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فن النسخ
التعرض
EXPOSURE
Getting the correct exposure is extremely important in copy work , but simply using an exposure meter in the normal manner as you would for ordinary subject material is not enough to insure correct exposure for copy work . There are three factors which you must bear in mind and then adjust the indicated exposure as given by your exposure meter accordingly . These are :
1. Reciprocity law failure . Compensation for this is often necessary , particularly with color film , because exposures for copy work tend to be rather long .
2. Filter factors . ( If color filters are used . )
3. Lens extension .
We have discussed the first two of the above three points in considerable detail in earlier Assignments and you should be quite familiar with them . The third point - lens extension - is a new one and so we will discuss it at this time .
The f / number calibrations on any lens remain correct only if the lens is focused at infinity - that is , when the lens is at its minimum distance from the film plane . When the lens is focused on nearby objects , it is farther away from the film plane or focal plane so that its true or effective f / number is actually larger than the marked f / number , and the exposure must be increased accordingly .
For ordinary work up to about 5 feet or so , the required increase in exposure is quite small and can be neglected because the latitude of the film is sufficient to take care of it . However , for close - up distances , the exposure increase is too great to come within the normal exposure latitude of the film , and it is necessary for the photographer actually to increase the exposure he gives .
As an example , when photographing something full size , as in copying a document to the same size as the original , the lens is actually at twice the distance from the film as it would be when focused on infinity . This means that the true or effective f / number is exactly twice that of the indicated f / number , and this , as you know means that the film is receiving only one - fourth the exposure indicated by the f / number marked on it . To put it another way , if you copy something full size , you must give four times the exposure indicated by the exposure meter if you want to get correct exposure .
For close - up photography , therefore , you must increase the exposure by a factor which is determined by the actual lens extension . There are a number of ways in which this can be done .
If you are copying with a 35mm camera using extension tubes , the instruction book will usually give you the exposure increase required for each section of the extension tube assembly . If you are using a 35mm camera with a bellows extension made for it , you may find that one of the side rods of the bellows unit is marked off to show either the extension in millimeters or the exposure factor for the lenses most frequently used in such work .
If you do not have the exposure factors , you must figure them out for yourself , and the first step is to determine the actual extension either in millimeters or in inches . If you are using a set of extension tubes , the problem is quite simple - you simply measure the length of the tubes and add that length to the focal length of the lens to give you the total extension . For example , let us say the focal length of the lens in your 35mm camera is two inches ( 50mm ) and the extension tube is one inch long . When using that particular tube , the extension is , therefore , three inches .
If you are using a camera such as a Speed Graphic which is equipped with a distance scale , you can determine the lens extension for close - ups by measuring the distance between infinity mark on the focusing scale and the distance indicator and adding that to the focal length of the lens to give the total extension .
If you are using a view camera without a focusing scale in a homemade copying stand as illustrated in Figure 20 , point the camera out of the window and focus on something at least 300 feet away . Mark off some point on the camera front or back , whichever you move for focusing , and make another mark on the camera bed or gear rack so that these two marks line up exactly . Write down the focal length of the lens on the gear rack or camera bed at this point and use it as a start of the extension scale . Now mark off the remainder of the gear rack or camera bed in inches . For example , let us say the focal length of your lens is 82 inches so that the first mark on the camera bed would be labeled 8½ . The second mark should be ½ inch away from it , and it would be labeled 9. The third mark would be 10 and so on for the entire length of the bed . You can now read off the lens extension directly from the camera bed and , if necessary , you can easily estimate the half inch or quarter inch positions accurately enough for all practical purposes .
When you have the lens extension , it is a simple matter to calculate the exposure increase . There are a number of ways this can be done . Perhaps the easiest is to obtain a simple exposure calculator for close - up work from your dealer . If you don't have such a calculator , use the following formula :
= Effective f / number Marked f / number x lens extension Focal length
For example , a 10 - inch lens racked out 5 inches from the infinity mark extension 15 inches ) and stopped down to f / 8 would have an effective f value of 12 as follows :
8 x 15 /10 = f / 12
فن النسخ
التعرض
EXPOSURE
Getting the correct exposure is extremely important in copy work , but simply using an exposure meter in the normal manner as you would for ordinary subject material is not enough to insure correct exposure for copy work . There are three factors which you must bear in mind and then adjust the indicated exposure as given by your exposure meter accordingly . These are :
1. Reciprocity law failure . Compensation for this is often necessary , particularly with color film , because exposures for copy work tend to be rather long .
