle of 53 degrees (for which image width equals perpendicular viewing distance), the optimal resolution is 2000 line pairs.  It is wasteful, and unnecessary,  to provide more resolution in the display!!

3)   What equipment or system (imaging plus projecting, say) is required to generate 2000 line pairs across the image that is being viewed? 

• In printing, consumer inkjet printers can replicate about 150 lines per inch (when source material fed to the printer has about 300 d.p.i.).  Thus for any print over about 14 inches wide, the system will not be limited by the printer.

• In printing small stereo cards, where the image width is three inches or less, an inkjet printer can deliver only about 450 line pairs.  This is well below the optimal resolution of 2000 line pairs.  Small prints (that will probably be viewed with magnifying glasses) should be made on high quality paper or film using a high resolution printing system such as the LightJet5000.

• For projected images there is about a 10% to 20% loss of resolution between the film and the screen.  This number is typical of Ektagraphic projectors with high-quality lenses.  This loss is not perceptibly significant in most situations.

• In all cases it will be necessary to choose a film and camera-lens combination that can capture in excess of 2000 lp across the image.  For example, Fuji Provia in combination with good optics can resolve up to about 80 line pairs per millimeter, which, when multiplied by 36mm film width, yields about 2800 line pairs.  Projection results in some degradation (see table below), but typically can meet the goal.

• It is clear that it will be hard to meet the optimal-resolution condition with a digital projector.  On the face of it one would need a 4000 pixel wide unit!  Indeed, Hollywood executives have stated that digital theatrical presentation will require 4K x 2K projectors.

Table 1.  Resolution of film and projected film (*  lines across whole image, with approximately similar contrast. See section II for samples).

Using the optimal-resolution guideline ( 2000 lines across the image), slide projection (with Kodak Ektagraphic) is adequate, when coupled with projection using glass mounts.  Unmounted slides behave poorly near the edges with these lenses.  One can help some this by using slower projection lenses (e.g. f3.5), but for large-screen projection the substantial loss of illumination (almost a factor of 2, or a whole stop)  is objectionable.

The last two bullets (above)  argue against using digital equipment for optimal - resolution presentations.  This is somewhat deceptive, since closely paired lines are not resolved by the human eye with 100% contrast.  On the other hand, if the pixels in a digital display alternate between full on and full off, these line pairs are shown with much higher contrast than the lower values typically associated with quoted resolution limits for film.  Therefore, practical digital equipment (cameras and projectors), are able to present high contrast at modest resolution, but will miss subtle low-contrast details at high resolution.  For example, a true 1600 x 1200 projector (just becoming available) can output high contrast data at ~ 800 lines, but will miss low contrast details at 1600 lines.  On the other hand 35mm film can output low contrast details at up to 2800 lines, but at 800 lines it will not have any advantage over digital projection, and in fact may appear poorer due to grain and less than 100% contrast.

One practical exception to the 2000 line optimal-resolution guideline is in the viewing of panoramas.    These may be made with  super-wide-angle lenses.  For example a 20mm lens on 35mm film leads to an ortho-location at about 0.57 image widths.  The corresponding viewing angle is about 80 degrees.  For this the required resolution is 3500 line pairs (across the print or projected image).  Obtaining optimal resolution in this configuration will require the use of medium format (2 1/4 inch, or 54mm wide) film.   Such large viewing angles are also often encountered when gallery patrons walk up close to a 2D display print.  If the patron is allowed or encouraged to do this, then medium or large format film and high quality printing will be required.  But, as indicated above, one should not do this in viewing 3D (because the perspectives and ability to fuse get bad).

*  Ogram, G.R., Magical Images:  A Handbook of Stereo Photography, page 245. 

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