> Mickey Trageser <mickeytr@xxxxxxxxxxxxxxx> wrote:
>
> Seems to me that even at the same 'magnification', the 90mm is
> further from
> the subject than the 50mm. Therefore, a 1 degree movement of the film
> plane
> will cause the image to shift more on the 90 than the 50. To put it
> another
> way, look at the hands of a clock at 12 o'clock, both pointing
> straight up.
> If you move the hour hand so that the minute and hour hand tips both
> touch
> the same point (the subject), they now each have a different axis
> (film
> plane)on the line from the center of the clock to the top point. If
> the
> minute hand moves 6 degrees, it now points to 1 minute past the 12.
> If the
> hour hand now moves 6 degrees in the same direction, it will end up
> parallel
> with the minute hand, but it's point (view of the subject) won't have
> moved
> as far. If this made any sense at all, you'll see that the 50 will
> show less
> motion given the same degree of movement. The 90mm, given the same
> degree of
> movement sweeps across more image area causing MORE blur of the
> image. It
> isn't the angle of movement, it's the shift in the field of view that
> makes
> the difference. The 90 will shift more than the 50 for the same
> angular
> movement.
>
> OK, I've tired myself out. Hope this helps... ;-)
Using your analogy of the clock hands:
If the magnification of the 90mm and the 50mm are the same then the length of
the minute hand and the hour hand are the same.
Wayne Harridge
http://members.optusnet.com.au/~w_harridge
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