cyclespeed Loc: Calgary, Alberta Canada

Good day,
At our last club meeting we were discussing uploading photos to our website. When resizing we must not exceed 800 pixels on the longest side of the image in order to have it displayed properly. However when it got to resolution dimension we received different advice from two members who have some IT experience.
One said 150 d.p.i. was good and allowed for quality display i.e. little if any pixelation on screens.
The other claimed 72 d.p.i. worked best since screens maximum resolution display is 72.
They both agreed that fine prints should be at 300 d.p.i.

Can you help clarify this issue for us please. Thank you

paulrph1 Loc: Washington, Utah

It depends on the size of the print and with larger prints 300 DPI is inadequate. Shoot in the highest resolution and buy another memory card they are cheap. You can always adjust down but not up. There is a reason the photo club allows up to 800 pixel so use that one. If they want smaller they can adjust it down. I simply do not understand why people would want to use the smaller D.P.I. unless they are trying to save memory on cards. When in doubt use the largest available. You will never regret using the larger but can regret using the smaller.

TheDman Loc: USA

It's sad that in this day and age even those that claim to have "I.T. experience" still think dpi matters. Here's all you need to know: it's completely meaningless. It doesn't make the files larger or smaller, it doesn't have anything to do with your screen, it doesn't have anything to do with anything. It's an old internet myth that just will not die.

BobHartung Loc: Bettendorf, IA

I am not sure that there is a 'correct' answer to the first statement: screen resolution. You have to consider the 4K and 5K monitors some are now using. Most use well over 100 ppi. See http://www.photoshopessentials.com/essentials/the-72-ppi-web-resolution-myth/.

As for prints, I have read that the native Epson is best at 360 dpi. See about half way down this page http://www.gballard.net/psd/sharpening.html.

So now you'll have to make up your collective mind on what to use.

MT Shooter Loc: Montana

DPI stands for Dots per inch and is a printing resolution ONLY, having nothing at all to do with PPI which is a digital image resolution measuring the number of pixels on your cameras sensor and how they are used to create a viewable image size.
And image from a 24MP sensor is 6000x4000 pixels in size, the PPI resolution it is displayed at electronically will determine its VIEWABLE size. 300 PPI means you will be viewing a 20" x 13.66" image, but that same photo can still be printed up to 48" x 32" with no loss of resolution at 300 DPI on any quality large format printer. I do it every day.
Pay no attention to those "experts" who say DPI & PPI are the same thing, they absolutely are not!

Apaflo Loc: Anchorage, Alaska

The Exif tag for DPI/PPI has no meaning at all when an image is displayed on a computer monitor. the monitor is probably displaying at about 100 PPI, and that is fixed in hardware that cannot be varied.

Printers also print at a fixed PPI value. HP and Canon print at 300 PPI and Epson uses 360 PPI. That is not varied.

The image has to be resampled to appropriate pixel dimensions for each display device. The hardware of the monitor or printer determines the value, not the Exif tag.

Here are two images that demonstrate what that means. Same image, and the only difference is that one is tagged for 10 DPI/PPI and the other for 10000 DPI/PPI. Either way there are 900x646 in the download image.

Exif PPI tag set to 10

(Download)

Exif PPI tag set to 10000

(Download)

Apaflo Loc: Anchorage, Alaska

The value in the Exif data can be called either DPI or PPI. It is exactly the same thing either way, and is totally meaningless.

Somebody suggested shooting at the highest value. That is impossible. It can't be changed, and has no meaning anyway.

BebuLamar

When the image is on a webpage the DPI has no meaning. The viewer will see the image at the DPI of his/her monitor. If one has a retina display then the DPI would be about 300DPI and an image with the long side of 800 pixel would be 2 2/3 inch on the screen. Most screens in use today have about 90 DPI so that most would see your image at and the image is about 10 inch wide. But the DPI is totally dependent on the viewer monitor and not your file.

TriX Loc: Raleigh, NC

Along with the 72 dpi or ppi screen "myth". If you doubt that it's bogus, then measure your display, take the displayed resolution, and do the math...

BebuLamar

While a good number of people have 4K or high resolution monitors. I noticed that most people still use 1920x1080 monitor. A monitor with that number of pixels and 21.5" diagonal would have the resolution of 102 DPI. A larger one like 27" with the same number of pixels would have about 80 DPI. The 72DPI is popular with older monitors.

jerryc41 Loc: Catskill Mts of NY

You should keep these two images and re-post them every time the DPI question comes up.

Apaflo Loc: Anchorage, Alaska

Yep. Actually I've posted a pair, just not that pair, several times! I'm sure that at least a few people have caught the significance, but honestly it has never stirred up any conversation. Most people are wired for something other than logic...

burkphoto Loc: High Point, NC

The only thing that really EVER matters is the size of the image in pixels... especially when you are talking about posting images on web pages.

Web designers base their standards — and the specifications they ask you to follow — on the average monitor in use at the time they are designing the web page. It is very likely in this case that the designer wants you to limit the long side to 800 pixels so it will fit on a 1920x1080 HDTV monitor, with room to spare for other design elements. Depending upon the operating system and browser, the image will appear larger or smaller on higher resolution monitors. Some monitors SCALE smaller pages to make them larger on "retina" (4K, 5K, high resolution) displays.

