Digital image resolution

<p>I'm baffled by this whole PPI business. I get that DPI is an old printing term and has no relevance. But what is PPI? Pixels per inch?<br />So if an image is 100 x 100 pixels it has a resolution of 1000 pixels right?<br>

People keep saying to me they make their images 300 ppi for printing and 72 ppi for the web. But how can you change the resolution of an image without changing it's physical size?<br /><br />Dpes anyone understand this subject?</p>

PPI and DPI make absolutely no difference for displaying images on the web or any type of monitor. For example, a 600 x 900 pixel image will be exactly the same size on any given monitor whether the DPI/PPI is set to 72 or to 300 or to 3,000. Only the dimensions matter.

<p>Henry-I think you will find this very informative and interesting. cb :-)<br>

<a href="http://kelbytv.com/photographytnt/2014/02/13/photography-tips-and-tricks-pixels-image-resolution-and-printing-episode-51/">http://kelbytv.com/photographytnt/2014/02/13/photography-tips-and-tricks-pixels-image-resolution-and-printing-episode-51/</a></p>

<p>Lets say I upload an image of 72 ppi, 4 x 6 inches. Doesn't that mean that at such low ppi it cannot be printed very well?</p>

 

<p>watch the video Henry,</p>

<p>Thanks Charles I watched it. What I got from it was that image resolution is nothing more than image dimension. If you change the dimension, the density of the pixels changes, but the amount doesn't. <br>

My camera automatically has raw files which open at 240 ppi. Why 240? Industry standard for printing?<br />Now, if I reduce 240 to 72 ppi, he image size increases, so if I am saving for the web I need to both decrease ppi and also decrease image size?</p>

<p>I just opened a raw file. It is 240 PPI. I then 'saved for web' and the resultant JPG was 96 PPI. I opened that image in Photoshop and PS said it was 72 PPI. I don't understand what's going on here.</p>

 

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<p>I just opened a raw file. It is 240 PPI. I then 'saved for web' and the resultant JPG was 96 PPI. I opened that image in Photoshop and PS said it was 72 PPI. I don't understand what's going on here.</p>

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<p> <br>

None of those numbers matter. They are placeholders. The only thing that matters, unless you are using a printer that uses the PPI setting, is the pixels x by pixels y. You can change those numbers all you want and the file size can stay the same. It means nothing.</p>

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<p>For example, a 600 x 900 pixel image will be exactly the same size on any given monitor whether the DPI/PPI is set to 72 or to 300 or to 3,000. Only the dimensions matter.</p>

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<p>The displayed size depends on the attributes and styling (CSS) of the img element used to display the image. The actual size of the image matters only if the img element has no sizing of its own.<br>

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However, you are correct about PPI not mattering.</p>

<p>Maybe ignore all the PPI until you need them? - Most Online stuff has a limited dimension to display as in 800 x ** pixels and you downscale accordingly, no matter if some half blind folks will spread the result over a full 32" screen to get an idea of it. So yes! - you do change the physical size of an image for the web<br>

"So if an image is 100 x 100 pixels it has a resolution of 1000 pixels right?" -Multiplying both sides of an image results in Megapixels - Image size... Not sure if "resolution" is the right term for that.<br>

"Lets say I upload an image of 72 ppi, 4 x 6 inches. Doesn't that mean that at such low ppi it cannot be printed very well?" - Exactly! But it could be downsized to 1.2x1.8 and look stunning. - Enough for a headshot on your business or ID card.</p>

<p>For quality offset printing some 60 x squareroot of 2 pixel per cm would be nice to have, thats something above 215PPI. - Thats a lowest European industry standard I am aware of - I am pretty sure that the US do it a tad differently. - A quality factor of 2PPI per LPI was more conservative and means something around 300PPI. Considering that your printshop will need 3mm image seam for cutting a borderless end product, has to convert your RGB to their CMYK with dotgain and will have the 4 screens 15° angled to each other for printing, its probably best that you send in plenty of pixels and let their gear do the number crunching instead of downscaling at home just to hear "Ooops we had to upscale it 1.5%"...<br>

