Pixels-per-inch or Dots-per-inch
This comes up OVER and OVER and OVER again and people do not understand it.
The pixels-per-inch (ppi) box is only useful to people who specify their image sizes in INCHES and not in pixels. If you tell Lightroom (or Photoshop or any other program) that you want a picture to be 10 inches wide and 300 ppi, that means you are telling it to give you a 3000 pixel wide image. If you want you can do that, but you could also tell it you want to print a 100 inch wide image at 30 ppi and you'll get EXACTLY THE SAME IMAGE. Or you could just tell it to give you a 3000 pixel wide image and it doesn't matter what you put in the ppi box! Here:
One of these images is 819 pixels x 1024 pixels at 1 (yes, that's ONE) ppi. The other one is 819 pixels x 1024 pixels at 1,000 ppi. Can you tell which is which? No you can't. THEY'RE THE SAME. Keep reading...
Added after the fact: This post is getting a LOT of hits because this question seems to come up a lot on Facebook. Both the Aspect Ratio and the Print Resolution questions. If you find this useful, please consider clicking the donation request at the bottom – purely optional but it would help! At least, I would ask you to subscribe to my newsletter (it's free and you can unsubscribe with one click if you must). Here's the link
Now let's look at the ASPECT RATIO, or in plain English, the SHAPE of the picture.
Suppose this is the original picture:
This image is copyrighted and the property of the author. If you want to use it for any purpose whatsoever, you need written permission. Email me. If you want a print, I'd be happy to accommodate you at a very reasonable price. Please don't rip it off.
It's ASPECT RATIO (that's the technical term: maybe it's easier to understand the word "SHAPE") is 2:3. What does that mean?
For every 2 units high, it is 3 units wide. When it came out of the camera it was 4912 pixels high x 7360 pixels wide. That's out of a D800 so it's 36 million pixels if you multiply it out. If it came out of an old D70 it would be 2000 x 3008 pixels: only 6 Megapixels, but exactly the same shape — 2:3*.
* for all intents and purposes. I know neither one is EXACTLY 2:3 but we're splitting hairs here.A Canon 60D has a resolution of 3456 x 5184 pixels. Do the math: guess what the shape of the image is? That's right, 2:3. Pretty standard in the DSLR world. Your camera might be different but let's go with this example.
OK, what if you wanted to print this picture without cutting ("cropping") anything off the height or the width of the image? What shape would the paper you're printing on have to be?
If you said 2:3 then you're staying with me! Can you think of some examples of paper that is that ratio? Look at this list:
- 4" x 6" (10 cm x 15 cm)
- 5" x 7" (12.5 cm x 17.5 cm)
- 8" x 10" (20 cm x 25 cm)
- 8" x 12" (and so on...)
- 11" x 14"
- 12" x 12"
- 16" x 20"
- 16" x 24"
- 20" x 30"
Which ones are the right shape? Do the math, I'll wait right here...
If you said the 4x6, the 8x12, the 16x24 and the 20x30, now you're cooking!
So what happens if you try to print that picture on an square piece of paper, say 12"x12", for example. One of three things:
- the image can completely fill the paper, but some of the width will get cut off. In other words, you're printing the full 12" height, but there's only room for 12" of width so the other six inches are going to get cut off (remember your picture is 2:3 shape, so if it's 12" high it's going to be 18" wide).
- you can squeeze the 18" width of the picture into the 12" width of the paper, but then you'd be squeezing the height too, so the height will only be 8" high and there'll be a 2" band of unprinted paper above and below the picture, or
- you can squeeze the width of the picture without changing the height, but this distorts your picture (you'd have to get fancy in Photoshop to do this, though)
The red arrow shows you where to click to get the crop tool. The blue arrow shows where to click to get the dropdown menu that you see with the different shapes
** This isn't limited to printing, you know. For example, the shape of the header at the top of this blog is 1280 x 350 pixels. If you look closely at the list, you'll see "3.657 x 1" which is that shape. You don't actually have to do the math, you could just enter 1280 x 350 and Lightroom will do it for you!
A good workflow practice in Lightroom would be to create a "Virtual Copy" of the image if you're going to produce different crop shapes. Then work your magic on that copy without affecting your original. You can't rename the virtual copy without renaming the original but you can put something in the "Copy Name" field in the metadata, like "square version" so that you can search for it later.
So now you should understand why, when you try to print your 2:3 ratio picture on a piece of 5x7 paper, some of it is getting cut off. The only way to prevent that is to print it like the second example above. Or you can decide to cut it off wherever YOU want by cropping in Lightroom. So far so good?
Now let's look at RESOLUTION.
You're probably looking at this article on a computer screen (OK, maybe on a tablet or phone, but let's go with the easy example!)
When you set up your computer, you chose the screen resolution (or, let's face it, you accepted whatever you got when it came home from the store). Depending on what kind or shape of monitor you have, you might have selected 640x480 pixels (if you did, you're older than me. And trust me, I'm OLD!). Or 1600x900. Or 1280x1024. Or some other number. My 26" Samsung monitor is set for 1920x1050 pixels.
Now just to confuse you further, my "26-inch" monitor is really only 23½" wide. The 26" is a diagonal measurement. But let's divide that 1920 pixels into 23½", we get about 81 pixels per inch. The universal standard for computer monitors and the Internet is 72 pixels per inch but who's counting? Close enough.
