Digital cameras do a lot of things that film cameras could never do. But in a way, film photographers had it kind of easy. There was no such thing as "file size", because everything you shot on a roll of film was exactly the same "size" as everything else you shot on that roll of film.
Today we are blessed - or perhaps cursed - with the ability to shoot photos at any resolution and quality setting we want. But all of these choices come at a cost, and that cost is most obvious when you try to print the photos that you take with your digital camera. Let's see why....
All resolutions are not created equal
Have you ever uploaded a batch of photos to your favorite printing service, and then been appalled by what they ended up looking like? Instead of those clear, sharp photos you thought you'd shot, you've got photos that are covered with JPG artifacts—blocky, pixelated-looking areas that make every single shot look like it's not only out of focus, but also occupied by a series of Minecraft characters. Here's what happened: either you shot your photos at a resolution that wasn't compatible with the print size you ordered, or you chose a compression setting when you uploaded the files (that just means that you agreed to upload a smaller version of the file, so the upload time would be less). Now, you may or may not have meant to do either one of those things, but that's what happened. And here's the reason why it completely escaped your notice, until you actually had those photos in-hand and the terrible truth was dawning on you.
Low resolution images look great on your computer, even though they'll look terrible in print. Why is that? To understand the answer to that question, you must first understand what exactly "resolution" means.
A digital photo is basically just a series of very, very small dots, which are called pixels. You probably already knew this. Resolution, then just refers to the number of pixels there are in any given photo. Now, you may have heard both the term "dpi" and "ppi" used when talking about image resolution, but there is actually an important distinction between the two. "dpi" stands for "dots per inch," while "ppi" stands for "pixels per inch." "dpi" is an older term, which is supposed to only refer to the number of dots per inch that a printer can lay down on a piece of paper. It's not really something you need to be worried about in terms of image resolution, but you may hear it used interchangeably with ppi so I mention it here so you won't get confused. When people talk about "dpi" when discussing image resolution, they mean "ppi."
Now, here's the reason why your low resolution images look great on screen, but not on paper. The average LCD computer screen has a resolution of somewhere between 67 – 130ppi. It's a low-resolution way of displaying images, so when you look at a low-resolution file on screen it looks fine because it's been optimized to be displayed on that low resolution screen.
But print is a higher resolution format. On paper, there need to be a lot more pixels per inch before our eyes will stop seeing those individual dots and start interpreting an image as being an image rather than a series of points of color. On paper, we need about 300 ppi before we get to this point. So that 72 dpi photo that looked great on your monitor is either going to look really bad on paper, or it's going to have to be printed at postage-stamp size in order to make our eyes start seeing it as an image instead of a series of dots.
Now, a lot of people who are still familiarizing themselves with this idea think, OK, let's just increase the ppi for that 72ppi image to 300ppi, and problem solved. Unfortunately, this doesn't actually do anything to solve the problem because all you're doing is adding pixels that aren't actually there—so what you end up with is an image that's soft and blurry around the edges. Increasing the physical print size of the file won't work either, because now you're just telling your printer to increase the size of the pixels on the printed page. So instead of that smooth image you had at the smaller size, now those pixels are three times as large and your viewer can easily make them out.
In short, you can't go back and fix a low resolution image. Low resolution is low resolution. You can't add pixels, and you can't add physical size. And that's why it's important to make sure that you get the resolution right in camera. If you're going to print at 4x6, you need to use an image quality setting that will give you 300dpi at 4x6. If you're not sure, you can open up your files in post processing and look at the file size (in Photoshop, go to Image > Image Size).
Here's another thing that may be confusing. Sometimes, when you shoot those images on your digital camera and then open them up in post processing, your software may tell you that you've got a 72ppi image. This can be confusing until you look at the size of the file in inches—a photo shot on a full frame camera at "fine" resolution may be showing 59.111 x 39.333 inches at 72ppi, but that just means that at 300ppi you should have a perfectly printable image up to a print size of 14.187 x 9.44. If you want to see this for yourself, go to Image > Image size and uncheck "Resample Image," then replace "72" in the "Resolution" field with "300." The width and height fields should update, telling you the maximum print size you can use if you want a high-quality printed image.
Just in case that completely confused you, here's another way of looking at it: there is really no difference between the 72ppi version of the file and the 300ppi version—it's the exact same resolution. In other words, there are the same number of pixels in both versions. So in post processing the information about ppi is mostly irrelevant (unless the file size in inches is also very small), because when you physically print the image it's going to be compacted to fit whatever paper size you're printing it at. That means that there's no need to change the ppi from 72 to 300 (thus decreasing the file size in inches) before sending it to the printer, because that should all happen behind the scenes.
Where do megapixels fit in to all of this?
One megapixel amounts to exactly one million pixels in a single image. The number of megapixels your camera can do really only matters in terms of how large you want to print your files. So regardless of how often you've heard people say you have to upgrade your camera, because 12 megapixels is woefully inadequate for today's digital photographer, those megapixels actually don't matter one bit unless your goal is to print large photos.
So let's put this all together with a simple chart, which will hopefully clear up any questions you might still have about all of this:
|Megapixels||Image dimensions (in pixels)||Largest acceptable print size at 300ppi|
|0.3||640 x 480*||2.1" x 1.6"|
|1.2||1,280 x 960||4.2 x 3.2|
|2.0||1,632 x 1,224||5.4 x 4.1|
|2.6||2,000 x 1,312||6.7 x 4.4|
|3.5||2,275 x 1,520||7.6 x 5.1|
|5.0||2,590 x 1,920||8.6 x 6.4|
|6.0||3,008 x 2,000||10.0 x 6.7|
|12.1||4,256 x 2,848||14.2 x 9.5|
|13.7||4,536 x 3,024||15.1 x 10.1|
|22.8||5,782 x 3,946||19.3 x 13.2|
- No modern digital camera shoots images at this resolution, but I've included it here because an image you found online may have similar dimensions, and will therefore be unsuitable to print at a size greater than 2.1"x1.6".
So if you only ever print files at 4x6, the maximum megapixels you really need to have is 2.6. Does that surprise you? Now, if you like printing 8x10s, you need to have a camera that can do better than 6 megapixels, but that 24.2 megapixel camera the salesman at Best Buy tried to get you to buy? You don't need it unless you really, really think you might want to print posters.
Now, if you frequently shoot at lower quality settings in the name of saving hard drive or memory card space, I hope this chart will give you a reason to think about whether or not that's something you want to continue doing. Fortunately, hard drive and memory card space is pretty cheap, and still getting cheaper. So I always advise people to shoot at the quality setting that will give them 300ppis at whatever maximum print size they plan to use. For most people, that's 4x6. For others, a few 5x7s and the occasional 8x10 are often possibilities. If you're not sure, open up your photos in post processing and check the file size, or refer to your camera's manual—it should give you a clear explanation of image dimensions for each one of those quality settings.
Your digital camera probably represents an investment to you, and if you're not using it to capture images that will print well, you're definitely not getting the most out of your investment. Make sure you understand what that lower quality setting will mean for your prints, not just for the digital copies of your photos. Just because it looks great on screen does not mean it will look great on paper.
Most people think this post is Awesome. What do you think?