At 06:28 PM 9/23/97 -0600, Jim Gilley wrote: >Dave got my attention with yet another topic - Sharpness, or lack >thereof.
and a lot of deletion....... This motion blur really jumped up and got my attention back in the '60s and early '70s when I used a leaf-shutter Hasselblad for black and white photography. The mechanical differences between a leaf shutter and a focal plane shutter are not subtle and amazing in results. I'm sure that most readers know the differences between the operation of the two types of shutters. If not, a lot of bandwidth will be required to explain the difference in operations. The bottom line is, however, that the amount of light deposited on the emulsion of the film is the same for a given shutter speed; 1/500 sec has the same amount of light on the film regardless of shutter type (theoretically). Remember that anything mechanical is bound to have some degree of variance. While the focal plane shutter (s) expose film by a slit of light running across the film, the leaf shutter opens and closes from an aperture of nothing to the full f stop setting, then closes. So, instead of being a narrow slit or band of light across the film, it is light that begins as a tiny dot (nothing) to full f stop and back to nothing. OK, so what's the big deal? Actually, lots. The focal plane shutter moves very quickly across the film producing an even distribution of light, while the leaf shutter takes a lot longer to provide the same amount of light. An easy visualization (if I had a black board) is to draw a bell curve and measure all the light beneath the curve from the opening of the shutter to total closure. Because all the light required for 1/500 sec begins at the very beginning of the curve and ends at the very end, it becomes evident that the amount of time that the shutter is actually open is a lot longer than 1/500 sec. The 2" a 60 mph train moves in 1/500 sec doesn't change.....it is still moving those 2". The problem is that the leaf shutter is actually recording its motion longer than that; perhaps almost twice as long. If one is using a fairly sensitive film, bright objects are exposed more quickly (like locomotive number boards and headlights and white handrails). The darker regions expose more slowly. Hence, my nicely exposed negatives of fast-moving trains are sharp, except for all the up-front bright objects. The same shots with a focal plane Pentax 6X7 would yield sharp(er) negatives. Anyone with access to old publications with lots of b&w action shots will notice this phenomenon, and from photographers who are well known. Focal plane shutters changed the day. So while leaf shutter lenses are old technology and few people are probably ever going to be using them again in train photography, it is a good idea to remember that a faster shutter speed may be required to obtain the same sharpness image as that from a focal plane 35mm camera. JCL --> SPORRS: Serious Photographers of Railroad Related Subjects
