Sharpness vs Acutance


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StreetShooter

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Jan 17, 2002
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#1
OK all along I've been boasting that you can get good results with a cheap lens, because you can use always use Photoshop to sharpen the image.

Then I read somewhere a couple of months back that there is a difference between sharpness and accutance (also spelled acutance), so using a good lens DOES make a difference.

I would now like to find more about this difference. I'm Googling "accutance" now, but if any pro's out there can enlighten me further about "accutance" I would be glad to hear what you've got to say. The stuff that Goggle is trawling out is a bit beyond me. This is probably the least chim discussion:

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=000dhx

More specifically, I would like to know whether USM improves sharpness or accutance, and similarly what a good (expensive) lens does vs a cheap lens (not that I'm going to start buying expensive lenses, mind you...)

On a related note, I don't know if anyone has noticed this, but adding a bit of Gaussian Noise to a blurry image tends to sharpen it somewhat. Try it. Then tell me: How does THIS work?
 

#2
Originally posted by StreetShooter
OK all along I've been boasting that you can get good results with a cheap lens, because you can use always use Photoshop to sharpen the image.

Then I read somewhere a couple of months back that there is a difference between sharpness and accutance (also spelled acutance), so using a good lens DOES make a difference.

I would now like to find more about this difference. I'm Googling "accutance" now, but if any pro's out there can enlighten me further about "accutance" I would be glad to hear what you've got to say. The stuff that Goggle is trawling out is a bit beyond me. This is probably the least chim discussion:

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=000dhx

More specifically, I would like to know whether USM improves sharpness or accutance, and similarly what a good (expensive) lens does vs a cheap lens (not that I'm going to start buying expensive lenses, mind you...)

On a related note, I don't know if anyone has noticed this, but adding a bit of Gaussian Noise to a blurry image tends to sharpen it somewhat. Try it. Then tell me: How does THIS work?
My understanding has always been that acutance is a FILM property, sharpness can be lens or film property :dunno: The experts like Ian, Jed should be able to tell us more. ;p

Good lens DOES help. No amount of post processing is going to help a poor image by a poor lens. If nothing has been captured (e.g. lens resolution too low etc) then too bad. Of coz, cheap lenses stopped down can still perform reasonably well.

As to how the noise make the image sharper, well, there's something similar in the audio world. It's called Noise Dithering, something like adding noise to mask out noise. Sounds funny I know, but apparently it works, as you've seen.

Regards
CK
 

GitS

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#3
not a pro but some b/w developers are known as high accutance developers (although u lose speed), makes the grains clearly defined, gives the impression of sharpness.

on the lens front, there is a difference in contrast and sharpness, and one can easily be fooled by a high contrast lens, thinking that its actually a sharp lens (old battle between leica and leitz if i remember correctly)...the literature quickly spirals into resolving power and lines per mm.....interesting physics stuff... but hardly stuff you find in a typical "The Joy of Photography" book.:)
 

#4
Yup, from reading that thread, I infer that acutance is a film, photo paper or other optical/chemical recording media (dunno if CCD/CMOS included).

Actutance is a measure of how steep (fast) the transition from black to white is on a black/white edge (or any other edge). Higher acutance again results in higher (subjective) "sharpness".
and

Acutance: A numerical value that correlates to some extent with subjective image sharpness. The term "acutance" is generally reserved for photographic edge sharpness as measured with images formed in light-sensitive materials. as distinct from optical images. Acutance is determined by printing a sharp knife-edge in contact with the material being tested using a point source of light or a collimated beam. Scatter of the exposing light in the emulsion layer causes the developed image of the knife to be diffused producing an s-shaped density distribution. Acutance is the geometrical average of the slopes at different points on the edge trace divided by the density difference between the limits of the trace.

To summarize from these 2:

High Acutance : Transition from Black to White (and vice versa) is steep. Much like a chasm.

Low Acutance : The transition is more gradual. Imagine having a very small "grayscale" between the black and white. Much like a more gentle hill/slope.

Regards
CK
 

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