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تقنيات الطباعة المتقدمة
التحكم في الطباعة باستخدام المكبر
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ILLUMINATION SYSTEMS
Enlarger illumination systems fall into two main categories : condenser and diffusion . These differ markedly in construction , , and in results obtained , so that a full discussion of each is essential to any work on the subject of enlarging . Figures 7 and 8 show in diagrammatic form representative enlargers of each type . These diagrams to in the discussion that follows .
The two types of enlargers are here . Figure 7 shows how a diffusion enlarger works , and Figure 8 illustrates the principles of a condenser enlarger
DIFFUSION ENLARGER . In a diffusion enlarger , the object is to illuminate the negative by as large and diffuse a source of light as possible . This can be done by placing a diffusing screen , such as a sheet of flashed - opal glass or groundglass , in back of the negative and illuminating it either with a gas - filled tube or with an incandescent light bulb of some kind .
The diffusion - type enlarger shown in Figure 7 is of standard , acceptable design . The light source is an opal glass enlarger lamp which provides a fairly large initial light source . It has a large reflector , which is parabolic in shape to throw as uniform a beam as possible . The reflector may be brightly polished for increased efficiency if it is well designed and constructed , and care is taken to focus the lamp within it for maximum uniformity of illumination . However , since a polished reflector requires careful focusing , it is often given a matte or satin finish so that focusing the lamp will not require extreme accuracy . A sheet of diffusing glass is also placed between the lamp and the negative to serve as the final stage of diffusion . This glass may be frosted or ground on one side , on both sides , or it may be flashed opal ; the degree of diffusion and also the loss of illumination increasing in the order named . A simple expedient to provide diffusion with the least loss of light is to use a sheet of sandblasted glass that is heavily frosted in the center , with the frosting gradually diminishing toward the edges until the glass is clear .
With diffused illumination , only a small part of the total light transmitted by the negative reaches the projection lens . Figure 9 illustrates this point . Let us assume that the light striking the negative is perfectly diffused that the diffusion screen can then be considered as the light source . If we take just one point , P , on the diffusion screen , we see that of all the light it emits , only the small bundle within angle A , is intercepted by the projection lens and utilized in forming the enlarged image .
This system of projection is obviously very wasteful of light . It is not practical for small negatives , which require a high lighting efficiency , for two reasons the greater magnification calls for a much greater utilization of the available light , and the smaller size of the enlarger housing generally limits the lamp wattage that may safely be employed . Diffusion - type enlargers are more practical for negatives measuring 4 x 5 or larger .
CONDENSER ENLARGER . The condenser type of enlarger overcomes this drawback to a considerable degree through the use of an auxiliary lens system which gathers or condenses the light coming from the source and puts a relatively high percentage of it through the projection lens . Figure 10 represents an ideal condenser enlarger . The light source , S , is focused by the condensing lenses , L , so that the entire beam which illuminates the negative also passes through the pro jection lens , P , and onto the paper . The projection lens , P , in turn , focuses the negative so that when light the paper , the negative is in sharp focus
With a theoretically perfect system of this kind , all the light reaching the negative passes through the projection lens and is utilized in making the picture . In practice , of course , this ideal condition is never achieved . Nevertheless , practical condenser enlargers are considerably
faster than equivalent diffusion types .
Figure 9 The rather narrow . cone of light used by the lens of a diffusion enlarger accounts for its relatively . slow speed .
Figure 10 The wider cone of light used by the condenser enlarger gives this type a faster speed .
Principles of a condenser enlarger . Figure 11 shows that light from the lamp is picked up by the condensers , passes through the negative , and is brought to a point within the enlarger lens . In Figure 12 , the enlarger head is raised to make a larger print . The enlarger lens must now be refocused by being brought closer to the negative . In Figure 13 , the lamp has been raised with respect to the condenser . The light again focuses to a point in the enlarger lens , providing maximum light and uniform illumination .
