As we approach the New Year I thought Id write a post on resolutions. No, not the kind that you will eventually break but the kind that seems to cause so much confusion for beginners (and even some non beginners). Mostly its a case of terminology either gone wrong or brought forward from a dinosaur age. Thats like 15 years in digital life. Here, I’ll try to clear a few things up.
PPI DPI WTF? (where F=Fudge)
The two most common misused acronyms are PPI and DPI. Innocently of course they are interchanged but often the user, if a photographer, is trying to communicate the same thing – how many PIXELS there are per INCH. Some people will say DPI and mean PPI. D stands for DOTS so should not be used unless you are specifically referring to print output, as in, dots on paper. Since your average desktop inkjet doesn’t print dots anyway its kind of archaic terminology for the home user of digital cameras and printers. Take a really close look at an inkjet print next time and try to find dots of ink. You won’t, so don’t spend too long looking. Here are two samples form an Epson 3800. Hardly “dots” at all really.
Mind you, its not your fault, if I recall even Photoshop as far as version 5 (I think) referred to image dimensions in the Image Size dialog as dpi. Grr. So from now on you can resolve to use PPI when you mean Pixels per Inch and most of you reading this can forget about DPI altogether.
Hi Res vs Lo Res
Now we’re all so much wiser about dots and pixels, whats the problem? Well, its those other terms we tend to throw around in the resolution pool. Hi Res, meaning High Resolution or Lo Res, meaning Low Resolution. Exactly what is high resolution and when is an image considered to be hi res? I don’t know. But I do know how many pixels I need to either display or print an image on any particular device, and thats what counts.
Generally when someone asks for a Hi Res image they mean they want to print it and when they ask for a Lo Res image they expect not to print it. I’ll wager that most times what they are trying to say is “I want a big image” or a “small image”. Now we’re getting to the root of the problem. The terms Hi or Lo, on their own don’t mean much. They are only part of the equation. We also need to know the destination of the file.
Im going to talk specifically about what we need to focus on when outputting an image from Lightroom or Photoshop and you’ll see its not as difficult or as complicated as people make it seem.
When outputting images that are destined for screen display there is one simple thing to remember. It doesn’t matter how many pixels there are per inch. Why? Because the screen doesn’t know what an inch is, or a centimeter for the matter. It only knows what a pixel is and how many there are on any given display. Take a 15inch MacBook Pro as an example. Its default or native resolution is 1440 x 900. Thats pixels. Simply put if you display an image that is 1440 pixels wide it would take up the entire width of the display, assuming it was in an application that allowed full screen display like Photoshop or Lightroom do.
In the example below, (click for larger view) each of the three images have exact pixel dimensions but different resolutions. They appear the same size in a web browser because the browser only understands the pixel dimensions.
I’ll say again – It doesn’t matter how many pixels per inch there are in that image. So when you want to upload images to the web the only thing you have to consider is how big you want them to appear on screen.
A question I was asked recently was “whats a good size for the web?”. In my mind Im thinking, how long is a piece of string? A good size is whatever you want to display your images at. The point being, its pretty much a what-you-see-is-what-you-get world when it comes to image display on screen (with respect to dimensions. Not color). If you upload an image to the web and you have to scroll to see it then guess what, its too big. In my opinion 1500 pixels in any direction, especially the vertical is large enough. Remember thats bigger than a 15inch MacBook Pro screen but would sit quite nicely in a browser on a 21 or 27 inch iMac display that sport resolutions of 1920 and 2560 pixels in width. Ultimately, knowing your audience helps.
Most, if not all, WordPress themes and automated uploaders such as those used in Meetup.com will resize your images on the fly. The image above was 1400 pixels wide when I uploaded it but an alternate version at 700 pixels appears here. This is simply so they can display the images in various forms from thumbnails to main post images and still have an original size available for download. Well if thats the case then why not just upload all the pixels your camera produces? Because DSLRS will produce 3, 4, or 5000+ pixel images. Thats bigger than a 27 inch iMac display. Simply put, theres no point, plus, it just takes longer to upload, longer to download and longer to display.
So when you export an image from Lightroom or Save for Web from Photoshop just remember, its all about the pixels and NOT the PPI. In fact I dare you to set your resolution to 1ppi. Its about as arbitrary as 72 or 96.
If your destination is a printed piece then you have to determine how or where the image is going to be printed. The simplest thing to do is ask for specific criteria if you plan on sending an image to an online service or giving it to a local printer but there are some generic sizes we can discuss.
If your image is going to be printed on a commercial press (like a magazine) then its a safe bet that the amount of pixels per inch you need will be in the area of 300. If your image is going to be printed on a decent Inkjet printer then the range will be closer to 180-240 in most cases.
So, using the formula to make a print on a commercial press (300ppi) if you want a 7×5 inch image your file has to have 2100 pixels on the long side. (7 x 300 =2100)
Its just math. Not magic, and not a mystery. That same 2100 pixel wide image can easily be printed 8.75 inches wide on an inkjet printer because the inkjet printer requires less pixels in the image for each inch of paper. (2100 = 240 x 8.75)
So is 300ppi Hi Res? Well, yes and no. It is a high resolution but generally when someone asks for a Hi Res file they simply mean a “big” file. To be more precise, big enough to print. Print on what? Who knows?
If we look at the sample images above. Each of them have the same amount of pixels but the internal resolution is different. Notice how the 300ppi image is much smaller when printed. The same image with a resolution of 72ppi prints much larger but since we know that the printing device requires more pixels than 72 for each inch what we will end up with is a very unsatisfactory print. So 300ppi alone does not mean a big print.
What about File Size?
I like to refer to the file size as the weight of the file because size can often be confused with pixel or print dimensions. The weight of a file is measured in, kilobytes KB and megabytes MB. There are two factors that control the weight of a file. One is the amount of pixels they have and the other is the amount of compression they have. It is not about the amount of Pixels per Inch as you can see from the inset image above where all three files weigh the same 86KB regardless of the resolution.
The second factor of compression is of particular importance to the photographer displaying images on the web. The goal has always been to reduce the files weight via jpg compression without compromising the image quality. Photoshops Save for Web function allows you to view up to 4 different versions of the file before committing to any compression. With Lightroom its a case of trial and trial again, there is no preview. Once you get a setting that suits, save it as a preset.
A side note for geeks: Lightroom still produces jpg files that are heavier than a similar file from Photoshops Save for Web because they seem to require less compression (higher quality setting) in order to maintain image quality
The amount of pixels in a file contributes to its weight too. Thats kind of a no brainer. More sand in the bucket equals a heavier bucket. Now that we have larger displays available we can be tempted to create large files for display. Be cautious. A slow loading image is about as good as a no loading image. We have become quite impatient and while DSL, Cable, and Fiber Optics have improved speeds considerably there is a limit to what we will accept. Not to mention that having an image display large on screen also means that it can make a decent size print at home, for which you have no recourse. Flash sites are no longer considered protection as they in fact usually promote full screen viewing anyway, and who doesn’t know the shortcut to print screen?
File size for printing is less of a concern. Those files will always be large and suffer no compression. Its not a problem for a commercial press. If you’re printing at home and not going directly through Lightroom then a file that is too heavy simply wastes time.
For screen – its about the exact pixels and the compression WYSIWYG.
For Print – its about enough pixels in the right configuration for the device.