The resolution of a camera sensor is different from the resolution of a monitor which is different from the resolution of a printer. Adobe Photoshop brings all three together, giving you the ultimate control of how your image is presented. But only if you know how each different resolution works.
In this article, I’ll (1) briefly describe camera sensor resolution, monitor or screen resolution and print resolution, (2) explore how Photoshop manages each, and (3) lay out some of the most helpful settings for processing your images with Photoshop.
What is resolution anyway?
For photographers, resolution isn’t that thing you do each New Year’s Day (unless you’re resolving to go out and take more photos). Nor is it the kind of thing elected officials do, like a Congressional Resolution establishing July as National Ice Cream Month.
In terms of photography, resolution refers to “the capability of distinguishing between two separate but adjacent objects or sources of light or between two nearly equal wavelengths of light,” according to dictionary.com. In other words, we’re talking about the ability to resolve, or see, fine details in texture, color, lighting or shape. Photographic resolution is commonly measured in pixels. In general, the higher the resolution is, the sharper and more detailed the image will be.
Cameras and sensors
Camera sensors are composed of millions of pixels (megapixels or MP) each of which records the amount and wavelength (or color) of light to which it is exposed when the shutter is opened. Sensor resolution is measured in “effective megapixels” (some pixels on a sensor may not actually be used to record the image). My Nikon D750 has a 24.3 MP full-frame sensor. The sensor is 36 by 24 mm and records the data for an image that’s 6016 pixels by 4016 pixels.
In general, the more pixels in a sensor of a given size, the better the resolution, the more details that you can cleanly record and the larger you can print that image. The larger the files, the more space they take on your hard drive and, after a certain point the slower they might make post processing.
So far we’re talking about maximum potential resolution. In the real world, the quality of your lens and of our technique matter, too. All the potential resolution in the world won’t make a shaky, out-of-focus shot sharp. The key resolution in this situation is your resolution to go out and get a better lens and practice better technique.
Screen resolution is also measured in pixels and is most commonly cited as the length and width of the screen in pixels for example, 1920 x 1080 (also considered full HD). Newer 4K displays have a horizontal resolution of around 4,000 pixels, typically either 3840 or 4096). Older monitors once displayed 72 pixels per inch (PPI), but today a 24” Full HD monitor displays about 92 PPI, while a 24” 4K monitor displays about 186 PPI. More is (mostly) better, so a 4K monitor will deliver a sharper, cleaner image than an HD monitor but will also require more processing power from your computer.
Printer resolution measures how closely the printer lays down droplets of ink on paper, measured in dots per inch (DPI). For example, a Canon PIXMA PRO 100 lists a resolution of 4800 x 2400 dpi (printers can often print more dots vertically than horizontally). The higher the printer resolution, the finer the detail and the smoother the transitions between tones of color. Anything at or above 1440 dpi is good.
Some printers allow you to select the appropriate dpi setting for your needs, for example 300 dpi for a draft image or 1200 dpi for a finished print.
Confused yet? Then how about this: Many home inkjet printers have a default printing resolution of between 240 and 300 pixels per inch. Many print labs will ask for image files with a resolution of 300 ppi. That assumes printing on paper. Printing on canvas, for example, can take somewhat lower resolution files. The lab will tell you what resolution they require for a specific print medium.
PPI, DPI, Oh my! Hang in there—this will eventually make sense!
Photoshop & Resolution
Cameras, monitors and printers all measure resolution differently, but Photoshop makes it all to work seamlessly. Here’s how.
When you first open an image in Photoshop, go to Image > Image Size. You will see the sensor’s pixel dimensions up near the top and then the length and width of your image, in pixels or inches (or other units of measurement depending on what you last used).
Let’s assume you do all your editing and do not crop the image. Your image resolution will be the same as before, though your file size might have increased. So far, so good.
Now, it’s time to print your image. Unless you resample when you change the image size, the amount of data will remain the same regardless of how big or small you print.