Referring to Figure 11 , we see that there are two distinct lens systems : the condenser lens , L , which projects an image of the light source , S , within the projection lens , and the projection lens , which projects an image of the negative . onto the easel . Let us see what happens if we change the degree of enlargement . First of all , we change the distance from the lens to the easel . This of necessity requires refocusing the projection lens , which changes the distance between lens and negative . Now all the light from the condenser no longer enters the projection lens , but instead some of it by - passes the lens barrel , as shown in Figure 12 .
With a theoretical point source of light at S , this would only mean some loss of illumination . Actually , the projection lamp filament has appreciable size and , as a result , some of the light rays which may be cut off by the projection lens barrel will produce streaks and dark areas in the projection field . This requires that the light source be refocused until its image again falls within the lens . This new set of conditions is shown in Figure 13 .
Refocusing the lamp as well as the lens for every change in the degree of enlargement is obviously not practical . By introducing some diffusion between .
the lamp and condenser lens , its exact position is made less critical so that it needn't be refocused throughout the normal range of enlargement . The simplest means of introducing this diffusion is to use an opal enlarging bulb . Some enlargers also add a sheet of groundglass just before the condenser lens or even etch the flat surface of the back condenser to increase the diffusion still further .
Focal Length of Condenser Lens . The focal length of the condenser lens must be such that it will project an image of the light source within the projection lens . If the condenser focal length is too long for the projection lens , you will get a condition similar to Figure 12 ; if too short , the rays will converge to a point some distance in front of the lens and then fan out again so that when they reach the projection lens , the diameter of the cone of light will again be larger than the lens .
The condensers ordinarily supplied with any enlarger are designed to work with a lens of normal focal length for the full negative size taken by that enlarger . If for any reason you intend using an enlarger that is considerably longer or shorter than normal , it may be necessary to use a special condenser lens to match it , unless a considerable amount of diffusion is introduced . For example , a 4 x 5 enlarger is usually supplied with a condenser that is right for a 5 to 6½ inch enlarger lens . If you use such an enlarger for 35mm film and you use a 2 - inch projection lens , the condenser will no longer match , and you will get a condition similar to that shown in Figure 12. In that case , you must use a condenser with a shorter focal length .
FOCAL LENGTH OF PROJECTION LENS . In an enlarger , the focal length of the projection lens bears about the same relationship to the size of the negative as in a camera . It is , therefore , usual practice to choose a focal length that is approximately equal to the diagonal of the negative . This rule doesn't work for larger negatives ( 4 x 5 and up ) because the degree of enlargement is less and so the negative - to - lens distance is considerably greater than the focal length . This permits the use of relatively short - focus lenses for the larger negatives .
For any given height of the enlarger housing from the easel , the size of the enlargement is in inverse ratio to the focal length of the projection lens . In other words , a lens of short focal length will enable you to make larger blow - ups from an equal enlarger height . For that reason , it is advisable to use the shortest focal length that will cover the negative to the corners .
The maximum height to which an enlarger can be raised on its vertical supports is usually enough to permit a 16 x 20 enlargement from the full negative . However , it is often necessary to enlarge only part of a negative to 14 x 17 or even 16 x 20 , and in that case there may not be enough head room . In such instances , it is usual practice to project the image onto the floor but this is very clumsy and awkward . It is much more convenient to equip your enlarger with an extra short - focus lens for extreme blow - ups from small parts of the negative . The short focal length will give you proportionally greater magnification . Before you buy the second lens , however , try it out on your own enlarger to be sure you get even illumination over the small area . covers .
NEGATIVE ILLUMINATION AND PRINT QUALITY . The type of negative illumination provided by the enlarger will have a marked effect on the final print from the standpoint of its ( 1 ) contrast , ( 2 ) graininess , ( 3 ) spotting requirements , and ( 4 ) sharpness .
