Are Digital Images Sharp?

by ralph

It's not a question that many "film & darkroom" photographers have lost a lot of sleep over, but it does come up occasionally. The short answer is YES --- if you spend enough money. Resolution produced by digital cameras is closely tied to the price you've paid. On the low end, say cameras costing $300 or $400, the detail can actually be less than you have on your TV screen. Even a 4x6in. snapshot print would be unsatisfying. On the high end, near $30,000 or more, resolution can equal or surpass the best large format work you've ever seen. This is vastly different from the film based scene where quality is nearly price independent. A 120 size (2 1/4" square), $150 Yashicamat with a roll of T Max 100 can turn out eye popping, sharp 16"x16" prints and a used $500 field camera plus lens can do larger work that approaches the Ansel Adams level. Higher prices buy convenience, inter-changeability and bells and whistles, but the quality is inherent in the size.

Let's consider cameras ("digital capture") first. Resolution is measured by the number of pixels in the image -- the more you have the finer the image will be, just like finer grain or higher lines per mm. on film. Multiplying the number of pixels in the length and width of the image gives you the total for the image, or "file size". This also measures the resolution of the cameras image chip or array. For starters lets look at that TV screen again. It has 585 lines vertically and about 460 horizontally giving, if it were a still image, a file size of 270,000 pixels. This 270 Kilobyte ( 270 KB ) file is considered low resolution or "low res" and the image falls apart above 4"x6" looking jagged and "pixelated". There has been quite a flurry of new, low price digital cameras introduced lately but they don't indicate progress in the sense we still photographers would appreciate. These are aimed at the growing use of images on the internet and E-mail where high res images are a royal pain because they take to long to open on the screen. Ergo, lower is better and some of the new cameras drop down to well below 100 KB, good for little more than a postage stamp.

Going the other way we notice the $849 Sony DSC-F with a 640x480 chip giving a 307 KB file which betters video resolution by just a hair. The same chip is used in the Canon Powershot 350, Konica QE-2, Casio QV-120 which costs $500 and boasts a mini video viewscreen, and many others. Cameras in this range are such "bargains" because they use mass produced video and camcorder chips generally measuring about 1/3 of an inch. Quality looks up just under $1000 with the Canon Powershot 600 whose 832 x 608 pixel chip yields a 506 KB file and the Olympus P-300L where a 1024 x 786 chip gives an 804 KB file.

This still falls short of "medium res" which starts around 1.0 to 1.5 megabytes (MB), equal to 1000 to 1500 KB. The Fuji DS-300 for "under $3000" starts this class with a 1280 x 1000 chip giving 1.28 MB. Moving up we see the Kodak DCS 420 (now called the DCS 410 I believe) delivering about 1.5 MB at a list price just under $10,000 and the ingenious Minolta RD-750 achieving a 1.75 MD file by separating the three color images (RGB) onto three smaller, low priced chips, for a street price under $7000. The 1.0 to 1.75 MB file size becomes interesting for minimal professional uses. It can produce fairly smooth and sharp 5x7's, usable in newspapers and small catalog work. With computer enhancements (sharp-ness and interpolation) it can even produce a respectable full magazine page.

Going for more sharpness we get into the "nosebleed" stratosphere of prices. The "high res" file size begins with the Kodak DCS-460 whose 2000 x 3000 pixel, nearly full frame chip yields a 6 MB file and this nifty camera using a Nikon N-90 body, costs about $28,000 (the chip alone is valued at $22,000). A 6 MB file (18 MB in full color RGB) is considered minimally acceptab
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to express the full quality of a 35mm image (it is the highest res file on the Kodak Photo CD), though some workers prefer 24 MB or even 36 MB . Large high res files cannot be instantly scanned off and processed into memory. In the DCS-460 it takes twelve seconds -- but there is a buffer memory to speed up short bursts. In fact the DCS-460 reigned supreme for a couple years for producing the highest resolution image that could be captured in an instant, just like on film. Going beyond that level requires a rather slow scan over the CCD chip. The Leafscan 22 is a good example. It has a 2000 x 2000 pixel chip measuring 3 x 3 cm. (again, smaller than the 6x6 cm cameras like Hasselblad, in which it is used). It can shoot an instantaneous b&w image, but by using an almost antique RGB filter wheel on a very still, still life subject it can deliver a 12 MB color file of astonishing sharpness and color fidelity, but it takes about 2 minutes to scan. Looking at a 16 x 16 in. blowup from this camera back is a real eye opener. The detail is needle sharp and the depth of color in shadows and highlights, a ten stop range, surpasses anything you've ever seen on a 'chrome.

Why does such a modest file size, which would equal only 33 lines per mm if it were spread over the full 6x6 frame, look so good blown up? The answer lies with the different ways film and digital determine resolution. When a lens/film system is said to deliver, lets say 60 lines/mm. the number refers to the smallest line set where the lines can be barely distinguished, although mushy, dim and grainy. You might have to back down to a half or third of that number before finding a set of clear, sharp lines. In contrast the lines of the CCD chip are microscopically sharp, etched by the same technology that makes multimillion transistor computer chips, less than an inch wide. So even at the tiniest level, the lines are perfectly sharp.

Shooting studio still life subjects allows breakthroughs to higher resolutions without blowing the roof on costs. Since a full 4x5 in. CCD array would cost half the national debt the Dicomed and Phase One backs found a simple alternative, mechanically moving a triple strand (three color) of CCD receptors, called a tri-linear array, across the film plane. It takes several minutes but the $21,000 back can produce huge 6000 x 7500 pixel files totaling 129 MB (the PhaseOne does 6000 x 8400 yielding 144 MB) --- and there is even an 8x10 back that delivers a 600 MB file, enough information to store all the text in the Encyclopedia Britannica. Now that IS sharp. Dicomed has recently upped the ante for instantaneous quality with the Big Shot. Its full 6x6 cm chip produces an instant 48 MB RGB color file, displacing the Kodak DCS-460 as top dog. Hmm that's strange, I can't seem to find the price anywhere --- maybe we really don't want to know.

When it comes to manipulating images in the computer the costs are a bit more elastic. Most up to date, middle level computers salted with some extra RAM memory will handle a fairly sizable file, say an 18 MB image, although a machine with heftier specs will work faster.

How do you convert a film image into the image file the computer needs? While you can spend a bundle for high res, high quality scans at a specialty shop the top res scans in the Kodak Photo CD are considered pretty good especially for their low cost. Of course you hear "a buck per scan" in the advertising but these are cranked out at automatic-default settings okay for average work. The better photo labs offer higher priced "pro scans" for $4 or $5 that measure up to fairly high standards.

You can run into some expense at the output end also. Image files can be turned into slides, negatives or prints, again at service bureaus or mail order digital labs but the most likely output will be to Thermal Diffusion prints. These look every bit like high quality photographic prints but at twice the cost. That's because the consumables that go into an 8x10 print cost almost $2.00, as
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d to $.65 for a print from a negative, and the machines cost about $10,000.

Computer heads have been finding an interesting way around the high cost of thermal prints lately. Some ink jet and laser, home and office printers have been getting so good lately that they've been claiming "photo-realistic" quality. Some, like the Hewlett Packard Deskjet 690C and 693C appear to live up to the ads. Consumable cost per 8x10 here is about $.50 for inks with another dime to $.50 for specially sized paper, potentially under a buck in all, and the big news is that these machines ( which only go up to 8 1/2 x 14 inches) cost under $500, a twentieth the cost of those thermal diffusion jobs. We may be seeing more of this.