Deen,
>QUESTION: Say i want to shoot moving subjects, I realise the amount of
>background blur will depend on the subject movement, distance from the camera,
>focal length used, etc, but is there a nice rule of thumb to work out an
>optimal shutter speed?
I don't know of a rule-of-thumb but I'll try to come up with something close.
First of all you need to determine how much blur you want - this is somewhat
subjective I would think. Using your camera's viewframe as the reference you'd
say that you want subjects in the background to blur a fraction of the size of
the frame. Let's say you decide you want blur to be something like 1/10th of
the frame's horizontal direction (which is also the direction along which you
are panning). Since the frame is 36mm wide this would mean you want the blur to
be about 3.6mm (or 4mm rounded off).
Now you need to determine how long it takes subjects in the background to move
across your frame from edge to edge as you are panning. Estimating is OK. This
will obvioulsy vary depending on many of the factors you mentioned above plus
more. Let's say that given the subject you are planning to photograph this is
about 1/4 second.
The implication of this is that images of subjects in the background move 36mm
in 1/4 second or that they are moving at a rate of about (36mm / 1/4 second)
150mm per second.
To determine which exposure time will provide 4mm of blur when the image is
moving 150mm per second you divide the desired background blur by the rate of
movement of the background images and out comes the required exposure time.
In this case it would be 4mm / 150mm per second or .0266 seconds which is about
1/3.75 or rounded off about 1/4 second.
I hope this helps and is not too far off!
Andy Davidhazy
andpph@rit.edu
