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Altering the tonal range
The image recorded on your film results from the various light intensities reflected from the subject . Each light level records as a separate tone , different density of black silver , on the film when it is developed norm- ally , as shown on the graph , right . For any given subject and type of film , the lighting , exposure , and development time all affect the tonal range in the negative .
Normal black and white film can repro- duce a wider range of tones than you will find in most subjects . But near the top and bottom end of the range on the graph ( extremes of exposure ) you can see that tone differences flatten out . One tone becomes difficult to distinguish from the next . A pro- perly used light meter will suggest camera settings that give the film sufficient exposure to use the lower middle section of its range .
It is best to aim for a negative of slightly low density , where tone values in shadow areas are still definable . Detail is often lost in excessively dark tonal areas of negatives due to light spread and increased graininess so most films are designed to use more of the low end of the scale .
Over- and underexposing locate the range of recorded subject light intensities very high or very low on the performance graph . You can counteract this by respectively decreasing or increasing development . This changes the steepness of the curve on a representative graph of recorded tones , thereby decreasing or increasing contrast . By deliberately underexposing and over- developing , you can get acceptable nega- tives on film that is not fast enough for the lighting conditions . This effect - " pushing " development to uprate film speed is shown opposite .
Film performance graph
Density
Range of tones produced by " normal " exposure
Underexposure
The graph below right remains the same shape as the normal perfor- mance curve but shows how underexposure only uses part of the tonal range - giving you a pale negative , above right . The darkest shadow parts of the sub- ject now fall at the lightest end of the tone curve , where it flattens to almost a horizontal line . It comes impossible to distinguish one tone from another on the negative here - shadows are transparent and " empty " . Only the brightest parts of an underexposed subject record with differentiated tones . When you print the nega- tive , these midtone - to - highlight parts of the subject require normal or hard grade paper ; the remain- ing tones require a very hard grade , even then will give little more than black or flat gray .
Overexposure
Overexposure causes subject tones on the negative to fall toward the top end of the film performance graph , below right . All the tones appear very dark on the negative , but now it is the highlights in the subject which merge , as shown above right . When you print the negative , normal grade paper is required for the shadow areas , which still have well separ- ated tones . The lighter subject areas will print with a bleached or , if printed - in , a muddy gray appear- ance , even on hard grade paper . An overexposed negative appears very grainy and fine detail is not clearly resolved . This is because more light - sensitive grains than necessary have been affected by the long exposure .
The simplified diagram above shows how normal black and white film converts parts of the image at different brightness levels into separate tones on the nega- tive , left . It shows the evenly graded tones recorded by a theoretically ideal exposure . The range is exten- sive and tonal values only merge at the extreme ends of the graph .
Pushing
When you know that a film has been underexposed , try " pushing " -increasing the development time . The performance curve , right , is much steeper than the normal curve . The difference between each tone is more marked and so the negative is more con- trasty . This increased contrast helps to prevent tones from merging in shadow areas on the negative , as shown far right .
This correction can be used up to the point when mid - tone - to- highlight range becomes so con- trasty that it begins to require soft grade paper to print well .
Holding back
To compensate for overexposure you can " hold back " , or reduce the development time . As shown right , the film performance graph is flattened so that the processed negative appears less black and grainy , far right . Tone values in the highlight areas are still poorly separated , but there is now less difference between this range and the mid - tones - to - shadows range . The negative will give acceptable results overall if you print it on a hard grade paper . If the subject itself was of low contrast this compensation could result in an unacceptably flat negative .
Compensating for lighting conditions
A film performance curve helps you decide how to change the ex- posure and development of your film to improve the tonal range on your negatives .
The subject shown near right had very flat , frontal lighting . The brightness range between the darkest shadow and lightest high- light is slight . Overdeveloping would result in a steeper perform- ance curve , shown center , near right . This gives an expanded , more normal tone range on the negative . But the extra develop- ment would also make the nega- tive darker overall . To avoid a very dark result , you can slightly under- expose the film and increase the development time proportionally , to produce an image such as shown bottom , near right .
Compensating for a harshly lit , high contrast subject , such as shown top far right , works the other way round . You overexpose the film , and then reduce the develop- ment to produce a flatter negative . with a more normal tone range as shown bottom . far right .
In low light levels with slow rated film , you can increase the rating by underexposing and push- ing . Set the film speed dial on your camera to two or three times the real film speed . 25 or 50 per cent extra development time will produce near normally graded tones .
Subject lighting conditions
The quality and direction of the lighting in the subject , far left , has produced a low contrast negative . Hard lighting conditions in the subject , near left , have produced a contrasty negative .
Compensating for the lighting
You can compensate for the light- ing , above far left , by slightly underexposing then overdeveloping the negative . Hard contrast , above near left , can be improved by over- exposure and underdevelopment Exposure determines the part of the graph you use , the development alters its shape .
