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تحديد الوضع الصحيح ..
أنواع خاصة من عدادات التعرض ..
SPECIAL TYPES OF EXPOSURE METERS
SPOTMETERS
In the previous discussion , we discussed ways of arriving at an overall correct exposure by measuring the brightness of various areas throughout the camera field of view . There are times when this may be very difficult or entirely impossible with the ordinary type of exposure meter . For example , you may not be able to get close enough to all the areas you wish to measure , or the picture takes in such a large area that it simply is not practical to spend the time to go from one spot to another . To solve both of these conditions , a spotmeter may be used . Such a meter is simply a highly sensitive exposure meter which takes in a very narrow angle of view , usually from 1º to 3 ° . A typical spotmeter is shown in Figure 16 .
Because of the extremely narrow angle of view , the spot meter must be aimed with great accuracy . It is , therefore , pro vided with a viewfinder some what like a camera , and this viewfinder shows exactly the area being read . To use a spot meter , simply point it at the area you wish to measure and take the reading .
You can use a spotmeter in one of two ways . The quickest is to select an area which seems about average for the overall scene and read it directly . For more accurate results , use the brightness range method . With a spotmeter , this can be done quite easily from the camera position , thus saving you a considerable amount of time and trouble .
Fig . 16 A typical spotmeter .
BUILT - IN METERS
We have discussed built - in meters in a general way in Assignment 1. However , at this point , we can consider built - in meters as coming under two types . In one type , the meter is built into the camera in such a way that the photocell is placed close to the lens but is entirely independent of it . Such a meter is used in much the same way as a separate meter , except that it is coupled to the aperture and shut ter speed controls of the camera .
Fig . 17 This illustration shows a number of possible locations for the photo - cell in a through the - lens metering system . The cell may be located on the back of the mirror and fine slots cut through the silvered surface , or one or more cells may be located in the pentaprism housing at positions labeled 1 , 2 and 3 .
The second type of built - in meter is often referred to as a TL meter because it reads the light through the lens of the camera TL being an abbreviation of through - the - lens . There are a number of ingenious methods of accomplishing this result . In one the photocell is attached to the back of the mirror and a number of very fine lines are cut through the surface of the mirror so that a small percentage of the light goes through the mirror on to the cell instead of being reflected upward to the pentaprism for viewing purposes . In other TL systems , two or three tiny photocells are placed in the pentaprism housing in such a way that they read the image on the groundglass . screen . Figure 17 shows the placement of the photo - cells in both of these systems .
Both of the above methods take an average or integrated reading of the entire image area . However , in some cameras a tiny photocell is positioned just in front of the film and takes in only the small central area of the field of view . Such a meter is in essence a spotmeter . When using such a meter , the operator must be sure that the central area represents a typical or average area of the picture or his exposures will be incorrect . If the central area is not truly representative of the entire field of view , the photographer must shift the camera position slightly when taking his exposure reading so as to get a truly average area . After the reading is taken in that way , the camera is again aimed to properly frame the desired picture .
The photocell , of course , is mounted in such a way that it drops out of the field of view just before the shutter opens .
STUDIO EXPOSURE METERS
For use in a commercial studio where portability is of relatively minor importance , the exposure meter can be much more elaborate so as to provide a much wider range of usefulness and much greater sensitivity . We need not go into too much detail in a discussion of studio expo sure meters here , but we should point out one of their most valued uses . In studio exposure meters , the photocell is mounted in a separate probe , which is connected by cable to the box containing the meter and ampli fier . The probe can then be used to take readings of very tiny areas , of areas which are very dimly illuminated , and even of the image on the groundglass itself . In a sense , a studio expo sure meter can actually be used as a through - the - lens meter . The student photographer is unlikely
Fig . 18 Studio exposure meter mounted in a view camera film holder . The probe can reach any area of the focal plane .
to be interested in elaborate and expensive studio meters , but there are compromise arrangements in which attachments may be added or clipped on to a standard sensitivity to give it much greater versatility so that to a large extent it will provide the same function as a studio type exposure meter . Such a meter is shown in Figure 18 .
