the light under the cross will get brighter and less well defined as the exposure increases
recprocity: come from the word reciprocal: carries the meaning of being "inversely related" :
in plain maths, 1000 = 1x1000=5x200=10x100=20x50=25x40=500x2=..etc rite? ok so far so good...
in plain exposure terms: for the same ISO: f1.4, 1/15 sec = f2, 1/8 sec = f2.8, 1/4 sec =f4, 1/2 sec = f5.6, 1 sec = f8, 2 sec = f11, 4 sec =f16, 8 sec...etc. yup?
now, in theory going by plain maths, this means that 1000 units of light falling on the film for a duration of 1 sec gives the same exposure as 500 units of light falling on the film for 2 sec which also gives the same exposure for 250 units of light falling on the film for 4 seconds...until blah blah blah ..= same as 1 unit of light falling on the film for 1000 seconds?
no.
it is found out that if 1000 units of light falling for 1 sec on film gives different results from 1 unit of light falling for 1000 seconds. essentially u get soem weird colour shifts. when the reciprocally equivalent relation gets skewed towards the "less light for longer time" end of the spectrum, the relation starts to break down and go haywire. hence the term "reciprocity failure" ie the reciprocal equation fails to hold true as we approach nearer to the case of "less light for longer time"
more light with less time~ soemthing like daylight shooting
less light with more time ~ something like long exposures at night.