Altering the tonal range
The image recorded on your film results from the various light intensities reflected from the subject . Each light level records as a separate tone , different density of black silver , on the film when it is developed norm- ally , as shown on the graph , right . For any given subject and type of film , the lighting , exposure , and development time all affect the tonal range in the negative .
Normal black and white film can repro- duce a wider range of tones than you will find in most subjects . But near the top and bottom end of the range on the graph ( extremes of exposure ) you can see that tone differences flatten out . One tone becomes difficult to distinguish from the next . A pro- perly used light meter will suggest camera settings that give the film sufficient exposure to use the lower middle section of its range .
It is best to aim for a negative of slightly low density , where tone values in shadow areas are still definable . Detail is often lost in excessively dark tonal areas of negatives due to light spread and increased graininess so most films are designed to use more of the low end of the scale .
Over- and underexposing locate the range of recorded subject light intensities very high or very low on the performance graph . You can counteract this by respectively decreasing or increasing development . This changes the steepness of the curve on a representative graph of recorded tones , thereby decreasing or increasing contrast . By deliberately underexposing and over- developing , you can get acceptable nega- tives on film that is not fast enough for the lighting conditions . This effect - " pushing " development to uprate film speed is shown opposite .
Film performance graph
Density
Range of tones produced by " normal " exposure
Underexposure
The graph below right remains the same shape as the normal perfor- mance curve but shows how underexposure only uses part of the tonal range - giving you a pale negative , above right . The darkest shadow parts of the sub- ject now fall at the lightest end of the tone curve , where it flattens to almost a horizontal line . It comes impossible to distinguish one tone from another on the negative here - shadows are transparent and " empty " . Only the brightest parts of an underexposed subject record with differentiated tones . When you print the nega- tive , these midtone - to - highlight parts of the subject require normal or hard grade paper ; the remain- ing tones require a very hard grade , even then will give little more than black or flat gray .
Overexposure
Overexposure causes subject tones on the negative to fall toward the top end of the film performance graph , below right . All the tones appear very dark on the negative , but now it is the highlights in the subject which merge , as shown above right . When you print the negative , normal grade paper is required for the shadow areas , which still have well separ- ated tones . The lighter subject areas will print with a bleached or , if printed - in , a muddy gray appear- ance , even on hard grade paper . An overexposed negative appears very grainy and fine detail is not clearly resolved . This is because more light - sensitive grains than necessary have been affected by the long exposure .
The simplified diagram above shows how normal black and white film converts parts of the image at different brightness levels into separate tones on the nega- tive , left . It shows the evenly graded tones recorded by a theoretically ideal exposure . The range is exten- sive and tonal values only merge at the extreme ends of the graph .
Pushing
When you know that a film has been underexposed , try " pushing " -increasing the development time . The performance curve , right , is much steeper than the normal curve . The difference between each tone is more marked and so the negative is more con- trasty . This increased contrast helps to prevent tones from merging in shadow areas on the negative , as shown far right .
This correction can be used up to the point when mid - tone - to- highlight range becomes so con- trasty that it begins to require soft grade paper to print well .
Holding back
To compensate for overexposure you can " hold back " , or reduce the development time . As shown right , the film performance graph is flattened so that the processed negative appears less black and grainy , far right . Tone values in the highlight areas are still poorly separated , but there is now less difference between this range and the mid - tones - to - shadows range . The negative will give acceptable results overall if you print it on a hard grade paper . If the subject itself was of low contrast this compensation could result in an unacceptably flat negative .
Compensating for lighting conditions
A film performance curve helps you decide how to change the ex- posure and development of your film to improve the tonal range on your negatives .
The subject shown near right had very flat , frontal lighting . The brightness range between the darkest shadow and lightest high- light is slight . Overdeveloping would result in a steeper perform- ance curve , shown center , near right . This gives an expanded , more normal tone range on the negative . But the extra develop- ment would also make the nega- tive darker overall . To avoid a very dark result , you can slightly under- expose the film and increase the development time proportionally , to produce an image such as shown bottom , near right .
Compensating for a harshly lit , high contrast subject , such as shown top far right , works the other way round . You overexpose the film , and then reduce the develop- ment to produce a flatter negative . with a more normal tone range as shown bottom . far right .
In low light levels with slow rated film , you can increase the rating by underexposing and push- ing . Set the film speed dial on your camera to two or three times the real film speed . 25 or 50 per cent extra development time will produce near normally graded tones .
Subject lighting conditions
The quality and direction of the lighting in the subject , far left , has produced a low contrast negative . Hard lighting conditions in the subject , near left , have produced a contrasty negative .
Compensating for the lighting
You can compensate for the light- ing , above far left , by slightly underexposing then overdeveloping the negative . Hard contrast , above near left , can be improved by over- exposure and underdevelopment Exposure determines the part of the graph you use , the development alters its shape .
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