> Could someone please refresh my memory. What is the aperature/shutter speed
combination I would use if I wanted to photograph a busy intersection and not
record the moving items(i.e. the cars, et al) ?
Bill, I would suspect that it might be something like a few minutes to an hour
or two ... although the presumption is made that things are, in fact, moving
so that they do not remain in the same spot for longer than maybe 1/100 of the
exposure time.
Another way to say this is that if you know how long it takes anything in the
scene to move a distance equivalent to its own dimension in the direction it is
moving, then use an exposure time that is 100 times longer than that. To be
safer use one that is 1000 times greater than that.
The truth is that I just made this up and have no references to back this up.
It is based on a presumption that most anything that is underexposed to the
tune of 7 stops or so will be so underexposed as to be virtually invisible on
the record.
For example, if the scene contains people moving at maybe a foot per second
and we assume that a person is about two feet wide then it takes them 2 seconds
to cover a distance equivalent to their size and thus you would use an
exposure time of 2 x 100 or 200 seconds.
Now about exposure. Since you are going to be using such a long exposure time
even the smallest aperture on your lens may not be small enough to allow you to
use a time of 200 seconds ... so what do you do? Well, place a 2.0 ND filter
over the camera lens and "recalibrate" you lens aperture settings to be 7 stops
less than what is marked on them. So, f/16 would become ... let's see .... 22,
32, 45, 64, 90, 128 ... roughly f/180
So suppose it is a bright sunny day and you would find that your exposure would
need to be f/16 at 1/125 second (you are using ASA 125 film and Sunny 16 "rule"
applies) but now you want to make an exposure of 200 seconds ... hmmmm ...
So .... correct exposure is 1/125 - 1/60 - 1/30 - 1/15 - 1/8 - 1/4 - 1/2 - 1 -
2 - 4 - 8 - 16 - 32 - 64 - 128 ... ~ 256 or 15 stops more than 1/125 at f/16.
So, by placing the 2.0 ND filter over camera lens you get 7 stops ... now you
need 7 (or so) MORE stops. - one way to get this is to use 2 2.0 filters.
This will make f/16 "look" like f/2000 which is roughly the equivalent of 15
stops less than f/16. So since you are using 15 stops more TIME than what is
required now you place enough ND filtration over the lens to get 15 stops LESS
light at the film plane and it all works out OK.
And then bracket!
Another thing you might want to do is to consult your reciprocity failure
charts to find the effective film speed of the film at an exposure time of 200
seconds and meter and adjust lens aperture accordingly.
Photographs like what I think you are driving at can also be made by making a
series of short exposure time photographs instead of keeping the shutter open
for a long time.
I think this is sort of right ... if not I am sure some of you will set me
stright, eh?
andy
