Tips About Film Acutance
Sharpness is quite difficult to define precisely because it’s based on subjective evaluations, which are influenced by the contrast along edges in a photo. This characteristic is known as ‘acutance’. We judge an image to be sharp when the edge contrast differences we see are relatively large. While any photographer can recognise a sharp image, we often disagree about how sharp that image is. Different individuals perceive sharpness differently ““ and each person will make a different assessment of the sharpness of an image at different times.
Perception may be more acute first thing in the morning before the eyes become tired ““ or in the middle of the day when the brain/eye interface is at its most efficient. Assessments can also be biased by the evaluator’s pre-conceptions.
Sharpness also varies with different subjects ““ as do our expectations of what should appear sharp. We generally expect landscapes and shots of buildings to be sharp edge-to-edge but are much more forgiving for portraits and close-ups.
‘Soft’ (unsharp) images can be aesthetically satisfying in many cases and images in which only part of the frame is sharp can be pleasing to many viewers’ eyes. Whereas perceived sharpness is a combination of resolution and acutance, only one of these parameters can be adjusted by photographers. Captured resolution is defined by the sensor, lens and focus in the image and can’t be changed. But acutance can be adjusted in post-capture processing.
People tend to judge images with higher acutance as being sharper, even though this is not necessarily associated with higher resolution. By increasing tonal differences along edges in an image, we can make it appear sharper, even though the actual resolution of the image may be reduced. Resolution defines the amount of detail in an image, regardless of how it was captured. Of the three qualities we’re looking at, resolution is the only one that is quantifiable (measurable).
Perception may be more acute first thing in the morning before the eyes become tired ““ or in the middle of the day when the brain/eye interface is at its most efficient. Assessments can also be biased by the evaluator’s pre-conceptions.
Sharpness also varies with different subjects ““ as do our expectations of what should appear sharp. We generally expect landscapes and shots of buildings to be sharp edge-to-edge but are much more forgiving for portraits and close-ups.
‘Soft’ (unsharp) images can be aesthetically satisfying in many cases and images in which only part of the frame is sharp can be pleasing to many viewers’ eyes. Whereas perceived sharpness is a combination of resolution and acutance, only one of these parameters can be adjusted by photographers. Captured resolution is defined by the sensor, lens and focus in the image and can’t be changed. But acutance can be adjusted in post-capture processing.
People tend to judge images with higher acutance as being sharper, even though this is not necessarily associated with higher resolution. By increasing tonal differences along edges in an image, we can make it appear sharper, even though the actual resolution of the image may be reduced. Resolution defines the amount of detail in an image, regardless of how it was captured. Of the three qualities we’re looking at, resolution is the only one that is quantifiable (measurable).
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