تويت
الخطوة 3: التحكم في حدة الصورة / عمق المجال .. تقنية الكاميرا .. المرشد خطوة بخطوة للتصوير الفوتوغرافي
STEP 3 : CONTROLLING PICTURE SHARPNESS / Depth of field
Usually you will want to produce sharp , clear pictures , but even when you want an image to be blurred for a particular effect it is still important to know how to produce the right amount of sharp detail . The two main ways of controlling how much of the subject appears sharp are : use of depth of field ( the zone that is sharply in focus ) con- trolled by the aperture setting , and the shutter speed ( discussed on pp . 34-5 ) .
Aperture and depth of field :
The two photographs of the chess pieces on this page illustrate how dramatically a pic- ture can change by altering the depth of field . If your subject is a distant landscape , or has little depth , such as a face - on view of a building , changing depth of field has little effect . Because the elements of these subjects are all about the same distance from the camera , they will appear equally sharp at any aperture setting . However , a subject with close foreground elements and distant background elements will appear differently when the aperture setting ( and therefore the depth of field ) is altered . Both the fore- ground and background will grow much sharper as the lens aperture is reduced , that is , as the f number is increased .
Single lens reflex cameras usually have a pre - selective aperture control whatever f number you set , the lens stays at its widest aperture for focusing , so that the subject is bright and clear ( when the shutter fires it closes down to the chosen aperture ) . Most of these cameras have a " preview " button which closes the diaphragm to the working aperture so that you are able to check depth of field before exposure . If you have such a camera , you can test this for yourself - when you press the preview button the picture will darken , but ignore this , and notice how those parts previously out of focus now appear sharper . It is important to remember this change in depth of field when using a camera at small apertures your photographs may include more sharp detail in the foreground and background than you expect , or want .
Although rangefinder focusing cameras clearly show the point of focus , they do not show the depth of field . This is where a depth of field scale on the lens is helpful . This shows the extent of the depth of field either side of the focused subject at each aperture .
The aperture is the main way of con- trolling depth of field , but subject distance is also a contributing factor . It is always easier to produce sharpness throughout a picture where everything is a long way from the camera than in close - up shots .
How aperture affects depth of field :
In the two diagrams , right , the lens remains focused for the figure , but at different apertures the amount of the subject that is sharp on the film plane ( depth of field ) changes . At a wide aperture , top , the tree comes to focus in front of the film plane and so would record unsharply . ( Equally , fore- ground elements would come to focus behind the film and so would also be unsharp . ) By reducing the aperture , bottom , the cones of light are narrowed . As a result , the tree ( and any foreground elements ) now form smaller circular patches of light and so give a more acceptably sharp image on the film plane .
Depth of field and sharpness :
As the diagram , below , and the two pictures , right , show , depth of field extends equally in front of and behind the focused subject when it is close - up . In both photo- graphs the focus set on the center chess piece - remained unchanged ; only the aperture was altered . In the top photograph . taken at f16 , the depth of field is wide so that the chess pieces in the foreground and background are also sharp and clear . Simply by widening the aperture to 12-8 ( i.e. reducing the depth of field ) , below right , the foreground and
background chess pieces become unsharp . So , with the same subject . you can produce quite different results by altering the aperture set- ting ( and depth of field ) creatively .
Depth of field is also altered by the camera - subject distance . As this increases the proportion of the sharp zone in front of , and be hind , the subject changes . On a standard 50 mm lens at distances of about 3 ft ( 0-9 m ) and over , the depth of field extends about one third in front and two thirds behind the subject .
Using depth of field :
Often lens bodies have a depth of field scale next to the focusing ring so that you can more easily predict the depth of field at each aperture setting . This scale is marked in f numbers to show how far the depth of field extends in front of and behind the focused sub- ject . For example , when the lens shown below is focused for 20 ft ( 6.6 m ) the depth of field extends from 10 ft ( 3-3 m ) to infinity at f16 . At 12-8 it only extends from about 18 ft ( 5-4 m ) to 24 ft ( 7-3 m ) . The scale acts as a guide to the f numbers you should select to get a particular depth of field in your picture .