تحديد الوضع الصحيح ..
أنواع خاصة من عدادات التعرض ..
SPECIAL TYPES OF EXPOSURE METERS
SPOTMETERS
In the previous discussion , we discussed ways of arriving at an overall correct exposure by measuring the brightness of various areas throughout the camera field of view . There are times when this may be very difficult or entirely impossible with the ordinary type of exposure meter . For example , you may not be able to get close enough to all the areas you wish to measure , or the picture takes in such a large area that it simply is not practical to spend the time to go from one spot to another . To solve both of these conditions , a spotmeter may be used . Such a meter is simply a highly sensitive exposure meter which takes in a very narrow angle of view , usually from 1º to 3 ° . A typical spotmeter is shown in Figure 16 .
Because of the extremely narrow angle of view , the spot meter must be aimed with great accuracy . It is , therefore , pro vided with a viewfinder some what like a camera , and this viewfinder shows exactly the area being read . To use a spot meter , simply point it at the area you wish to measure and take the reading .
You can use a spotmeter in one of two ways . The quickest is to select an area which seems about average for the overall scene and read it directly . For more accurate results , use the brightness range method . With a spotmeter , this can be done quite easily from the camera position , thus saving you a considerable amount of time and trouble .
Fig . 16 A typical spotmeter .
BUILT - IN METERS
We have discussed built - in meters in a general way in Assignment 1. However , at this point , we can consider built - in meters as coming under two types . In one type , the meter is built into the camera in such a way that the photocell is placed close to the lens but is entirely independent of it . Such a meter is used in much the same way as a separate meter , except that it is coupled to the aperture and shut ter speed controls of the camera .
Fig . 17 This illustration shows a number of possible locations for the photo - cell in a through the - lens metering system . The cell may be located on the back of the mirror and fine slots cut through the silvered surface , or one or more cells may be located in the pentaprism housing at positions labeled 1 , 2 and 3 .
The second type of built - in meter is often referred to as a TL meter because it reads the light through the lens of the camera TL being an abbreviation of through - the - lens . There are a number of ingenious methods of accomplishing this result . In one the photocell is attached to the back of the mirror and a number of very fine lines are cut through the surface of the mirror so that a small percentage of the light goes through the mirror on to the cell instead of being reflected upward to the pentaprism for viewing purposes . In other TL systems , two or three tiny photocells are placed in the pentaprism housing in such a way that they read the image on the groundglass . screen . Figure 17 shows the placement of the photo - cells in both of these systems .
Both of the above methods take an average or integrated reading of the entire image area . However , in some cameras a tiny photocell is positioned just in front of the film and takes in only the small central area of the field of view . Such a meter is in essence a spotmeter . When using such a meter , the operator must be sure that the central area represents a typical or average area of the picture or his exposures will be incorrect . If the central area is not truly representative of the entire field of view , the photographer must shift the camera position slightly when taking his exposure reading so as to get a truly average area . After the reading is taken in that way , the camera is again aimed to properly frame the desired picture .
The photocell , of course , is mounted in such a way that it drops out of the field of view just before the shutter opens .
STUDIO EXPOSURE METERS
For use in a commercial studio where portability is of relatively minor importance , the exposure meter can be much more elaborate so as to provide a much wider range of usefulness and much greater sensitivity . We need not go into too much detail in a discussion of studio expo sure meters here , but we should point out one of their most valued uses . In studio exposure meters , the photocell is mounted in a separate probe , which is connected by cable to the box containing the meter and ampli fier . The probe can then be used to take readings of very tiny areas , of areas which are very dimly illuminated , and even of the image on the groundglass itself . In a sense , a studio expo sure meter can actually be used as a through - the - lens meter . The student photographer is unlikely
Fig . 18 Studio exposure meter mounted in a view camera film holder . The probe can reach any area of the focal plane .
to be interested in elaborate and expensive studio meters , but there are compromise arrangements in which attachments may be added or clipped on to a standard sensitivity to give it much greater versatility so that to a large extent it will provide the same function as a studio type exposure meter . Such a meter is shown in Figure 18 .
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