You could quibble with me and tell me you have a fancy 'Retina' display or a giant monitor, or a $500 video card that lets you see 4000 pixels on your screen but hey...Now go to the top of this article and read what I said about the resolution of the owl picture as it came out of my camera. Wow. 7360x4912 pixels! If I tried to look at every pixel of that on my computer, it would be dripping out of my monitor and down both sides of my tabletop! So guess what? I CAN'T look at them all. I can only squeeze them into the 1920 pixels (wide) that are available to me.
OK, now go find a magnifying glass. I'll wait right here. Are you back? Now look really closely at your computer screen. See how it looks like a bunch of little square dots? Those are called pixels.
So if I want to output a picture for use online or just to be viewed on a computer or tablet, I don't need it to be any bigger than about 2000 pixels (wide). Otherwise it's a waste anyway, you can't see the in-between ones. Since you probably aren't looking at any pictures at the full width of your monitor, even that big is a waste. Make them smaller!
Facebook is special. They have some nasty compression algorithms that mess up pictures. If you research it, you'll find that the best size to make a picture, to optimize how it looks on Facebook, is 2048 pixels. Don't even think of asking why.Your computer is smart. You don't have to tell it how many pixels per inch to make the picture, that's determined by the computer and the monitor. Even if you tell it you want 10,000 pixels per inch it can't do it! It's going to change it to 72 ppi (Pixels per inch) for you. The important thing for you to decide is how many pixels (wide, for example) you want the image to be. Half a screen width is about 800 pixels. What about if you only output your picture at 200 pixels wide, and then tried to blow it up to full-screen (your browser can do it, for instance)? Then it's going to look all unsharp and fuzzy because there's not enough information to exactly light up all of the pixels. Get it?
So if you're outputting a picture for the internet, somewhere between 800 and 2000 pixels is best. Of course the bigger you make it, the bigger the image file size becomes, the slower it is to load, etc. Your call...
I choose to make my pictures 1280 pixels wide for this blog. Here's what my settings look like in the Lightroom export module:
The internet, and your computer monitor, all work in the sRGB colour space. If you set it to something else, your colours are going to look funny. And generally you're going to want JPEG image files, the internet likes them the best.
Understand that your files inside Lightroom are probably not going to be that size. For that matter, they won't necessarily be JPEG's, they may not be in that colour space... they're your NEGATIVES. You need to EXPORT the pictures from Lightroom to use them for anything, and this is where you get to tell it what kind of files, what size, etc.
Now what about for printing?
Go get your magnifying glass and a photographic print. Do you see a bunch of little pixels? (If you do, you're not looking at a photo print, you're looking at something reproduced on a printing press, not an inkjet or giclée printer). Your printer has a bunch of little nozzles that spray ink on the paper but in a really teeny-tiny pattern. Some of the better ones might spray up to 1400 ink dots per inch! (DPI stands for "DOTS per inch". PPI is "PIXELS per inch". One's for ink, the other for screens).
I'm not going to get into how the ink spreads as it soaks into the paper, or how it blends together with nearby dots. Suffice it to say: the universal standard for not being able to see the little individual dots is 300 DPI. Some people use 240 DPI, some use 360... but let's call it 300 to be safe. If you have less information than that the in-between dots are going to take on an average value, not the right value, so your picture will get fuzzy.
Here's the same owl shot but I saved it at only 128 pixels wide then blew it up. The same thing happens if you don't have enough pixels to print! And you need a lot more for printing than you do for displaying on-screen!
Doing the arithmetic, if you want a sharp 8x10 print, you are going to need 300x8 dots by 300x10 dots, or 2400x3000 pixels. For a 16x20, double that. For a 20x30, you need 6000x9000 dots. That's more than most cameras can do but Adobe, who makes Lightroom and Photoshop, and a bunch of other companies know how to intelligently upsize your pictures. The person who REALLY knows how is the company that you're sending your images to print! Always ask them what they want when your image is undersized.
Where were we... by now, you understand that the really important number is how many pixels (or dots) your picture contains. Just like your monitor, that printer will change the number of dots per inch to whatever it has available. If you give it only 1500 dots and you ask for a 10" print, you're only going to get 150 dots per inch (which surprisingly, isn't bad most of the time!).
If you give them too many pixels, don't worry! The printer is smart (not necessarily the person pushing the buttons, the machine!) and it will figure out what to do with the in-between ones, just like your monitor does if you give it an oversized picture. The files might be too big, though!
Why does Lightroom have that dreaded resolution box I marked with an "X" above? It's for people who would prefer to output pictures of a certain size in inches (or cm) and then you have to tell it how many pixels per inch to render. If you do the math yourself, and specify the image size in pixels, that box doesn't matter.
Here's one of my typical settings for going to print (it does 14" on the long side at 300ppi). Note that most printers like JPEG/sRGB as well, but talk to your print shop!
So get in the habit of figuring out how many pixels or dots you want. Let the computer do the rest. Or, choose the size of the picture you want in inches (or cm) and tell it 72 ppi for screen or 300 ppi for printing.
In Lightroom, first choose what SHAPE you want the image to be using the crop tool in the Develop module. Next choose what SIZE it should be in pixels wide x pixels high, using 72 ppi for use on screen or 300 ppi for use in print in order to make your calculations.
If you're going to be printing different sizes and shapes, it's probably smart to create separate output files for each version and virtual copy is one way of doing that and staying organized.
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