2. Filter factors . ( If color filters are used . )
3. Lens extension .
We have discussed the first two of the above three points in considerable detail in earlier Assignments and you should be quite familiar with them . The third point - lens extension - is a new one and so we will discuss it at this time .
The f / number calibrations on any lens remain correct only if the lens is focused at infinity - that is , when the lens is at its minimum distance from the film plane . When the lens is focused on nearby objects , it is farther away from the film plane or focal plane so that its true or effective f / number is actually larger than the marked f / number , and the exposure must be increased accordingly .
For ordinary work up to about 5 feet or so , the required increase in exposure is quite small and can be neglected because the latitude of the film is sufficient to take care of it . However , for close - up distances , the exposure increase is too great to come within the normal exposure latitude of the film , and it is necessary for the photographer actually to increase the exposure he gives .
As an example , when photographing something full size , as in copying a document to the same size as the original , the lens is actually at twice the distance from the film as it would be when focused on infinity . This means that the true or effective f / number is exactly twice that of the indicated f / number , and this , as you know means that the film is receiving only one - fourth the exposure indicated by the f / number marked on it . To put it another way , if you copy something full size , you must give four times the exposure indicated by the exposure meter if you want to get correct exposure .
For close - up photography , therefore , you must increase the exposure by a factor which is determined by the actual lens extension . There are a number of ways in which this can be done .
If you are copying with a 35mm camera using extension tubes , the instruction book will usually give you the exposure increase required for each section of the extension tube assembly . If you are using a 35mm camera with a bellows extension made for it , you may find that one of the side rods of the bellows unit is marked off to show either the extension in millimeters or the exposure factor for the lenses most frequently used in such work .
If you do not have the exposure factors , you must figure them out for yourself , and the first step is to determine the actual extension either in millimeters or in inches . If you are using a set of extension tubes , the problem is quite simple - you simply measure the length of the tubes and add that length to the focal length of the lens to give you the total extension . For example , let us say the focal length of the lens in your 35mm camera is two inches ( 50mm ) and the extension tube is one inch long . When using that particular tube , the extension is , therefore , three inches .
If you are using a camera such as a Speed Graphic which is equipped with a distance scale , you can determine the lens extension for close - ups by measuring the distance between infinity mark on the focusing scale and the distance indicator and adding that to the focal length of the lens to give the total extension .
If you are using a view camera without a focusing scale in a homemade copying stand as illustrated in Figure 20 , point the camera out of the window and focus on something at least 300 feet away . Mark off some point on the camera front or back , whichever you move for focusing , and make another mark on the camera bed or gear rack so that these two marks line up exactly . Write down the focal length of the lens on the gear rack or camera bed at this point and use it as a start of the extension scale . Now mark off the remainder of the gear rack or camera bed in inches . For example , let us say the focal length of your lens is 82 inches so that the first mark on the camera bed would be labeled 8½ . The second mark should be ½ inch away from it , and it would be labeled 9. The third mark would be 10 and so on for the entire length of the bed . You can now read off the lens extension directly from the camera bed and , if necessary , you can easily estimate the half inch or quarter inch positions accurately enough for all practical purposes .
When you have the lens extension , it is a simple matter to calculate the exposure increase . There are a number of ways this can be done . Perhaps the easiest is to obtain a simple exposure calculator for close - up work from your dealer . If you don't have such a calculator , use the following formula :
= Effective f / number Marked f / number x lens extension Focal length
For example , a 10 - inch lens racked out 5 inches from the infinity mark extension 15 inches ) and stopped down to f / 8 would have an effective f value of 12 as follows :
8 x 15 /10 = f / 12
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