Getting maximum depth of field If you are photographing a sub- ject which extends over a con- siderable distance , such as a deep landscape , it is not always best to focus the lens for infinity . The top picture , right , was taken with the lens set for infinity and the aperture at f16 . Everything is sharp from the horizon ( infinity in your picture ) down to a near limit of sharpness ( the hyperfocal point ) at about 18 ft ( 5.4 m ) and fore- ground elements are blurred However , the depth of field can be maximized by refocusing for the near limit . This increases the depth of field so that the horizon is the furthest point that is sharp and elements in front of the near limit also become sharp . In the bottom right picture , focused for the near limit , the depth of field extends down to about 10 ft ( 3 m ) away from the camera .
Depth of field and subject distance :
The closer you get to your subject the shallower the depth of field be- comes . The picture below was taken with lens focused for its near- est subject distance and set to its widest aperture . The depth of field is only about 3 ins ( 76 mm ) - everything nearer or further away is completely out of focus . At the same aperture the same lens would give a much greater depth of field if it focused for a more dis- tant subject . Focused for a subject 15 ft ( 4-5 m ) away , for example . the depth of field would be 14-17 ft ( 4.2-5.1 m ) .
Even at a small aperture setting it is difficult to get a great depth of field when working close - up . This means that you must be particularly careful to focus accu- rately since there is little latitude , either side of your subject , for error . Therefore if you want the subject to be sharp throughout , try to choose a viewpoint from which most of its elements are equidistant from the lens .
Assignment : exploring depth of field :
Select an interesting subject , preferably outside , and position yourself so that the foreground and background ( up to about 20 ft . 6-6 m ) includes detail . In good lighting take the following pictures .
A. With the lens focused for the main sub- ject and set to its widest aperture .
B. With the lens at the same aperture . using the depth of field scale to focus on the hyperfocal point .
C. With the lens focused for the main sub- ject again , using the smallest aperture on your camera . ( Increase the shutter speed proportionally . )
D. With the focus and aperture settings just making the foreground and main subject sharp . leave the background un- sharp . Use the depth of field scale .
E. Using focus and aperture settings to render the subject and background sharp , and foreground unsharp . Again use the depth of field scale .
F. Selecting a detailed part of the main subject , move in very close - up and repeat A and C again .
Compare your results and decide which picture ( s ) of your subject works best .
STEP 3 : CONTROLLING PICTURE SHARPNESS / Depth of field
Usually you will want to produce sharp , clear pictures , but even when you want an image to be blurred for a particular effect it is still important to know how to produce the right amount of sharp detail . The two main ways of controlling how much of the subject appears sharp are : use of depth of field ( the zone that is sharply in focus ) con- trolled by the aperture setting , and the shutter speed ( discussed on pp . 34-5 ) .
Aperture and depth of field :
The two photographs of the chess pieces on this page illustrate how dramatically a pic- ture can change by altering the depth of field . If your subject is a distant landscape , or has little depth , such as a face - on view of a building , changing depth of field has little effect . Because the elements of these subjects are all about the same distance from the camera , they will appear equally sharp at any aperture setting . However , a subject with close foreground elements and distant background elements will appear differently when the aperture setting ( and therefore the depth of field ) is altered . Both the fore- ground and background will grow much sharper as the lens aperture is reduced , that is , as the f number is increased .
Single lens reflex cameras usually have a pre - selective aperture control whatever f number you set , the lens stays at its widest aperture for focusing , so that the subject is bright and clear ( when the shutter fires it closes down to the chosen aperture ) . Most of these cameras have a " preview " button which closes the diaphragm to the working aperture so that you are able to check depth of field before exposure . If you have such a camera , you can test this for yourself - when you press the preview button the picture will darken , but ignore this , and notice how those parts previously out of focus now appear sharper . It is important to remember this change in depth of field when using a camera at small apertures your photographs may include more sharp detail in the foreground and background than you expect , or want .
Although rangefinder focusing cameras clearly show the point of focus , they do not show the depth of field . This is where a depth of field scale on the lens is helpful . This shows the extent of the depth of field either side of the focused subject at each aperture .
The aperture is the main way of con- trolling depth of field , but subject distance is also a contributing factor . It is always easier to produce sharpness throughout a picture where everything is a long way from the camera than in close - up shots .
