At  high  speeds  this  is  not  the  case,  for  if  we  set  the  lens  aperture  to  the  maximum
opening  of  the  shutter,  say  /  5.6,  and  give  an  exposure  at  top  speed,  and  then
stop  the  lens  down  to /1 1.3,  and  repeat  the  same  speed,  the  plate  will  by  no
means  receive  exposures  in  ratio  to  the  two/  values,  viz,,  4  to  1.  The  expla-
nation is  simple — at  the  large  stop  there  is  no  period  of  full  opening  of  the  lens.
Let  us  assume  the  efficiency  to  be  then  J/j  (in  reality  it  will  be  somewhere
between  50  per  cent  and  S3}/^  per  cent).  At  the  aperture  of  /1 1.3,  as  the
stop  has  a  considerable  period  of  full  opening — represented  by  the  time  the
leaves  take  to  open  beyond  this  aperture  to/ 5.6,  and  close  down  to  it  again —
the  efficiency  is  increased  to  two-thirds;  in  fact,  just  doubled.  Under  ordinary
circumstances  if  a  reduction  of  aperture  is  made  from/ 5.6  to /1 1.3,  the  light-
intensity  is  reduced  to  one-quarter,  but  in  the  case  supposed  the  reduction
of  aperture  has  doubled  the  efficiency  of  the  shutter  and  the  reduction  of
light-intensity  is  halved  only.
These  figures  were  given  as  an  illustration  of  what  happens  with  an  iris
shutter  in  the  *'B.  J.*'  some  years  ago,  but  they  apply  approximately  to  all
shutters  opening  from  the  centre  to  a  constant  diameter.  They  show  there
is  no  definite  ratio  between  the  duration-of-exposure  and  the  efficient  exposure
at  high  speeds,  and  the  stops  lose  their  relation  value  to  each  other.  The
point,  perhaps,  is  of  more  interest  than  of  real  importance,  as  at  high  speeds
the  full  aperture  of  the  lens  is  usually  employed;  still  it  is  useful  to  bear
it  in  mind.
