Sharpness, resolution and accutance and my lack of clarity

As best as I can understand sharpness is a function of both accutance and resolution. Resolution refers to the fineness of the transition of one detail or element to another. For example with white and black bars the resolution refers to how immediately the white transitions to the black. Accutance refers to the degree of contrast between elements or details of an image. I still am having a difficult time separating the two in tangible way. Sharpening of an image in photoshop simply increases accutance, but not resolution. Is this correct? I'm a little unclear on these concepts.

Help, anyone?

brianentz said:

Sharpening of an image in photoshop...

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You might want to direct that to one of the digital forums out there.

I think resolution is a measure of how small an image can be where you can still really count the bars. The bars can be quite low contrast and fuzzy but you can still reliably count them in a film that attains very high-resolution.

I think you understand it rather well, Brian.

The reasons that you still feel you do not fully understand may be because of 1) the somewhat interchangeable nature of some of the terms used (not by you in particular, but generally in books and forums), such as 'contrast', 'resolution', 'resolving power', 'acutance', 'micro contrast', 'clarity', 'sharpness', etc., and with 2) human vision not being as straightforward as testing devices used for measuring the above quantities may suggest.

The properties of an image are difficult to qualify. Then throw in human vision, and what you're left with is your feeling of confusion.

But again, your initial description where resolution and acutance (and perhaps overall contrast) together determine a certain perceived sharpness is perfectly workable, in that it enables you to get a feel/measure for the quality of say a print you've made.

brianentz said:

For example with white and black bars the resolution refers to how immediately the white transitions to the black.

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That is acutance. High acutance means the transition is more acute.

Resolution refers to the capacity to capture fine detail- that is, how narrow the white and black bars can be and still be discernible as separate.

brianentz said:

As best as I can understand sharpness is a function of both accutance and resolution. Resolution refers to the fineness of the transition of one detail or element to another. For example with white and black bars the resolution refers to how immediately the white transitions to the black. Accutance refers to the degree of contrast between elements or details of an image. I still am having a difficult time separating the two in tangible way. Sharpening of an image in photoshop simply increases accutance, but not resolution. Is this correct? I'm a little unclear on these concepts.

Help, anyone?

Click to expand...

The three components of "sharpness" are resolution, contrast and accutance. Accutance refers to the degree of visibility of the edges of details. You are correct about the effect of the "sharpening" function in Photoshop et al.

There are some excellent observations about "sharpness" in the early parts of Ctein's "Post Exposure" which is now available on the web: http://ctein.com/booksmpl.htm

This was Geoffrey Crawley's summing up of the terms in his precis of his longer article on Film developers published in 1960/1 "Notes on Present-day Monochrome Emulsions and their Development".


" Sharpness " - the overall impression of a print or projected image, measured scientifically as "acutance ", seen from normal viewing distance.

" Definition " - the extent to which fine detail is recognisably rendered in a print, etc. When acutance of fine detail is good, then definition is good.

" Acutance " - the contrast at the edge of significant detail, a scientific measurement of the density gradient at that point.

" Resolving Power " - the scientific measurement of the actual fineness of detail recordable by a lens, film, or developer, or any combination of these three.

Ian