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أنظمة التركيز .. تقنية الكاميرا .. المرشد خطوة بخطوة للتصوير الفوتوغرافي
Focusing systems
If your camera allows lens focusing you will need to measure or estimate the subject distance ; this takes time and may not be too accurate . Some camera designs include a simple built - in focusing aid such as a large tinted head outline which appears in the viewfinder when the lens is set for por- traits . You then move the camera to the distance where the subject's head just fills the outline . More advanced direct vision viewfinder cameras sometimes have a coupled rangefinder for focusing . This is an optical device linked to the focusing ring that produces a double image in the view- finder until the subject is correctly focused .
The single lens reflex camera , right , offers a viewing system with the important advan- tage of showing you the image formed by the taking lens itself . You can adjust the lens until the image you see appears sharp . The system completely eliminates parallax error , and the image is always seen the right way up and corrected left to right . As shown below , right , an angled mirror just behind the lens reflects the image up to the horizon- tal and a pentaprism a five sided block of glass - corrects the left - to - right reversal . When the shutter release is pressed the mirror hinges upward out of the way , briefly blocking the image from the focusing screen , and the shutter in front of the film opens to expose the film .
Direct vision rangefinder camera :
A more advanced direct vision viewfinder camera , above , includes a rangefinder for accu- rate focusing . The viewing system is still separate from the taking lens , but instead of estimating the distance , you align two separate images of the subject for the correct focus , above right . When you look through the camera view- finder you see a pale tinted area in the center of the pic- ture . This area , top right , shows a double image . To set the correct distance you point the camera so that the most important part of the subject ap- pears in the tinted area , then turn the lens focusing ring until the image in this area shifts left or right to coincide with the main image . The cen- tral area of the image will now photograph in focus .
A rangefinder system is very quick and clear , but , unlike a focusing screen , does not show you how the image will actual- ly record on the film , since out- of - focus parts of the back- ground and foreground always appear sharp .
The SLR viewing system :
Single lens reflex cameras are so designed that the distance between the lens and the focusing screen ( via the mirror ) is exactly the same as between the lens and film . Con- sequently whatever parts of the image appear in focus on the screen will also record in focus on the film . The same applies if you change lenses or fit various lens accessories - there is no parallax or focus error . To make it easy to use at eye - level , the focusing screen on most SLRs is viewed through a " folded up " reflecting system formed by a five- sided , partially silvered , glass prism . A small magnifying lens in the eye- piece helps to make the screen look large and clear .
Sometimes two small prisms are incorporated in the central zone of the screen when out of focus , the image appears split and offset in this area , below . Other screens have a central grid of minute prisms which give a shimmering effect when the image is not sharp . Devices like this are helpful if you have difficulty checking sharp focus , particularly in poor lighting . But beginners sometimes find such marks on the screen distracting when composing pictures .
Focusing systems
If your camera allows lens focusing you will need to measure or estimate the subject distance ; this takes time and may not be too accurate . Some camera designs include a simple built - in focusing aid such as a large tinted head outline which appears in the viewfinder when the lens is set for por- traits . You then move the camera to the distance where the subject's head just fills the outline . More advanced direct vision viewfinder cameras sometimes have a coupled rangefinder for focusing . This is an optical device linked to the focusing ring that produces a double image in the view- finder until the subject is correctly focused .
The single lens reflex camera , right , offers a viewing system with the important advan- tage of showing you the image formed by the taking lens itself . You can adjust the lens until the image you see appears sharp . The system completely eliminates parallax error , and the image is always seen the right way up and corrected left to right . As shown below , right , an angled mirror just behind the lens reflects the image up to the horizon- tal and a pentaprism a five sided block of glass - corrects the left - to - right reversal . When the shutter release is pressed the mirror hinges upward out of the way , briefly blocking the image from the focusing screen , and the shutter in front of the film opens to expose the film .
Direct vision rangefinder camera :
A more advanced direct vision viewfinder camera , above , includes a rangefinder for accu- rate focusing . The viewing system is still separate from the taking lens , but instead of estimating the distance , you align two separate images of the subject for the correct focus , above right . When you look through the camera view- finder you see a pale tinted area in the center of the pic- ture . This area , top right , shows a double image . To set the correct distance you point the camera so that the most important part of the subject ap- pears in the tinted area , then turn the lens focusing ring until the image in this area shifts left or right to coincide with the main image . The cen- tral area of the image will now photograph in focus .
A rangefinder system is very quick and clear , but , unlike a focusing screen , does not show you how the image will actual- ly record on the film , since out- of - focus parts of the back- ground and foreground always appear sharp .
The SLR viewing system :
Single lens reflex cameras are so designed that the distance between the lens and the focusing screen ( via the mirror ) is exactly the same as between the lens and film . Con- sequently whatever parts of the image appear in focus on the screen will also record in focus on the film . The same applies if you change lenses or fit various lens accessories - there is no parallax or focus error . To make it easy to use at eye - level , the focusing screen on most SLRs is viewed through a " folded up " reflecting system formed by a five- sided , partially silvered , glass prism . A small magnifying lens in the eye- piece helps to make the screen look large and clear .
Sometimes two small prisms are incorporated in the central zone of the screen when out of focus , the image appears split and offset in this area , below . Other screens have a central grid of minute prisms which give a shimmering effect when the image is not sharp . Devices like this are helpful if you have difficulty checking sharp focus , particularly in poor lighting . But beginners sometimes find such marks on the screen distracting when composing pictures .
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