Image Sharpness

There are no answers here, just some factors to consider. NASA says they can read license plates from space. We don't yet have that technology available in the film business. With film image sharpness is limited by the weakest link. I am not discussing resolving power and modulation transfer function testing.
Test factors. A single frame of motion picture film will look grainy if double printed or shown as a still. The difference in grain pattern of each frame at 24 FPS (actually shown 2 or 3 times in projection) gives a much greater perception of sharpness. Almost all films can resolve sharper images than we can see on a ground glass, so we must trust focus calibrations, depth of field charts, hyperfocal distance, zooming in to focus and correct lens seating. Wide angle lenses are the biggest problem. Aerial images appear sharper, but are not a good representation of focus on an image plane because our eyes corrects for any difference in focus.

I like the Siemans Star focus chart best to test focus. (Down-load one from here and blow it up on a copier.) If a zoom lens "hold focus" of a test pattern at 5 to 10 feet and the focus marks agree with the object distance the lens is probably OK. Many zooms do not track center position perfectly, but this will usually not effect sharpness. Use printed material or a measuring tape at an angle to see if the focus creeps closer or farther as you zoom. Older zooms didn't hold perfect focus throughout their whole range and some focal lengths aren't as sharp as others.

Quick "slop tests" can be done on short strips of negative. Develop them like regular B and W still film, fix and wash. Wipe the anti-halation backing off while still wet. Project or view with a loupe or a 50 mm lens. An alcohol wash speeds drying.

Film Size. Of course 70 mm film is better than 16 mm.

The Film Gate. Registration or not, position of the registration pins, correct pressure for the film thickness, pitch adjustment, flatness of film during exposure, aperture size, and correct threading.

The Camera. Steadiness of camera during exposure; harmonic motor vibration, solid support, movement from operator, vehicle, floor, etc.

The Lens. Focus, quality, format the lens was designed for, condition of the lens, optimal F-stop, optimal focus distance, flatness of filters, (longer lenses effected most), and circle of confusion used.

The Film. ASA, grain pattern, ideal exposure, development, age and storage of film, quality of perforations, and backing.

The Scene. Contrast, brightness, steadiness of test patterns,

Evaluation. Eye sight of viewer, still or motion, quality of projector, brightness of image,

Most field shooting situations are worse than test situations. Remember, the image is only as good as the weakest link.

For registration tests we rotated the test pattern a few degrees rather than displace the pattern for the second pass. This gives an easy to see gap is there was a registration problem. A wider lens and closer pattern will reduce support problems such as building vibration. Narrow white stripes on black read well.

© Copyright 1999-2004 Ron Dexter. All Rights Reserved.