How aperture affects depth of field :
In the two diagrams , right , the lens remains focused for the figure , but at different apertures the amount of the subject that is sharp on the film plane ( depth of field ) changes . At a wide aperture , top , the tree comes to focus in front of the film plane and so would record unsharply . ( Equally , fore- ground elements would come to focus behind the film and so would also be unsharp . ) By reducing the aperture , bottom , the cones of light are narrowed . As a result , the tree ( and any foreground elements ) now form smaller circular patches of light and so give a more acceptably sharp image on the film plane .
Depth of field and sharpness :
As the diagram , below , and the two pictures , right , show , depth of field extends equally in front of and behind the focused subject when it is close - up . In both photo- graphs the focus set on the center chess piece - remained unchanged ; only the aperture was altered . In the top photograph . taken at f16 , the depth of field is wide so that the chess pieces in the foreground and background are also sharp and clear . Simply by widening the aperture to 12-8 ( i.e. reducing the depth of field ) , below right , the foreground and
background chess pieces become unsharp . So , with the same subject . you can produce quite different results by altering the aperture set- ting ( and depth of field ) creatively .
Depth of field is also altered by the camera - subject distance . As this increases the proportion of the sharp zone in front of , and be hind , the subject changes . On a standard 50 mm lens at distances of about 3 ft ( 0-9 m ) and over , the depth of field extends about one third in front and two thirds behind the subject .
Using depth of field :
Often lens bodies have a depth of field scale next to the focusing ring so that you can more easily predict the depth of field at each aperture setting . This scale is marked in f numbers to show how far the depth of field extends in front of and behind the focused sub- ject . For example , when the lens shown below is focused for 20 ft ( 6.6 m ) the depth of field extends from 10 ft ( 3-3 m ) to infinity at f16 . At 12-8 it only extends from about 18 ft ( 5-4 m ) to 24 ft ( 7-3 m ) . The scale acts as a guide to the f numbers you should select to get a particular depth of field in your picture .
Getting maximum depth of field If you are photographing a sub- ject which extends over a con- siderable distance , such as a deep landscape , it is not always best to focus the lens for infinity . The top picture , right , was taken with the lens set for infinity and the aperture at f16 . Everything is sharp from the horizon ( infinity in your picture ) down to a near limit of sharpness ( the hyperfocal point ) at about 18 ft ( 5.4 m ) and fore- ground elements are blurred However , the depth of field can be maximized by refocusing for the near limit . This increases the depth of field so that the horizon is the furthest point that is sharp and elements in front of the near limit also become sharp . In the bottom right picture , focused for the near limit , the depth of field extends down to about 10 ft ( 3 m ) away from the camera .
Depth of field and subject distance :
The closer you get to your subject the shallower the depth of field be- comes . The picture below was taken with lens focused for its near- est subject distance and set to its widest aperture . The depth of field is only about 3 ins ( 76 mm ) - everything nearer or further away is completely out of focus . At the same aperture the same lens would give a much greater depth of field if it focused for a more dis- tant subject . Focused for a subject 15 ft ( 4-5 m ) away , for example . the depth of field would be 14-17 ft ( 4.2-5.1 m ) .
Even at a small aperture setting it is difficult to get a great depth of field when working close - up . This means that you must be particularly careful to focus accu- rately since there is little latitude , either side of your subject , for error . Therefore if you want the subject to be sharp throughout , try to choose a viewpoint from which most of its elements are equidistant from the lens .
Assignment : exploring depth of field :
Select an interesting subject , preferably outside , and position yourself so that the foreground and background ( up to about 20 ft . 6-6 m ) includes detail . In good lighting take the following pictures .
A. With the lens focused for the main sub- ject and set to its widest aperture .
B. With the lens at the same aperture . using the depth of field scale to focus on the hyperfocal point .
C. With the lens focused for the main sub- ject again , using the smallest aperture on your camera . ( Increase the shutter speed proportionally . )
D. With the focus and aperture settings just making the foreground and main subject sharp . leave the background un- sharp . Use the depth of field scale .
E. Using focus and aperture settings to render the subject and background sharp , and foreground unsharp . Again use the depth of field scale .
F. Selecting a detailed part of the main subject , move in very close - up and repeat A and C again .
Compare your results and decide which picture ( s ) of your subject works best .
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