Back to your various softwares: Their PPI outputs are most likely random. They could for example refer to the entire image as seen in a less than full screen window.<br>

"Now, if I reduce 240 to 72 ppi, he image size increases, so if I am saving for the web I need to both decrease ppi and also decrease image size?" - Aye. But most likely reducing the image size as needed Pixels wise does the job well enough.<br>

I guess all the confusion here origins from monitors being way too small to emulate prints, so the software has to quote image size at a given resolution for prepress folks. - If you work for the web stick to pure pixels and forget about inches and cm.</p>

Isn't 100x100=10,000? Not that it particularly matters. Just seems nice to get the math right.

<p>What you're really saying when you say, "an image 72ppi, 4 x 6 inches", is an image 288 x 432 pixels (72 x 4 by 72 x 6). Here the ppi given along with a physical dimension in inches means something, i.e. gives the pixel dimensions when multiplied out. What makes it "unprintable" are the small pixel dimensions.</p>

100 x100 = 10,000 not 1,000.

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<p>People keep saying to me they make their images 300 ppi for printing and 72 ppi for the web. But how can you change the resolution of an image without changing it's physical size?</p>

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<p>They are wrong and confused. Take an Epson printer for example. You want a 8x10 image. That's the <strong>size</strong> you want! Digital images have no size other than the space they take up on a drive. And a JPEG with exactly the same number of pixels as a TIFF is a different size in that respect. Back to the Epson. As long as you size the image to 8x10 <em>somewhere</em>, you can send it out at 180-480 pixels per inch and still get a great 8x10 print. Work in pixels! 10 inches at 180ppi 1800 total pixels. If you wanted a 10 inch print at 480, you'd need 4480 pixels. You divide up the pixels you have on each axis to produce a print size or other output you desire, in this case 10 inches. You want a 10 inch print using 203 pixels per inch, no issue. You'll need 2030 pixels to do so. There is no specific, correct number of pixels for any output per se, it depends on the output device. With our Epson, we can send out less than 180 PPI but the quality suffers, more than 480, quality too can suffer. <br>

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Maybe this will help too: http://www.digitaldog.net/files/Resolution.pdf</p>

 

 

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<p>Henry-I think you will find this very informative and interesting. cb :-)<br /><a href="http://kelbytv.com/photographytnt/2014/02/13/photography-tips-and-tricks-pixels-image-resolution-and-printing-episode-51/" rel="nofollow" target="_blank">http://kelbytv.com/photographytnt/2014/02/13/photography-tips-and-tricks-pixels-image-resolution-and-printing-episode-51/</a></p>

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Confusing, he mixes dots and pixels, recommends 240 (which of course depends on the output device). Doesn't explain resample and the critical role of the algorithm options, what Auto does. Well if you are NAPP and are hawking yet <em>another</em> product (Perfect 8, not necessary), just tell folks to buy it.

 

No, resample doesn't make pixels bigger. Ignore the 'real pixels', more are sharper etc. I'd pass...

 

 

<p>A lot to understand, but think I'm getting it. I appreciate all your help everyone </p>

<p>Try not to use area calculations and things will be easier. Think about total pixels along a single edge. For example, if you have a camera that has 3000 pixels along the longest of its edges, then use that in your calculations. If you want to make a 10 inch print, it will be at 300 dpi. If you want to make a 20 inch print, you only have 3000, and 3000/20 = 150, so the resulting printing dpi is 150. Most people like a bit more. If you only make a 5 inch print, you can do it at 600 dpi... keep it simple.</p>

<p>Of course, the next step most people take is to look at resolution, which has nothing to do with the number of pixels a device can generate. There is stated resolution, which is this number of pixels, and then there is optical resolution, which is a factor of how much the image can actually resolve, using those Air Force test targets with the parallel bars. Most scanner companies and camera companies lie about the actual optical resolution (especially Epson). </p>

