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Author Topic: Best way to downsample?  (Read 1004 times)
texshooter
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« on: June 19, 2013, 09:21:29 PM »
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I've read several threads here on the subject and am still confused. Some say I should not downsample. "Why throw away pixels?" they say. Some say I should downsample in Lightroom instead of PS because PS does a crappy job at it. And others prescribe special downsampling techniques, like downsampling incrementally several times over.

My photo lab's printer's native resolution is 300dpi, but my images' resolution is higher than that (given the print sizes I have planned). [Note: I think my lab is bullshitting me about their Chromira printer's maximum resolution capablilty. They say 300dpi, I say 425dpi.  I read somewhere
 http://www.zbe.com/products/chromira/chromira_5x30/faq
that this printer can go up to 425dpi. But, hey, what do I know. I don't think they want to switch back and forth from 300 to 425 because it will interupt their work schedule. That's the nature of mass printing services, I guess. But their prices are decent.]

If I don't downsample, I'm sure the lab's printer will automatically do it for me; then the sharpening will be all screwed up.

So how exactly should I downsample? Or should I?
« Last Edit: June 19, 2013, 09:23:12 PM by texshooter » Logged
hjulenissen
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« Reply #1 on: June 20, 2013, 01:05:40 AM »
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If, for a given print size, your camera is a different resolution from the native resolution of your printer (and practically, it is), then your image will have to be resampled at least once. So the question then, perhaps, is : "should I resample one or several times? Should I trust my printer/drivers to do resampling, or should I do it myself?".

I believe that for many people, for many cases, it does not matter all that much. A good picture will still be a good picture. Good focus and good sharpening is important. I let Lightroom take care of it for me. If I was printing to jpeg for delivery to a photo lab, I might choose to resample to "their" dpi, but what are the chances that they will resample anyway to make sure that the paper is printed edge-to-edge?


-h
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BartvanderWolf
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« Reply #2 on: June 20, 2013, 01:59:48 AM »
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I've read several threads here on the subject and am still confused. Some say I should not downsample. "Why throw away pixels?" they say. Some say I should downsample in Lightroom instead of PS because PS does a crappy job at it. And others prescribe special downsampling techniques, like downsampling incrementally several times over.

Hi,

Lightroom does a better job of down-sampling, with fewer aliasing artifacts than e.g. Photoshop (CS6 and before). There are algorithms that do a slightly better job.

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My photo lab's printer's native resolution is 300dpi, but my images' resolution is higher than that (given the print sizes I have planned). [Note: I think my lab is bullshitting me about their Chromira printer's maximum resolution capablilty. They say 300dpi, I say 425dpi.  I read somewhere http://www.zbe.com/products/chromira/chromira_5x30/faq
that this printer can go up to 425dpi. But, hey, what do I know. I don't think they want to switch back and forth from 300 to 425 because it will interupt their work schedule. That's the nature of mass printing services, I guess. But their prices are decent.]

Actually their FAQ suggests that their printing technique has the visual effect of a 425 PPI print, because they use a diagonal (diamond) shaped interpolation or maybe just a printing grid. In general, diagonal resolution is indeed a factor of Sqrt(2) higher in a rectangular grid, but the input raster image grid is still 300 PPI (horizontally/vertically). It's probably an internal printer driver recalculation for the printing mechanism that you cannot influence, so you should deliver 300 PPI input files.

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If I don't downsample, I'm sure the lab's printer will automatically do it for me; then the sharpening will be all screwed up.

I'd output-sharpen for 300 PPI, do not overdo it as you would for matte inkjet materials but aim for the glossy output setting in Lightroom if you use that, or export unsharpened and do some proper deconvolution sharpening e.g. in Photoshop (using FocusMagic, which is hard to beat, or a similar Plugin) after the resampling to 300 PPI. Just try and make sure that the lab does not use excessive sharpening themselves on top of that (not likely, but better be safe than sorry). You can always ask to print a crop you take out of the full output resampled and sharpened output file, and see how that works out. If you do a test print crop (e.g. 8x10 inch), you can also sharpen half of it, and leave the other half unsharpened, and see how that difference looks in the final product they deliver.

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So how exactly should I downsample? Or should I?

With the proper procedures, you will probably be able to get better output quality than they do, because after resampling you can control the output-sharpening much more tuned for the specific image than they will. The only variable is what they will additionally do to your output file.

Resample to 300 PPI, and optionally work on the overall details of the output file with tools like Topaz Detail, or similar, especially if the viewing distance is relatively predictable. Then use deconvolution sharpening to compensate for the slight resampling blur you also created with the resampling (unless you already used output sharpening in Lightroom). Don't over-sharpen to pre-compensate for output losses too much because there will be few, unless the output medium is very textured.

