I often read and hear opinions relating to how 'sharp' a lens is, or is not - but what is 'sharpness'?
Sharpness is based on subjective evaluations influenced by edge contrast, or acutance, in an image that might be said to be sharp when the edge contrasts are clear and defined.
Subject plays a role in how we perceive sharpness, as do our expectations of what should be sharp and what should not. We might expect photographs of landscapes or buildings to be sharper edge-to-edge but might expect different qualities in portraits and other subjects.
We might expect photographs of hard edged landscapes a buildings to be sharp edge-to-edge, but it does not mean that they are.....
And our expectations of sharpness will differ with between subjects - this image is still sharp
Soft (unsharp) images can still be considered sharp and aesthetically pleasing..
Sharpness is a combination of resolution and acutance, but only one of these parameters can be enhanced by photographers - resolution is defined by the film/sensor, lens and focus in the image and can’t be changed, but acutance might be more obvious in a subject then enhanced or adjusted.
So images with higher acutance are generally percieved as being sharper, even when this is not a product of higher resolution lens - increased edge contrast in an image makes it appear sharper, even though the actual resolution of the image may be less.
An image is said to be sharp when the edge contrast differences we see are clear and defined.
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With a high resolution lens parallel lines are distinct pairs of black and white lines.
If a lower resolution lens is used, the same lines become less distinct.
Some subjects like landscapes might be expected to have lots of detail and are often photographed using wide-angle lenses, thus more information and detail is captured within a wider field of view - in the image below the rocks and other hard edged features have a high acutance, the sea and sky does not, yet we perceive this image as sharp even if it was made using a lens with a lower resolving power.
Portraits and close-ups might exploit our perception of sharpness to emphasise a key feature of the subject within the frame. By allowing other areas or backgrounds to blur out so as not to compete for our attention, we might exploit the softness in an image to over stimulate our perception of a sharpness which may not actually be there.
Film grain and Digital Image noise may have a very high acutance, which can give an illusion of sharp detail. Sharpness can also be influenced by viewing distances - a Billboard may be sharp when viewed across a street, but have very low resolution when viewed close-up.
It's also worth remembering that camera movement - intentional or not, poor focus, subject movement, depth of field, filtration, atmospheric conditions, viewing media, eyesight and a whole bunch of other factors can simply negate the resolving powers of the very best lenses and cameras.
So it's easy to claim an image is sharp, or not sharp, but impossible to say how sharp it really is or is not because our expectations and perceptions differ, they are influenced by subject, distance, size, viewing media and many other factors - and some might simply have better eyesight than others! "There is nothing worse than a sharp image of a fuzzy concept". Ansel Adams So instead of spending time and money on the ultimate sharpness and clarity, perhaps we should spend time and effort getting in front of interesting subjects and photographing them with purpose. Rob