1. Contrast . A condenser enlarger will produce a more contrasty print from any negative than will a diffusion - type enlarger . The difference in contrast will depend on the extent to which the condenser enlarger approaches the ideal system illustrated in Figure 11 and , also , on the negative itself - but with an average negative and a good commercial condenser enlarger , the difference may be as much as one contrast grade of paper . This increase in contrast arises from the fact that in a condenser enlarger the light passes through the clear negative areas . with undiminished intensity , but suffers a double loss when passing through areas containing some image deposit . Some of the light is absorbed by the image and some is diffused and scattered by reflection from the surface of the individual silver grains in much the same way as light is diffused and scattered by a sheet of groundglass .
In a diffusion enlarger , the light reaching the negative is already well . diffused . The further scattering effect of the silver grains makes but little difference . The loss of light by scatter is , therefore , more nearly uniform between the clear and dense portions of the negative , and the density difference ( or contrast ) is not increased .
2. Graininess . When a negative is enlarged more than ten diameters , the print may be noticeably grainy . The degree to which this grain is reproduced from any given negative depends upon the sharpness of the projection lens and the degree of diffusion in the enlarger - the greater the diffusion , the less noticeable the grain . All commercial condenser enlargers use a considerable amount of diffusion so that the difference in grain is not as pronounced as it would otherwise be .
3. Reproduction of Scratches . ( Spotting Requirements . ) Very fine scratches and abrasion marks on the negative show up prominently when an undiffused condenser enlarger is used . Such scratches scatter the light going through them so that less of it is focused on the print . As a result , they show up as white lines , which must be painstakingly spotted out on the print . It is generally necessary to introduce a certain amount of diffusion in all commercial condenser enlargers to reduce the effect of such scratch marks as much as possible , consistent with speed and definition .
4. Sharpness . Condenser enlargers will produce sharper prints than diffusion enlargers . The difference becomes more noticeable as the magnification increases . Since commercial condenser enlargers have a considerable degree of diffusion , the difference in sharpness between the average condenser and diffusion enlarger is not sufficiently great to be noticeable , although it may become so in very exact work .
تقنيات الطباعة المتقدمة
التحكم في الطباعة باستخدام المكبر
أنظمة الإضاءة
ILLUMINATION SYSTEMS
Enlarger illumination systems fall into two main categories : condenser and diffusion . These differ markedly in construction , , and in results obtained , so that a full discussion of each is essential to any work on the subject of enlarging . Figures 7 and 8 show in diagrammatic form representative enlargers of each type . These diagrams to in the discussion that follows .
The two types of enlargers are here . Figure 7 shows how a diffusion enlarger works , and Figure 8 illustrates the principles of a condenser enlarger
DIFFUSION ENLARGER . In a diffusion enlarger , the object is to illuminate the negative by as large and diffuse a source of light as possible . This can be done by placing a diffusing screen , such as a sheet of flashed - opal glass or groundglass , in back of the negative and illuminating it either with a gas - filled tube or with an incandescent light bulb of some kind .
The diffusion - type enlarger shown in Figure 7 is of standard , acceptable design . The light source is an opal glass enlarger lamp which provides a fairly large initial light source . It has a large reflector , which is parabolic in shape to throw as uniform a beam as possible . The reflector may be brightly polished for increased efficiency if it is well designed and constructed , and care is taken to focus the lamp within it for maximum uniformity of illumination . However , since a polished reflector requires careful focusing , it is often given a matte or satin finish so that focusing the lamp will not require extreme accuracy . A sheet of diffusing glass is also placed between the lamp and the negative to serve as the final stage of diffusion . This glass may be frosted or ground on one side , on both sides , or it may be flashed opal ; the degree of diffusion and also the loss of illumination increasing in the order named . A simple expedient to provide diffusion with the least loss of light is to use a sheet of sandblasted glass that is heavily frosted in the center , with the frosting gradually diminishing toward the edges until the glass is clear .
