Understanding Sharpness

What Are We Talking About?
Photographers as a group are pretty hung-up on sharpness. I‘m no exception.Unfortunately for some seeking the holy grail of ultimate image sharpnessbecomes a substitute for creating interesting and worthwhile photographs. Sincethe concept of sharpness is a means, not an end it itself, I thought itworthwhile to look at what is meant by the word sharpness as used inphotography.
It actually consists of two separate concepts —  resolution and acutance.
With this article I hope to explain these two concepts and the role eachplays in our perception of what is sharp.

Photographed with Canon D30 at ISO 400.1/250th sec @ f/5.6 with a Canon 300mm f/2.8L IS .  RAW Mode.

What You Get May Not Be What You See
The traditional measure of image sharpness is stated in line pairs per millimeter— abbreviated as lp/mm. In other words the ability of the human eye to discernedthe number of high-contrast pairs of lines appearing in the space of a singlemillimeter.
You may have read that some very high resolution B&W films can reach over150 lp/mm. That‘s amazing. Some really terrific lenses can actually resolvesomewhat over 100 lp/mm. Wow.
But are you also aware that even the very best colour printing papers can reproducelittle better than 75 lp/mm? Humm. Are you also aware that the human eyeisn‘t able to resolve any more than between 5—10 lp/mm under the most optimumconditions? Say what?
Forgive me, but I‘m having a bit of fun with you by my editorial comments because so manyphotographers get hung-up on theory and specs rather than what they actually canperceive with their own eyes.
Think about the variables standing between a scene in front of the camera anda print hanging on the wall. A large number of factors determine whether or notthe print will be sharp.
The lens‘ resolution capabilities
The film‘s resolution capabilities
Camera and/or subject motion
Aperture used and consequent depth of field as well as use of optimum aperture
Film flatness — is the pressure-plate doing its job properly?
Film thickness — are all layers in the film (colour) bringing the image into focus
Film grain — sometimes grain makes images look sharper — sometimes the opposite!
Enlarger parallelism — many aren‘t, and sharpness suffers
Negative carrier precision and film flatness
Enlarging lens quality and aperture used
Resolution of the printing paper used
Of course digital post-processing has its own issues, including...
The film flatness capabilities of the scanner‘s carrier
The native resolution of the scanner chip and firmware
The Sharpening level applied in post-processing
The native resolution of the printer
The resolution of the printing paper used
For digital cameras many of the same issues as for film cameras apply, aswell as those for digital image processing.
Arggggh!
Remarkably, in the face of this adversity we usually end up with crisp printsthat we‘re happy to show to others and hang on display.

Resolution & Acutance
North Window Panorama, 2000
Photographed with a Hasselblad XPan and 30mm lens on Provia 100F.Scanned with an Imacon Photo at 3200 dpi
Resolution is the most familiar of the two perceptual factorscontributing to sharpness. We‘ve alreadylooked at how much resolution is needed. You may start at 100 lp/mm (thoughtypically not more than 50 lp/mm) but along the way if you end up with 10lp/mm on a print you‘ll have a very crisp image indeed, and even 5lp/mm on a print isconsidered critically sharp by many observers. (To be scientifically accurateyou actually should have somewhat more resolution than this (maybe 30 lp/mm) ona low contrast image because of acutance effects).
Acutance is the less understood characteristic of sharpness. Acutanceisn‘t about resolving detail, it‘s about the transition between edges. In otherwords when an edge changes from one brightness level to another. This is whatSharpening in digital parlance is all about. Scanning and digital capturesoftens acutance and so we apply a (ill-named) process called an UnsharpMask to increase edge sharpness back to what it should be. Remember,this has nothing to do with resolution, the other aspect of sharpness.Unfortunately some anti-digital Luddites still confuse the two.

