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تجهم الصور ..
المصراع ..
THE SHUTTER
No matter how simple and inexpensive the camera , it wouldn't be very practical without a shutter to regulate the length of exposure . In the early days of photography when sensitive material was very slow and exposures were long , it was the general practice to expose by removing the lens cap for a short time and putting it on again after the correct interval of time . Today this method is wholly inadequate for most circumstances , although it can still be used in an emergency under poor light with the lens stopped down so as to require an exposure of several seconds .
Shutters are highly important with modern fast film because exact timing , particularly for short exposures , is so necessary to good work , and because in many instances the shutter case maintains the spacing and alignment of the lens elements . A very fine , highly corrected lens cannot give the performance for which it is designed if it is not properly mounted in its barrel or shutter case .
Modern shutters may generally be classified into three categories : ( 1 ) Between - the - lens ; ( 2 ) Behind - the - lens ; and ( 3 ) Focal - plane .
BETWEEN - THE - LENS SHUTTER
This shutter is mounted in the lens barrel between the lens elements , hence its name . It consists of a number of very thin metal blades so arranged that they normally prevent any light from getting past them . By an elaborate system of levers , gears , and springs , the blades snap open , remain open for a predetermined interval of time , and then snap shut again . The length of time they stay open constitutes the shutter speed , and this speed can be adjusted by moving a lever or rotating a ring . In the lower - priced shutters , pressing down on the lever builds up the necessary spring tension to trip the shutter , so only one release lever is provided . In the better and more accurate shutters , one lever is provided to cock the shutter , and exposure is made by pressing a separate trigger release . This permits smoother action and prevents jarring the camera when the exposure is made .
Most shutters used with lenses designed primarily for commercial work have settings for time and bulb exposures and for a wide range of short instantaneous exposures . With the time exposure setting ( marked T on the shutter ) , the shutter blades fly open when the release is pressed and stay open until the release is pressed second time . With the bulb exposure ( marked B ) , the shutter blades fly open when the release is pressed , but stay open only as long as pressure is maintained . The blades close again as soon as the exposure lever or button is released . It is generally customary to use the bulb setting for not over five seconds exposure , and time exposure for anything longer than that .
With the smaller hand cameras , particularly the 35mm size , the true time setting is often omitted and only the bulb setting is provided . In such cases , it is usually marked " T " and referred to as the time exposure setting , even though it is really a bulb exposure setting . In other words , if there is only a T setting and not a B setting , the T setting will keep the lens open only so long as the release is pressed . The shutter will close when pressure is released .
Some between - the - lens shutters have built - in flash synchronizer switches so they can be used to fire flashbulbs at exactly the right instant and thus permit very short flash exposures . Such shutters , of course , do not require flashgun synchronizers . This subject is discussed in detail in the Assignment on flash photography much later in this course .
Between - the - lens shutters are often called leaf shutters because of the individual blades or leaves which open and close to make the exposure . The term leaf shutter is actually beginning to replace the term between - the - lens shutter , even though it is not as accurate a term because some behind - the - lens or even focal - plane shutters are also leaf shutters .
BEHIND - THE - LENS SHUTTER
This type of shutter was at one time the most common for all studio work when films were quite slow . In studio work , the lens was stopped down con siderably for depth of field and unless a film was quite fast , exposures ran into seconds rather than fractions of seconds . Under those circumstances , the accuracy of a shutter was relatively unimportant . It was , therefore , customary to use an inexpensive shutter mounted on the lens board inside the camera , and to purchase lenses mounted in simple barrels containing only the diaphragm . This made it possible to use the same shutter with a number of lenses , thus reducing the cost considerably . Today , film is much more sensitive so that exposures are too short to be timed with a watch and old - fashioned behind - the - lens shutters are no longer . practical . As a result , except for strictly portrait work with large cameras and very long focal length lenses , the old - fashioned behind - the - lens shutter is a thing of the past .
There are , however , certain highly specialized shutters sold as accessories which are normally mounted behind a lens . The modern highly accurate electronic shutter is a case in point . Electronic shutters are quite expensive and are still not in general use . Because of their highly specialized nature , they will not be discussed here .
FOCAL PLANE SHUTTER
The focal - plane shutter derives its name from the fact that it is located just in front of the focal plane - that is , just in front of the film . The conventional focal - plane shutter is like a roller blind or curtain with a slit or aperture in it the full width of the film . This curtain works on rollers and is made to travel . immediately in front of the sensitive surface , the exposure being made as the opening or slit in the curtain passes across the film . The exposures can be varied by altering the width of the slit , as well as by increasing or decreasing the tension of the spring that moves the curtain . Usually the curtain is made of an opaque rubberized cloth , but in some cameras , the curtain is made of metal .