<p>Multiply one edge's total pixels by another (area calculations) gets you megapixels. Not a direct indicator of quality, there are many other factors, lenses, quality and size of sensor, etc.</p>

<p>When you have an image with 3000 pixels, you can make it any size you want. However, the total number of pixels doesn't change. The only way to change them upwards is to interpolate, which means to create new pixels using a "best guess" approach.</p>

<p>I hope this helps....</p>

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<p>Think about total pixels along a single edge. For example, if you have a camera that has 3000 pixels along the longest of its edges, then use that in your calculations. If you want to make a 10 inch print, it will be at 300 dpi. If you want to make a 20 inch print, you only have 3000, and 3000/20 = 150, so the resulting printing dpi is 150. Most people like a bit more. If you only make a 5 inch print, you can do it at 600 dpi... keep it simple.</p>

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<p>That's my way of thinking about resolution, but I include the size of the sensor which those 3000 pixels are packed into which will make them almost microscopic. The same is consider for scanners that use a linear array which can be thousands of pixels across on a sensor a number of inches long and use a step motor to fill in the other dimension to scan the area. Now how big are those pixels?</p>

<p>My 3000x2000 pixel/6MP APS-C sized sensor on my DSLR is close to 7/8in. wide or 23.5mm. Let's round it off to an inch for simplicity sake. If you want to enlarge a 1 inch area packed with 3000 very tiny pixels to 10 inches, divide 3000 by 10 and you get 300 pixels per inch on a 10 inch long print. If you enlarge any higher without resampling (IOW creating new pixels that aren't on the original sensor) the ppi must decrease while the inches increase.</p>

<p>Some folks like to maintain 300ppi and make sure Resample is checked in "Image Size..." in Photoshop and increase the inches which represents the size of the enlargement of the print in physical form so they can compare apples to apples and what upsampling algorithms do to detail at 100% zoom view on their display.</p>

<p>240ppi is just a random number I have no idea why it was considered as default in ACR but it's really meaningless unless you know the other dimensions.</p>

<p>Think of pixels as the thread count on a percale bed sheet where 200 percale on a queen size sheet is going to have the same size thread as on a twin size. 400 percale on a twin size sheet is going to be finer than on queen size at 200 percale.</p>

<p>When you try to stretch the twin size size sheet larger than it's original dimensions which will spread out the threads and create holes within checkerboard pattern grid you either add more threads by upsampling/interpolating or you allow the holes to show which in pixel language can be visually described as sawtoothed edges on a print.</p>

<p>I make a point to not resample. I am using film and a drum scanner which gives me a tremendous amount of pixels (always more than I need). I also use a RIP to print, and it accepts happily any dpi, regardless of it being some multiple of 300 (or not). I don't see a benefit to resampling for the most part and I never do it - I just send what I have to the printer. There are those who say its great and others who say its terrible. I'm not going to suggest one or the other. However, its important to not that you don't get any more detail from resampling. IMO, if you do it, it should be a last step, just before printing, on a copy meant for printing.</p>

<p>One good reason to resample is when a publisher demands a certain dimensionally sized photograph at a certain DPI. I had a photo that required a 12 X 12 requirement at 300 DPI. I didn't quite have enough pixels to meet the requirement and had to resample, fairly conservatively to meet the standard because that is what the submission required. So re-sampling is a valid tool but you have to be aware of its limitations, especially when up-rezing a photo.</p>

<p>Sometimes you do have to resample <strong>up</strong>, invent data. Depending on the input data and output, you should have no issues going 200% without any real issues. How you handle the data is important:<br /> http://www.digitalphotopro.com/technique/software-technique/the-art-of-the-up-res.html?start=1</p>

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<p>240ppi is just a random number I have no idea why it was considered as default in ACR but it's really meaningless unless you know the other dimensions.</p>

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<p>It's always been somewhat inkjet output non-agnostic. Just like the sharpening settings in the Print module. They had to put some value there, just like those cases where output to a display will pop 72 in the field. Something has to go there, meaningless/ambiguous as the values actually are. </p>