Folks,
This note is from the Film Scanners list. I thought it might be of interest
here.
Michael
Date: Tue, 14 Nov 2000 10:02:44 -0600
From: "James L. Sims" <jlsims@xxxxxxxxxxx>
Subject: Re: filmscanners: Chemical Film Resolution.
Chris,
It does seem to be a hot topic but I would like to share my experience and
observations on this subject. I have been involved in a number of studies
regarding resolution as it applies to process cameras, still, motion
picture, and television (including HDTV).
Film resolution is only one part of the product resolution equation. Lens
resolution (at the specific f:number used for the exposure), film
flatness, accuracy of focus, and camera movement during exposure also play
a major role in the resolution of the final product.
Through the years my company has conducted numerous resolution test on
film. In the early tests, with 35mm SLR cameras, the camera was placed on
a good tripod and carefully focused on an SMPTE resolution chart at a
distance of 26 times the lens focal length. The exposures were made
(throughout the lens¹ f:stop range) using the camera¹s self timer to
release the shutter. It was determined, however, that the impact of the
reflex mirror (as it flipped up against the top of the mirror cage),
introduced enough vibration to affect the resolution of horizontal lines.
The exposures were subsequently made in darkness, with the shutter open,
and an electronic strobe was used to make the exposure. Other measures
taken with regard to the test camera included; precise adjustment of the
focusing screen to assure coincidences with the film plain, installing a
lapped pressure plate to insure flatness, and reducing the clearance
between the camera¹s stock rails and the pressure plate to .008² (from the
.010² standard) to maintain film flatness over the imaging aperture. A
magnifier was used to aid focusing and a dial indicator gauge was used to
monitor lens position. The focusing of each tested lens was a tedious and
time consuming process.
We found that the film/lens product resolution of (most popular brand) 50
mm lens¹ ranged from about 12 line pairs/millimeter at f:1.4 to about 70
line pairs/millimeter at f:8. At f:16, the resolution was back down to
about 40 line pairs/millimeter. Kern, Leitz, and Zeiss lens¹ faired
significantly better with about 40 line pairs/millimeter at f:2 to near
100 line pairs/millimeter at f:4. The resolution was down to about 50
line pairs/millimeter at f:16 with these lens¹. It is interesting to note
that 50 mm and 105 mm lens¹ consistently tested best.
The point of this is that under average conditions where focus is quickly
done, the camera is hand held, and camera specifications are within
manufacturer¹s specifications, maximum resolution cannot be achieved. A
consensus of opinion (from those who have studied this subject) seems to
be that a resolution of about 30 line pairs/millimeter would be tops and
that may be a stretch.
If you think about it, 30 lp/mm is not bad. That means a definable ³image
bit² is only .0006² (.0167 mm) in diameter on the film and (from a 35mm
image) only .0052² (.133mm) on an 8¹ X 10² enlargement (not cropped)
difficult to see with the naked eye.
Jim Sims
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