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مؤثرات خاصة ..
كتاب التصوير الفوتوغرافي المغلق
Polarized patterns
A pair of polarizing screens one placed beneath the subject and the other above it - are used by scientists to enable them to analyze the positions of weak places in plastic objects which are subjected to stress . The technique - known as photoelastic stress analysis - involves making a scale model in clear plastic of the engineered structure under investigation . The model , with weights and clamps positioned to simulate the natural stress on the structure , is then placed between two polarizing screens . When unstressed plastic is viewed through two such screens , it still appears colorless ; whereas spectacular color patterns appear in stressed plastic revealing the position where the maxi mum amount of stress occurred . The screen beneath the object polarizes the light reaching it , so that only some of the light waves pass through . Any parts of the plastic model which have been subjected to stress slow down part of the polarized light beam making two parallel beams , one of which travels faster than the other . The result , viewed through the second polarized screen , is that interference between the two beams creates the color patterns .
This technique can very easily be applied to create some exciting and unusual images of everyday plastic objects . Polarized light can also be used to reveal dramatic color patterns of crystallized substances by means of photo micrography . I took all the pictures shown on these pages in the same way . The light source was a color - corrected light box normally used for viewing color transparencies , so I was able to use Kodachrome 25 daylight filmstock . A piece of plastic polarized sheet measuring 7x7 inches was placed on top the light box and the object laid on it . The camera was attached to an overhead copying stand so that the magnification could be quickly adjusted . By using a polarizing filter on the camera lens , as the second screen , I could see the effect of rotating it in relation to the lower screen by directly viewing through the camera . In each case , I metered the light passing through the object into the camera and I also took some exposures by bracketing ( see p . 38 ) half a stop on either side . In most cases I found the metered reading was the preferred one , although for the petri dish ( below left ) I selected the exposure which was half a stop less than the metered one ; this gave me a darker back ground . The intensity of the background changes dramatically from a pale gray to an intense blue as the polarizing filter is rotated on the camera .
Revealing the stress pattern
The base of a transparent petri dish , which I use for containing small subjects for photography beneath a microscope , is completely transformed when viewed through two polarizing screens . The colored pattern radiates from the point where the plastic was injected into the petri dish mold . Lens 55mm micro - Nikkor Mag , on film x0.4 Mag , on page × 1.3
◄ Polarized pens
Everyday plastic objects such as these ballpoint pens become greatly enhanced when seen through two polarizing screens . I spent some time carefully arranging a boxful of pens into this abstract linear pattern . Lens 55mm micro - Nikkor Mag . on film × 0.5 Mag . on page × 1.75
An abstract design ▷ The clear acetate slipover sleeves used to protect 35mm color transparencies provided me with the subject for this abstract picture . I found that by folding the sleeves in half and allowing them partially to spring open , the angles of the acetate material could be varied to create a much more colorful image . The apparently haphazard arrangement is deceptive . It was achieved by repeated rearrangement and critical viewing through the camera until the best effect was found . Lens 55mm micro - Nikkor Mag , on film * 0.3 Mag . on page × 2.2
مؤثرات خاصة ..
كتاب التصوير الفوتوغرافي المغلق
Polarized patterns
A pair of polarizing screens one placed beneath the subject and the other above it - are used by scientists to enable them to analyze the positions of weak places in plastic objects which are subjected to stress . The technique - known as photoelastic stress analysis - involves making a scale model in clear plastic of the engineered structure under investigation . The model , with weights and clamps positioned to simulate the natural stress on the structure , is then placed between two polarizing screens . When unstressed plastic is viewed through two such screens , it still appears colorless ; whereas spectacular color patterns appear in stressed plastic revealing the position where the maxi mum amount of stress occurred . The screen beneath the object polarizes the light reaching it , so that only some of the light waves pass through . Any parts of the plastic model which have been subjected to stress slow down part of the polarized light beam making two parallel beams , one of which travels faster than the other . The result , viewed through the second polarized screen , is that interference between the two beams creates the color patterns .
This technique can very easily be applied to create some exciting and unusual images of everyday plastic objects . Polarized light can also be used to reveal dramatic color patterns of crystallized substances by means of photo micrography . I took all the pictures shown on these pages in the same way . The light source was a color - corrected light box normally used for viewing color transparencies , so I was able to use Kodachrome 25 daylight filmstock . A piece of plastic polarized sheet measuring 7x7 inches was placed on top the light box and the object laid on it . The camera was attached to an overhead copying stand so that the magnification could be quickly adjusted . By using a polarizing filter on the camera lens , as the second screen , I could see the effect of rotating it in relation to the lower screen by directly viewing through the camera . In each case , I metered the light passing through the object into the camera and I also took some exposures by bracketing ( see p . 38 ) half a stop on either side . In most cases I found the metered reading was the preferred one , although for the petri dish ( below left ) I selected the exposure which was half a stop less than the metered one ; this gave me a darker back ground . The intensity of the background changes dramatically from a pale gray to an intense blue as the polarizing filter is rotated on the camera .
Revealing the stress pattern
The base of a transparent petri dish , which I use for containing small subjects for photography beneath a microscope , is completely transformed when viewed through two polarizing screens . The colored pattern radiates from the point where the plastic was injected into the petri dish mold . Lens 55mm micro - Nikkor Mag , on film x0.4 Mag , on page × 1.3
◄ Polarized pens
Everyday plastic objects such as these ballpoint pens become greatly enhanced when seen through two polarizing screens . I spent some time carefully arranging a boxful of pens into this abstract linear pattern . Lens 55mm micro - Nikkor Mag . on film × 0.5 Mag . on page × 1.75
An abstract design ▷ The clear acetate slipover sleeves used to protect 35mm color transparencies provided me with the subject for this abstract picture . I found that by folding the sleeves in half and allowing them partially to spring open , the angles of the acetate material could be varied to create a much more colorful image . The apparently haphazard arrangement is deceptive . It was achieved by repeated rearrangement and critical viewing through the camera until the best effect was found . Lens 55mm micro - Nikkor Mag , on film * 0.3 Mag . on page × 2.2
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