With diffused illumination , only a small part of the total light transmitted by the negative reaches the projection lens . Figure 9 illustrates this point . Let us assume that the light striking the negative is perfectly diffused that the diffusion screen can then be considered as the light source . If we take just one point , P , on the diffusion screen , we see that of all the light it emits , only the small bundle within angle A , is intercepted by the projection lens and utilized in forming the enlarged image .
This system of projection is obviously very wasteful of light . It is not practical for small negatives , which require a high lighting efficiency , for two reasons the greater magnification calls for a much greater utilization of the available light , and the smaller size of the enlarger housing generally limits the lamp wattage that may safely be employed . Diffusion - type enlargers are more practical for negatives measuring 4 x 5 or larger .
CONDENSER ENLARGER . The condenser type of enlarger overcomes this drawback to a considerable degree through the use of an auxiliary lens system which gathers or condenses the light coming from the source and puts a relatively high percentage of it through the projection lens . Figure 10 represents an ideal condenser enlarger . The light source , S , is focused by the condensing lenses , L , so that the entire beam which illuminates the negative also passes through the pro jection lens , P , and onto the paper . The projection lens , P , in turn , focuses the negative so that when light the paper , the negative is in sharp focus
With a theoretically perfect system of this kind , all the light reaching the negative passes through the projection lens and is utilized in making the picture . In practice , of course , this ideal condition is never achieved . Nevertheless , practical condenser enlargers are considerably
faster than equivalent diffusion types .
Figure 9 The rather narrow . cone of light used by the lens of a diffusion enlarger accounts for its relatively . slow speed .
Figure 10 The wider cone of light used by the condenser enlarger gives this type a faster speed .
Principles of a condenser enlarger . Figure 11 shows that light from the lamp is picked up by the condensers , passes through the negative , and is brought to a point within the enlarger lens . In Figure 12 , the enlarger head is raised to make a larger print . The enlarger lens must now be refocused by being brought closer to the negative . In Figure 13 , the lamp has been raised with respect to the condenser . The light again focuses to a point in the enlarger lens , providing maximum light and uniform illumination .
Referring to Figure 11 , we see that there are two distinct lens systems : the condenser lens , L , which projects an image of the light source , S , within the projection lens , and the projection lens , which projects an image of the negative . onto the easel . Let us see what happens if we change the degree of enlargement . First of all , we change the distance from the lens to the easel . This of necessity requires refocusing the projection lens , which changes the distance between lens and negative . Now all the light from the condenser no longer enters the projection lens , but instead some of it by - passes the lens barrel , as shown in Figure 12 .
With a theoretical point source of light at S , this would only mean some loss of illumination . Actually , the projection lamp filament has appreciable size and , as a result , some of the light rays which may be cut off by the projection lens barrel will produce streaks and dark areas in the projection field . This requires that the light source be refocused until its image again falls within the lens . This new set of conditions is shown in Figure 13 .
Refocusing the lamp as well as the lens for every change in the degree of enlargement is obviously not practical . By introducing some diffusion between .
the lamp and condenser lens , its exact position is made less critical so that it needn't be refocused throughout the normal range of enlargement . The simplest means of introducing this diffusion is to use an opal enlarging bulb . Some enlargers also add a sheet of groundglass just before the condenser lens or even etch the flat surface of the back condenser to increase the diffusion still further .
Focal Length of Condenser Lens . The focal length of the condenser lens must be such that it will project an image of the light source within the projection lens . If the condenser focal length is too long for the projection lens , you will get a condition similar to Figure 12 ; if too short , the rays will converge to a point some distance in front of the lens and then fan out again so that when they reach the projection lens , the diameter of the cone of light will again be larger than the lens .
The condensers ordinarily supplied with any enlarger are designed to work with a lens of normal focal length for the full negative size taken by that enlarger . If for any reason you intend using an enlarger that is considerably longer or shorter than normal , it may be necessary to use a special condenser lens to match it , unless a considerable amount of diffusion is introduced . For example , a 4 x 5 enlarger is usually supplied with a condenser that is right for a 5 to 6½ inch enlarger lens . If you use such an enlarger for 35mm film and you use a 2 - inch projection lens , the condenser will no longer match , and you will get a condition similar to that shown in Figure 12. In that case , you must use a condenser with a shorter focal length .