Cheers,
Bart
« Last Edit: June 20, 2013, 02:54:21 AM by BartvanderWolf » Logged
texshooter
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« Reply #3 on: June 20, 2013, 02:56:49 AM »
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. Actually their FAQ suggests that their resampling technique has the visual effect of a 425 PPI print, because they use a diagonal (diamond) shaped interpolation (maybe printing) grid. In general, diagonal resolution is indeed a factor of Sqrt(2) higher in a rectangular grid, but the input raster image grid is still 300 PPI (horizontally/vertically). It's probably an internal printer driver recalculation for the printing mechanism that you cannot influence, so you should deliver 300 PPI input files.   

Might as well be Greek, but I trust you.

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.  I'd output-sharpen for 300 PPI, do not overdo it as you would for matte inkjet materials but aim for the glossy output setting in Lightroom if you use that, or export unsharpened and do some proper deconvolution sharpening e.g. in Photoshop (using FocusMagic, which is hard to beat, or a similar Plugin) after the resampling to 300 PPI.

I'd always thought Photokit Sharpener was the king of the hill for output sharpening, but I'll give Focus Magic a try.

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.     Resample to 300 PPI, and optionally work on the overall details of the output file with tools like Topaz Detail, or similar, especially if the viewing distance is relatively predictable. Then use deconvolution sharpening to compensate for the slight resampling blur you also created with the resampling (unless you already used output sharpening in Lightroom).   

I gather you recommend I do BOTH creative sharpening and output sharpening AFTER downsampling. i had always thought creative sharpening came before resampling (whether up or down) and output sharpening came after. Thanks for setting me straight.
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BartvanderWolf
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« Reply #4 on: June 20, 2013, 04:10:46 AM »
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I'd always thought Photokit Sharpener was the king of the hill for output sharpening, but I'll give Focus Magic a try.

PKS is pushed by some, I prefer an actual deconvolution sharpening as implemented by FocusMagic (a 64-bit update for Windows was released recently and they're supposed to be beta testing the Intel Mac version) or similar.

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I gather you recommend I do BOTH creative sharpening and output sharpening AFTER downsampling. i had always thought creative sharpening came before resampling (whether up or down) and output sharpening came after. Thanks for setting me straight.

Not exactly. The Creative sharpening part is part of the overall image improvement, adjusting local contrasts and more global dodging and burning to accentuate certain levels of detail. It was traditionally done by a form of USM sharpening or High Pass filtering, but there are much better methods available now (e.g. Topaz Detail and Topaz Clarity are great for that creative part, if not essential to lift a drab veil from the image). This is done before further resizing of the original data for output.

The actual output sharpening comes after resampling for output, but one can also apply an additional bit of Detail boost if the viewing distance is predictable, in order to boost the spatial frequencies for which humans are more sensitive. These spatial frequencies depend on magnification, which varies with print size (so can only be judged after final resampling) and viewing distance (if it doesn't vary too much, you can only optimize it for a certain distance, see the attachments for an demonstration of the underlying principle).

Cheers,
Bart
« Last Edit: June 20, 2013, 04:21:18 AM by BartvanderWolf » Logged
PeterAit
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« Reply #5 on: June 20, 2013, 11:14:39 AM »
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I don't suppose there's the slightest chance you could try it out and see for yourself.
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Peter
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texshooter
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« Reply #6 on: June 20, 2013, 11:45:39 AM »
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I don't suppose there's the slightest chance you could try it out and see for yourself.

I don't suppose you can can buy me a Chromira printer.
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PeterAit
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« Reply #7 on: June 20, 2013, 04:56:40 PM »
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I don't suppose you can can buy me a Chromira printer.

Not the slightest chance. So, send the images off to the printer and look at the results. Decide what you like.

I think that all this obsessing about sharpening is silly. No bad photo has even become good by sharpening. The photo world is full of photos that are tack sharp but otherwise banal and boring. Please spare me.

 Sharpening has its place, to be sure, but it is a minor place.
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Peter
"Photographic technique is a means to an end, never the end itself."
View my photos at http://www.peteraitken.com
BartvanderWolf
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« Reply #8 on: June 20, 2013, 05:14:41 PM »
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Sharpening has its place, to be sure, but it is a minor place.

Hi Peter,

It's comments like yours which make me wonder, have you ever seen a really well sharpened print and not preferred it over a normal/mediocre sharpened version of that print? It is not a criticism, but I sometimes wonder if I'm oversensitive to appreciating great quality that much (with nothing distracting from the message in the image it self), especially when it's so easy to achieve by following the basic rules (once they are understood) like proper down-sampling and proper (deconvolution) sharpening?

Cheers,
Bart
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