The whole PPI vs dpi thing is an historical nightmare, and a subject of confusion between two industry groups: The Graphic Arts community and the Photo Lab community. Since I worked in both for decades, I'll try to straddle the fence and explain.

Printers print DOTS. The term originally came from halftones, which were made with lined screens that broke an image up into fine dots of different sizes, depending on brightness and the number of lines per inch on the screen. The term, 'dots' had nothing to do with pixels. It referred to real, physical ink dots on paper, not digital representations of image brightness values. The term 'dots' persisted when scanners were developed. Scanner manufacturers STILL insist on saying their machines scan at "6400 dpi" or "600 dpi", when they are stuffing those samples of an image into FILES that contain DATA representing PIXELS. I guess you COULD say that if there were a grid of 600 square dots per inch on a page, the scanner would turn them into 600 pixels per scanned inch in a file.

The arbitrary "150 dpi" figure is an OLD inkjet printer rule of thumb. The relatively low resolution office inkjet printers and laser printers your IT friend understands need at least 150 PIXELS (not dots!) spread over each linear inch of output. The printer converts the pixels to a LOT more dots in its driver software. For example, my desktop Epson outputs/deposits up to 5760x2880 dots per square inch on paper, but it only needs 360 pixels per square inch of input to maximize the print resolution. And usually, 180 PPI images print fine, at 1440x720 resolution.

Any IT person who still thinks monitors are 72 dpi hasn't bought a monitor or a monitor graphics card in 15 years or so. Monitors come in many different shapes, sizes, and native resolutions. They can display anything from 72 dpi to over 300 dpi, depending on make, model, size, and the graphics card or chip in the computer, tablet, or phone. Most monitors have a native resolution, where one input pixel from the graphics card equals one screen pixel, but they can also emulate a range of other resolutions.

It is very important to understand that the human eye can resolve a limited amount of information. Kodak figured out through extensive testing in the 1980s that the average person cannot see more detail than can be represented by around 240 pixels per square inch in an 8x10 print viewed at 1x to 1.5x its diagonal dimension (about 12.8 to 19 inches). More input resolution than 240 PPI is wasted! As the normal viewing distance for MOST prints is 1x to 1.5x the diagonal dimension, SMALLER prints need MORE resolution (higher PPI), while LARGER prints need LESS resolution. View an 8x10 at 240 PPI image at 13 inches. Then view a 16x20 of the same exact file at 26 inches. Can you see ANY more or less information? No! It is the same effective resolution. If you made the 16x20 with four times as many original, from-the-camera pixels, and viewed it from the same 26 inches, your eye wouldn't see any more information. Try it. Now, OF COURSE, you can view the individual elements MORE CLOSELY in the 16x20 print made at high resolution, so it's important to consider the subject of the photograph when choosing both input and output resolution... If I have a group of 400 people, I want all the pixels I can possibly gather to represent them.

The old "300 dpi" rule of thumb was a graphic arts community standard. It applied to SCANNING. In other words, if you were going to reproduce an image at 4"x5", you had to have 1200x1500 pixels in the scan, so you scanned it at 300 dpi. (Again, scanner drivers use dpi, not PPI, even though they save pixels!) The 300 dpi rule of thumb came from designers who knew that their printing processes ONLY NEEDED about 200 PPI for quality reproduction, because that's all they could resolve! What they wanted was a sneaky way to get enough resolution that they could use if they changed the layout and had to enlarge a photo.

Much of this information comes from the books of Dr. Taz Talley, a speaker I met years ago at a GATF (Graphic Arts Technical Foundation). More of it comes from my attendance at Kodak DP2 Print Production Software training sessions in Rochester in the early 2000s.

My biggest frustration when I see some of these posts is the tendency of myths to be perpetuated and distorted and used to support other myths. There actually IS some truth about the resolution setting in the header of a file! While it has absolutely nothing to do with image reproduction size on the Internet, it does come into play with certain page layout software. SOME page layout software uses the resolution header value to size an image when it is imported and placed on a page. In other words, if the header says "300 dpi or PPI", and the image is 1200x1500 pixels in size, the layout software will use that resolution header value to scale the image to 4x5 inches when it makes it available to place on the page! Adobe PageMaker users were notorious for REQUIRING the resolution header be set correctly, because they wanted images submitted for publication "at reproduction size and nominal reproduction resolution." Of course, the page designer knew she could ENLARGE the image by 50%, because her cheesy offset printer could only resolve 200... 300 was cushioned.

I hope this helps.

JCam Loc: MD Eastern Shore

Are you sure someone wasn't confusing PPI for viewing on screens and DPI for printing? For up to about an 11X14 print done on an inkjet printer 300 DPI is more than adequate.

Al Beatty Loc: Boise, Idaho

I ditto what MT Shooter and several others covered better than I could. I'll only muddy-the-waters a little by speaking to anyone considering publishing their work via Kindle Direct Publishing. I don't understand why but any pictures headed for a Kindle book is supposed to be saved at 96 ppi if you are working on a PC or 72 ppi if you are working on a Mac. It has something to do with the conversion process from Microsoft Word > HTML > Kindle. Take care & ...