Let’s say you want to make a 4 x 6 inch print. If you have the Resample box unchecked, your resolution increases from 300 to 1004 ppi — the image dimensions are smaller but the pixel dimensions and the image size stayed the same. You have resized your image. This is a higher resolution than your printer can print, so it would operate at its native value of 240 or 300 ppi.
If you check the Resample box, the resolution stays the same (300) but the pixel dimensions decrease from 6016 x 4016 to 1798 x 1200 and the image size falls from 138.2 to 12.3 MB. Photoshop is taking away pixels. You have resized and “downsampled” your image. Notice that the image may appear smaller on your screen—more on that later.
Say you want to make a larger print, like 16 x 24. If the Resample box is unchecked, the resolution changes to 251 while pixel dimensions and image size stay the same. You have resized your image. It has the same number of pixels, but they are spread out a bit more.
If you have the Resample box checked, your resolution stays at 300 ppi, but the pixel dimensions have increased to 7190 x 4800 (the file size will increase as well). Photoshop is making up new pixels to enlarge the photo. You have resized and “upsampled” your image.
Both downsampling and upsampling can reduce sharpness and detail because you are removing or making up pixels. Photoshop’s ability to interpolate pixels up- or downsampling is so good that you shouldn’t notice any perceptible difference in image quality with modest changes in size.
We already mentioned that many printers have a default print resolution of 240 ppi. That’s also the default for Lightroom’s print module. Generally, sending an image file to your inkjet printer with a resolution between 240 and 300 ppi will give you a very nice print for most paper sizes. Jim Harmer has an article on megapixels and print size here.
For a print you can hold in your hand, fine details are important and this resolution will be adequate. As you get to larger prints, people will view them at more of a distance, so the resolution does not need to be as high. Resolution on a large billboard that you’d view from a hundred feet or more away, for example, can be 50 ppi or even lower!
Resolution and the web, or a pixel is a pixel is a pixel
A monitor displays your image pixel by pixel—that's all it cares about. Your pixel dimension is how the monitor will display the image. For use on the web, or for display in email, you can set any resolution in Photoshop — just doesn't matter. It has no effect on how the image is displayed. If you use Photoshop's legacy Save for Web feature, for instance, you don't even get a box for resolution.
Simply put, while I can order a printer to print at 240 or 300 ppi, there’s nothing I can do to make the pixels in your monitor move closer together or farther apart. So, setting resolution for the web is a waste of time.
But wait, my photo was 6016 x 4016 pixels. How will that display on a 1920 x 1280 monitor? Some years ago, it would have displayed at 6016 x 4016 pixels. It would have been larger than the screen and you would have had to scroll up and down and side to side to see it. Some of us older (er, “more experienced”) photographers remember doing that.
Today, there are code scripts and widgets on websites and browsers that will dynamically resize photos to fit the screen, eliminating all that scrolling. You can post one image that is automatically resized on the fly for each device and screen. Facebook, as an example, takes your photo and crunches it to fit your Facebook wall on a desktop or on your mobile device. Google resizes your image to fit the screen it is being displayed on.
One reason to bother with resizing your photo is to make it load faster. A website may be slow to render gigantic file sizes and, these days, no one has any patience to wait. You may want to reduce your image dimensions to 1920 x 1080. That’s big enough to display well on most monitors and small enough to render quickly.
The other reason you’d want to change the pixel dimensions of your image are when you want to place it in a template, like Word Press. In Improve Photography articles, for instance, we use images that are no wider than 900 pixels, so they fit the template. Facebook asks for profile pictures at 170 x 170 and cover photos at 851 x 315 to fit their page templates.
But in both of these cases, resolution doesn't enter into the equation. After all, a pixel is a pixel is a pixel.
Be it hereby resolved:
Set Resolution for Printing: 240 or 300 ppi works well for inkjet printers. Check with your lab to see what they require for printing.
Don’t set resolution for the web, but consider setting the dimensions of your photo to something like 1920 by 1080 (in landscape orientation). Also check for recommended image sizes for sites using a template, like Facebook, Instagram or WordPress blogs. Otherwise, it doesn’t matter!