For high speeds , the focal - plane shutter is more efficient than the between - the - lens shutter because in the latter type , part of the time is taken up by the shutter opening and closing . For slow speeds and small lens openings , however , the between - the - lens shutter may be just as efficient as the focal - plane shutter , if not more so .
A focal - plane shutter is capable of giving shorter exposures than is possible with a between - the - lens shutter , and therefore , it is more suitable for high - speed work . However , the modern between - the - lens shutters are now so much improved both in speed and efficiency that they are capable of dealing with all but the most rapid exposure .
Some 35mm cameras use a rather unusual focal - plane shutter which has some of the advantages of the leaf shutter . It has four blades positioned in the focal plane and exposure is made by causing these blades to open and close in much the same manner as the blades or leaves of the between - the - lens shutter .
The fastest shutter speed of this type of focal - plane shutter is generally 1 / 500th second as compared to the standard 1 / 1000th second exposure of the conventional focal - plane shutter .
WHAT SHUTTER SPEED TO USE
Whenever you make an exposure , you will be faced with the problem of the correct shutter speed to use even after you have determined the correct exposure . For example , you will get exactly the same exposure whether you give 1/250 second at f / 8 , 1/125 second at f / 11 , or 1/60 second at f / 16 . You cannot , however , use these three exposures interchangeably . As a very rough guide , you should use the various shutter speeds for the following purposes :
1/1000 to 1/250 for stopping action of rapidly moving subjects .
1/250 to 1/125 for most hand - held exposures .
Under 1/125 for still subjects when the lens must be stopped down for maximum depth of field .
A tripod should be used for all exposures longer than 1/60 second with the ordinary roll or hand camera . The average person cannot hold a small camera . steady for more than 1/50 and sometimes only 1/100 second . If the negative is to be greatly enlarged , it is always best to use the faster shutter speed or else set the camera on a tripod .
The above discussion of shutters is necessarily of a general nature , since you are just beginning your studies . More detailed information on this subject will be included in subsequent Assignments as your knowledge grows . As an example , Assignments covering 35mm photography and flash photography will have quite a bit to say about shutters and shutter speeds .
تجهم الصور ..
المصراع ..
THE SHUTTER
No matter how simple and inexpensive the camera , it wouldn't be very practical without a shutter to regulate the length of exposure . In the early days of photography when sensitive material was very slow and exposures were long , it was the general practice to expose by removing the lens cap for a short time and putting it on again after the correct interval of time . Today this method is wholly inadequate for most circumstances , although it can still be used in an emergency under poor light with the lens stopped down so as to require an exposure of several seconds .
Shutters are highly important with modern fast film because exact timing , particularly for short exposures , is so necessary to good work , and because in many instances the shutter case maintains the spacing and alignment of the lens elements . A very fine , highly corrected lens cannot give the performance for which it is designed if it is not properly mounted in its barrel or shutter case .
Modern shutters may generally be classified into three categories : ( 1 ) Between - the - lens ; ( 2 ) Behind - the - lens ; and ( 3 ) Focal - plane .
BETWEEN - THE - LENS SHUTTER
This shutter is mounted in the lens barrel between the lens elements , hence its name . It consists of a number of very thin metal blades so arranged that they normally prevent any light from getting past them . By an elaborate system of levers , gears , and springs , the blades snap open , remain open for a predetermined interval of time , and then snap shut again . The length of time they stay open constitutes the shutter speed , and this speed can be adjusted by moving a lever or rotating a ring . In the lower - priced shutters , pressing down on the lever builds up the necessary spring tension to trip the shutter , so only one release lever is provided . In the better and more accurate shutters , one lever is provided to cock the shutter , and exposure is made by pressing a separate trigger release . This permits smoother action and prevents jarring the camera when the exposure is made .
Most shutters used with lenses designed primarily for commercial work have settings for time and bulb exposures and for a wide range of short instantaneous exposures . With the time exposure setting ( marked T on the shutter ) , the shutter blades fly open when the release is pressed and stay open until the release is pressed second time . With the bulb exposure ( marked B ) , the shutter blades fly open when the release is pressed , but stay open only as long as pressure is maintained . The blades close again as soon as the exposure lever or button is released . It is generally customary to use the bulb setting for not over five seconds exposure , and time exposure for anything longer than that .
