what strategies do the commercial photo enlargers use?

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jrs5fg
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p.1 #1 · what strategies do the commercial photo enlargers use?

I run an ad-hoc photo printing operation for fellow students at my school (mainly to pay for the costs of my own printing)-- I print 8.5x11 or bigger (because those are the sizes for which I can compete with outside). Usually I print at 5000px or 2048px, but people often want to print something that's kinda low-res for that size, like 1000px or even 700px (that's 63 ppi :o). Now I've printed a 1024px HDR image at 8.5x11 before, but the HDR tone mapping kind of disguised some of the resolution issues.

What algorithms or strategies are out there that will allow a reasonable enlargement of a low-res image? The thing is, I've seen some impressive enlargements made before, so I wonder what the commercial enlargers use. My fellow students would also be willing to endure the use of postprocessing or filters to disguise the low resolution.

Oct 26, 2012 at 05:14 AM

colinm
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p.1 #2 · what strategies do the commercial photo enlargers use?

Going big is easy.

Your situation—going relatively small from woefully inadequate files—is much more difficult, and even throwing a specialty interpolation tool or a RIP at it, you're unlikely to get acceptable results. There's just no magic bullet for making up what's not there.

Heavily textured papers hide sins pretty well. And canvas hides a lot of sins really well. How do your classmates feel about canvas?

Oct 26, 2012 at 05:49 AM

redcrown
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p.1 #3 · what strategies do the commercial photo enlargers use?

I went to a pro printing workshop a couple years ago. Here are the key lessons taught there.

1. Minimum PPI is a function of viewing distance. A 4x6 print hand held at 12 inches from the eye needs a higher ppi than an 11x14 print hung on a wall and viewed at 3 feet.

2. Maximum ppi needs to be no higher than 180 to 200. The instructor had students view a series identical of 8x10 prints, hand held at "comfortable" viewing distance, which averaged 12 to 18 inches. One series was from a high end Epson inkjet, another series was from a Lab.

None of the 20+ students could tell the differences in prints made between 180 ppi and 360 ppi. Only a few could accurately identify the prints made between 100 ppi and 150 ppi. Almost all could identify the prints made below 100 ppi. After the initial "test", we were allowed to look at the prints under a loupe magnifer. Only then could we see a difference between 180 ppi and 240 ppi. Nobody could see a difference between 240 ppi and 360 ppi.

3. The "uprezing" function of modern printers is very good, as good as or better than what can be done in Photoshop or other tools, but up to a limit. The instructor claimed that limit was about 250%. So if your actual ppi is 64 and you want 180 ppi, that would be a 280% increase. In that case, you are better off to use software to uprez the 64 ppi. Otherwise, leave it alone.

4. The very early version of Photoshop did not to a good job of uprezing images. So special "stairstep" techniques were developed and promoted, and third party resizing software programs evolved. Then Adobe improved Photoshop with the "Bicubic Smoother" option in Image Resize, effectively equaling other techniques and products.

But those third party products have also continued to evolve. Each one has a group of loyal followers, and reviews abound on the net. The workshop instructor claimed (2+ years ago) that he could get 99% of the performance of any third party program by using Photoshop only. Uprezing with Bicubic Smoother followed by some noise reduction and sharpening when necessary.

Oct 26, 2012 at 06:59 AM

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