Theory & Practice
Keep in mind that 35mm and medium format cameras have a very hard timeproducing the resolution needed for absolutely ideal sharpness. Only 4X5"or larger can really put down on paper an image that approaches the ideal.Assuming that about 25 lp/mm is what‘s needed for a typical moderate acutanceimage with lots of fine detail, then you would theoretically need a 200 lp/mmimage on film to make this possible. ( This is equivalent to an 8X enlargement,or about an 8X10" print). I know of no lens / film combination that can achievethis.
Medium format cameras have a lower magnification hurdle, but only the finestlenses have resolution capability comparable to top 35mm lenses. But only withthe best lenses, the highest resolution films and the finest enlarging or digitalimage processing techniques can this be achieved. (So, instead of a combined200 lp/mm needed for 35mm you‘d "only" need 100 lp/mm from a 6X6cmsystem — a 4X ratio.)

Grain
Ourperception of sharpness depends to a large extent on how much detail an imagecontains. Frequently an image with moderately coarse but sharp grain (orcrisp digital noise) can appear as sharp or even sharper than a fine-grainedimage. This is why high-acutance film developers such as Rodinal havehistorically been preferred by some. They made grain somewhat bigger, but gaveit higher acutance and therefore aided in creating the appearance of a sharperimage.
Conversely an image with fine but soft grain (for whatever reason)will drag down the general appearance of sharpness that a given image mightotherwise have.
Interestingly, completely grain-free images can appear to beless sharp than the resolution numbers might lead one to believe. This is thecase with the old Panatomic-X and Tech-Pan films. They are sograin-free than they can appear less than critically sharp if the imagedoesn‘t contain a lot of fine detail.
Along the same lines, in the digital realm it‘s my experience that the CanonD30 at 100 ISO is so free of digital noise (the equivalent of grain)that in some images it can almost appear to be less sharp than a 400 ISO framefrom the same camera.

Inkjet Prints

Minnihaha Falls, Rabun County, Georgia. September 2000
Photographed with a Hasselblad ArcBody and 35mmRodenstock lens onProvia 100F.
Thereare some folks who haven‘t yet seen well-made high quality inkjet prints, andwho therefore mistakenly believe that these somehow can‘t equal traditionalchemical prints in terms of sharpness. It‘s worth noting that a 6 colour 1440dpi inkjet printer (like the Epson 1270 / 1280 / 2000P Photo printers)when fed a360dpi output file, is capable of about 16 pixel per millimeter.This translates to 8 lp/mm — right in the high-end of the ballpark for meetingthe limits of human vision‘s ability to discern maximum sharpness.
This also explains why the latest generation of printers speced at 2880 dpi(like theEpson 1280/1290) don‘tmake prints that look any sharper to the naked eye. Under a loupe, yes, butnot unaided. The reason why, we can now appreciate is because at about 8 lp/mmwe are already near the limits of the eye‘s ability to resolve fine detail.All we end up with is slower print speeds and greater ink usage.

Magnification — Bigger Is Better
Anytime you examine the issue of sharpness it must be seen in the context ofmagnification and viewing distance. Look at a billboard from across the street. See the pretty girlwith silky long hair standing on a beach, enticing you to flee winter and joinher in the Bahamas? Looks pretty sharp. Individual strands of hair are visibleand you can almost count the grains of sand.
Now climb up on thescaffolding and look at the billboard from a distance of 24 inches. All thatbeautiful detail has been reduced to half-tone dots the size of golf balls. Theimage likely isn‘t even intelligible any longer — it‘s just big blobs ofcoloured dots.
This explains why medium and large format images appear "sharper"than prints made from 35mm — even though the 35mm lenses may have as much astwice the resolution. They simply don‘t need to be enlarged as much. To make an8X10" print a 35mm frame needs to be enlarged about 8X. A 6X6 frame about3X. As long as prints are small it doesn‘t make much difference. Beyond about an8X10" print though even the best shot 35mm images start to lose quality,while a 6X6cm based image is usually fine up to about 16X20".
Size matters!

What Have We Learned?
Hopefully this brief discussion will have shed some light on a sometimes confusingand murky topic. Remember, there‘s more to the issue of image sharpness thanraw numbers alone. Hone your technique and trust your eyes.