With the smaller hand cameras , particularly the 35mm size , the true time setting is often omitted and only the bulb setting is provided . In such cases , it is usually marked " T " and referred to as the time exposure setting , even though it is really a bulb exposure setting . In other words , if there is only a T setting and not a B setting , the T setting will keep the lens open only so long as the release is pressed . The shutter will close when pressure is released .
Some between - the - lens shutters have built - in flash synchronizer switches so they can be used to fire flashbulbs at exactly the right instant and thus permit very short flash exposures . Such shutters , of course , do not require flashgun synchronizers . This subject is discussed in detail in the Assignment on flash photography much later in this course .
Between - the - lens shutters are often called leaf shutters because of the individual blades or leaves which open and close to make the exposure . The term leaf shutter is actually beginning to replace the term between - the - lens shutter , even though it is not as accurate a term because some behind - the - lens or even focal - plane shutters are also leaf shutters .
BEHIND - THE - LENS SHUTTER
This type of shutter was at one time the most common for all studio work when films were quite slow . In studio work , the lens was stopped down con siderably for depth of field and unless a film was quite fast , exposures ran into seconds rather than fractions of seconds . Under those circumstances , the accuracy of a shutter was relatively unimportant . It was , therefore , customary to use an inexpensive shutter mounted on the lens board inside the camera , and to purchase lenses mounted in simple barrels containing only the diaphragm . This made it possible to use the same shutter with a number of lenses , thus reducing the cost considerably . Today , film is much more sensitive so that exposures are too short to be timed with a watch and old - fashioned behind - the - lens shutters are no longer . practical . As a result , except for strictly portrait work with large cameras and very long focal length lenses , the old - fashioned behind - the - lens shutter is a thing of the past .
There are , however , certain highly specialized shutters sold as accessories which are normally mounted behind a lens . The modern highly accurate electronic shutter is a case in point . Electronic shutters are quite expensive and are still not in general use . Because of their highly specialized nature , they will not be discussed here .
FOCAL PLANE SHUTTER
The focal - plane shutter derives its name from the fact that it is located just in front of the focal plane - that is , just in front of the film . The conventional focal - plane shutter is like a roller blind or curtain with a slit or aperture in it the full width of the film . This curtain works on rollers and is made to travel . immediately in front of the sensitive surface , the exposure being made as the opening or slit in the curtain passes across the film . The exposures can be varied by altering the width of the slit , as well as by increasing or decreasing the tension of the spring that moves the curtain . Usually the curtain is made of an opaque rubberized cloth , but in some cameras , the curtain is made of metal .
For high speeds , the focal - plane shutter is more efficient than the between - the - lens shutter because in the latter type , part of the time is taken up by the shutter opening and closing . For slow speeds and small lens openings , however , the between - the - lens shutter may be just as efficient as the focal - plane shutter , if not more so .
A focal - plane shutter is capable of giving shorter exposures than is possible with a between - the - lens shutter , and therefore , it is more suitable for high - speed work . However , the modern between - the - lens shutters are now so much improved both in speed and efficiency that they are capable of dealing with all but the most rapid exposure .
Some 35mm cameras use a rather unusual focal - plane shutter which has some of the advantages of the leaf shutter . It has four blades positioned in the focal plane and exposure is made by causing these blades to open and close in much the same manner as the blades or leaves of the between - the - lens shutter .
The fastest shutter speed of this type of focal - plane shutter is generally 1 / 500th second as compared to the standard 1 / 1000th second exposure of the conventional focal - plane shutter .
WHAT SHUTTER SPEED TO USE
Whenever you make an exposure , you will be faced with the problem of the correct shutter speed to use even after you have determined the correct exposure . For example , you will get exactly the same exposure whether you give 1/250 second at f / 8 , 1/125 second at f / 11 , or 1/60 second at f / 16 . You cannot , however , use these three exposures interchangeably . As a very rough guide , you should use the various shutter speeds for the following purposes :
1/1000 to 1/250 for stopping action of rapidly moving subjects .
1/250 to 1/125 for most hand - held exposures .
Under 1/125 for still subjects when the lens must be stopped down for maximum depth of field .
A tripod should be used for all exposures longer than 1/60 second with the ordinary roll or hand camera . The average person cannot hold a small camera . steady for more than 1/50 and sometimes only 1/100 second . If the negative is to be greatly enlarged , it is always best to use the faster shutter speed or else set the camera on a tripod .
The above discussion of shutters is necessarily of a general nature , since you are just beginning your studies . More detailed information on this subject will be included in subsequent Assignments as your knowledge grows . As an example , Assignments covering 35mm photography and flash photography will have quite a bit to say about shutters and shutter speeds .
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