FOCAL LENGTH OF PROJECTION LENS . In an enlarger , the focal length of the projection lens bears about the same relationship to the size of the negative as in a camera . It is , therefore , usual practice to choose a focal length that is approximately equal to the diagonal of the negative . This rule doesn't work for larger negatives ( 4 x 5 and up ) because the degree of enlargement is less and so the negative - to - lens distance is considerably greater than the focal length . This permits the use of relatively short - focus lenses for the larger negatives .
For any given height of the enlarger housing from the easel , the size of the enlargement is in inverse ratio to the focal length of the projection lens . In other words , a lens of short focal length will enable you to make larger blow - ups from an equal enlarger height . For that reason , it is advisable to use the shortest focal length that will cover the negative to the corners .
The maximum height to which an enlarger can be raised on its vertical supports is usually enough to permit a 16 x 20 enlargement from the full negative . However , it is often necessary to enlarge only part of a negative to 14 x 17 or even 16 x 20 , and in that case there may not be enough head room . In such instances , it is usual practice to project the image onto the floor but this is very clumsy and awkward . It is much more convenient to equip your enlarger with an extra short - focus lens for extreme blow - ups from small parts of the negative . The short focal length will give you proportionally greater magnification . Before you buy the second lens , however , try it out on your own enlarger to be sure you get even illumination over the small area . covers .
NEGATIVE ILLUMINATION AND PRINT QUALITY . The type of negative illumination provided by the enlarger will have a marked effect on the final print from the standpoint of its ( 1 ) contrast , ( 2 ) graininess , ( 3 ) spotting requirements , and ( 4 ) sharpness .
1. Contrast . A condenser enlarger will produce a more contrasty print from any negative than will a diffusion - type enlarger . The difference in contrast will depend on the extent to which the condenser enlarger approaches the ideal system illustrated in Figure 11 and , also , on the negative itself - but with an average negative and a good commercial condenser enlarger , the difference may be as much as one contrast grade of paper . This increase in contrast arises from the fact that in a condenser enlarger the light passes through the clear negative areas . with undiminished intensity , but suffers a double loss when passing through areas containing some image deposit . Some of the light is absorbed by the image and some is diffused and scattered by reflection from the surface of the individual silver grains in much the same way as light is diffused and scattered by a sheet of groundglass .
In a diffusion enlarger , the light reaching the negative is already well . diffused . The further scattering effect of the silver grains makes but little difference . The loss of light by scatter is , therefore , more nearly uniform between the clear and dense portions of the negative , and the density difference ( or contrast ) is not increased .
2. Graininess . When a negative is enlarged more than ten diameters , the print may be noticeably grainy . The degree to which this grain is reproduced from any given negative depends upon the sharpness of the projection lens and the degree of diffusion in the enlarger - the greater the diffusion , the less noticeable the grain . All commercial condenser enlargers use a considerable amount of diffusion so that the difference in grain is not as pronounced as it would otherwise be .
3. Reproduction of Scratches . ( Spotting Requirements . ) Very fine scratches and abrasion marks on the negative show up prominently when an undiffused condenser enlarger is used . Such scratches scatter the light going through them so that less of it is focused on the print . As a result , they show up as white lines , which must be painstakingly spotted out on the print . It is generally necessary to introduce a certain amount of diffusion in all commercial condenser enlargers to reduce the effect of such scratch marks as much as possible , consistent with speed and definition .
4. Sharpness . Condenser enlargers will produce sharper prints than diffusion enlargers . The difference becomes more noticeable as the magnification increases . Since commercial condenser enlargers have a considerable degree of diffusion , the difference in sharpness between the average condenser and diffusion enlarger is not sufficiently great to be noticeable , although it may become so in very exact work .
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