The effect of DPI settings (for Jim Manganella)

<p>In <strong><a href="/beginner-photography-questions-forum/00bJ3A">another thread</a></strong>, Jim wrote this (among other things) about the DPI setting of an image:</p>

<blockquote>

<p>Were you shooting at 300 DPI resolution? Did you reduce the quality any when you resized & saved it for the internet? That's what it looks like. I bet the banding is not there in the high-res.</p>

</blockquote>

<p>and</p>

<blockquote>

<p>Looks like Rob & Keith are the experts... that can't get along!<br /> My bad I should have said processed at 300 DPI during the editing/resizing which MOST CERTAINLY plays a part in the final output quality for viewing & for printing.</p>

</blockquote>

<p>and</p>

<blockquote>

<p>You folks need to compare images between a camera that will shoot 300 DPI compared to a cheaper camera with only say 72 DPI. You are way wrong comrades. Apparently your "expertise" does not come from experience.</p>

</blockquote>

<p>While not laying claim to any "expertise", I will say that I don't write <em>anything</em> in a forum that I don't <em>know</em> to be correct, so - for him, and for any beginners that might be baffled by his assertions that the DPI value of an image has a visible impact on it when viewed on, say, a computer monitor - here's something that might be usefully educational.</p>

<p><strong><a href="http://www.capture-the-moment.co.uk/tp/tfu29/upload/tree_sparrow_RT_1_30DPI.jpg">Here</a></strong> and <strong><a href="http://www.capture-the-moment.co.uk/tp/tfu29/upload/tree_sparrow_RT_1_3000DPI.jpg">here</a></strong> are two <em>visually identical</em> versions of the same image. Have a good long look, there's <em>no difference whatsoever</em> between them, as far as your eyes are concerned.</p>

<p>Now dig into the Exif: you'll find that one is <strong>30</strong> DPI and the other is <strong>3000</strong> DPI.</p>

<p>And yet - no difference at all in how they look.</p>

<p>Why is that? Because, as we tried to explain, and these images conclusively prove, the DPI value is just a metadata tag that means nothing whatsoever until the file is sent to a printer.</p>

<p>Is that experiential enough for you, Jim? I created this demonstration for no less a photographic luminary than Art Morris, incidentally: he "gets" DPI now, so if you're still confused, feel free to ask questions. I'm no expert, but I <em>do</em> understand DPI...</p>

<p>(<strong>Mods, I'm not trying to be provocative with this, my first-ever Photo.Net thread, but I really think this point needs to be made <em>very, very</em> unambiguously. If you disagree however, I'll understand</strong>).</p>

<I>Moderator note:</I><P>

 

As a moderator of this forum I am going to let this stand. This Beginners forum is very tightly moderated to avoid alienating any newcomers. Since you or any responders are most likely not beginners, I am going to let you have a free-er go at it. As on all forums, you should conduct yourselves with the courtesy and decorum that is expected of Photo.net Members.

I am going to transfer this thread to the Casual Conversations.

<blockquote>

<p>DPI value is just a metadata tag that means nothing whatsoever until the file is sent to a printer</p>

</blockquote>

<p>And even then, it only matters if the printer is actually paying attention to it. Most of the time, they're paying attention to the <em>dimensions at which you told them to print</em>. If you put in an order for a 5x7 print, they're generally going to print at 5x7 no matter what the DPI stamp on the file happens to say.<br>

<br />Printing companies are usually looking at the actual number of pixels in the file (wide, and tall) to see there's enough data there to print the hardcopy dimensions you're asking them to print. But even then, they're not looking at the DPI value embedded in the file, they're looking at pixel dimensions, and comparing that to what standards they have in-house when it comes to the type and size of print you've asked them to make. DPI shmee-PI. <br /><br />All of this is generally true of using your own printer, too.<br>

</p>

<p>Agree totally. I have slides that are scanned at 6000 x 4000. The dpi is 4000 so the dimensions are 1.5 inches x 1 inch. If I then set the dpi to 400, the size goes to 15 inches x 10 inches, <em>but still the same number of pixels 6000 x 4000</em>. The image looks <em>exactly</em> the same because the number of pixels have not changed.</p>

<p>Showed the concept of resolution in relation to how it appears in a print here:</p>

<p>http://www.photo.net/beginner-photography-questions-forum/00acnx</p>

<p> </p>

<p>I wrote this for the site in 2004. Nothing has changed.<br>

http://www.photo.net/learn/resize/</p>

<p>I have been at times a photographer, photography enthusiast and visualization professional for a good 15 years anyway, and I can't remember the last time I knew what the DPI of an image file I was working with was set to. The number of pixels on each side and the desired print size are important, and the final PPI is by definition the ratio between those two numbers. I don't know why people get excited about this, it's just arithmetic.</p>

<p>Thanks Keith. I didn't contribute to the thread in question but was about to post a thread just like this with example images to make the issue clear.<br>

Not only is DPI irrelevant to digital photography it is pretty much irrelevant in printing too. Printing those images on my printer at home gives me two identical images. Take them to your local print shop and they will print the images at the size you request regardless of the DPI setting in the file. </p>

<p>It only influences printing in the sense that in some programs, like Photoshop, you can assign your image a size in inches, which will be the default print size, and from which a PPI value will be calculated, or you can assign a PPI value from which default print size will be calculated. PPI = size in pixels / size in inches (both measured in either direction because as far as we're concerned here pixels are always square). But that's just the default - you go into the print window, set a paper size, click size to fit or enter a size, and the default size and PPI are forgotten.</p>

<p>Thanks gents - valid points all, especially the idea that DPI isn't even necessarily relevant at the print stage.</p>

<p>I'm simply focusing on Jim's initial assertion that you can<em> actually see</em> the effect of the DPI setting in the image quality of an image viewed on screen, which we all (well <em>almost</em> all) know is demonstrably a fundamentally flawed premise.</p>

<p><strong>And thanks to James for letting this post stand - I know you were similarly baffled by the way the original thread headed too</strong>.</p>