Very good discussion around this topic in my opinion!
The 1.25 might indicate that the background was not totally black and during the exposure of the
moving image it added a bit of density to the negative or exposure to the area that was uncovered by
the moving subject. At the time the shutter opened the image was in one place but its trailing edge
moved almost immediately away from its position so it will be very underexposed and dark. Then it
rises to maximum possible level once it gets to where the leading edge was at that same (initial)
time. In addition the shutter is not 100% efficient so it adds yet another layer of complexity to
the situation. The opposite happens on the other side. The leading edge is at the farthest point for
a VERY short time and thus way underexposed as well.
The aspect ratio of the whole record is 1:4 if my measurement is correct. But the size of the
subject or its image needs to be subtracted out to determine the actual distance moved by any part
of it. So the ratio then is 1:3 and so it traveled 3 inches in 1/30 second so 90 inches per second.
If it can be assumed that the background was totally absorbent then the lighter areas of the record
would reproduce at close to 1.3 indicating a 1/30 second local exposure time in the mid-sections of
the record due to the motion of the subject. Then it can also be deduced that the camera shutter
must have been opened for about 3 x 1/30 or 1/10 second. I think Tim was driving at this.
In terms of "pondering" one might note that the density vs. exposure is not a "linear" matter. It
makes one think of a "characteristic curve". In the region of interest the relationship is linear.
roughly! - this is meant as a thinking problem and I may have my logic flawed so if it is so tell me!!
;)
andy
re: http://people.rit.edu/andpph/a-pix/2009-sensi-1.jpg
