Having trouble finding the "resize" button in Lightroom? It's not you: there isn't one.
This isn't some kind of facepalm oversight on the part of Lightroom's developers--far from it. Instead, Lightroom has more powerful, flexible, and safer way of dealing with image resizing. But it means a different way of thinking about the whole process. Instead of resizing the original image, you create a new copy with a combination of cropping and exporting that ends up at the size you want.
The key is understanding the idea of non-destructive editing. Like other leading image management systems like Aperture, Capture One, PhotoMechanic, or Corel Aftershot Pro--or pretty much any photo software that knows what it's doing--the idea is that the original image data itself is never edited or altered.1 That eliminates the risk of corrupting or degrading the original file. If you ever need to get back to the digital equivalent of a film negative, you can. Think of it like this: if you were using slide film and you wanted a small image, you wouldn't go and attack your slide with scissors.
It also means that unless you try quite hard you shouldn't be able to accidentally overwrite your original image with a lower resolution or somehow degraded version--your master version should always be preserved. You don't want to accidentally replace with your original image with a resized thumbnail, for instance. Of course, there are other ways you can accidentally delete files, so always, always be sure to have a robust, redundant backup system humming away, a topic I'll be dealing with here sometime soon.
So if you can't actually resize in Lightroom, how do you get an image at the size you want it?
This is where Lightroom's powerful export functionality comes into play. But first, you have to know the difference between cropping and resizing. They're not the same thing, and you can do both to an image.
Cropping in Lightroom
Cropping is choosing which parts of an image you want in the finished image. It's one of the tools in Lightroom's Develop module. You might zoom in to the center and eliminate some from all the sides, cut off just one side, create a square image, or create a long, thing panorama. What you're doing with the crop tool is telling Lightroom that that's the part of the image you want to work with.
I have a post specifically on the topics of cropping and straightening photos in Lightroom as well as rotating images. And if you're looking for more control over your cropping, Lightroom includes a number of very useful crop tool overlays to help as guides.
Exporting from Lightroom
To get an image of a particular size, whether it's a certain number of pixels wide or a particular number of inches tall, you approach things a bit differently. Rather than trying to resize the original image, a much better and safer practice is to export a new version of the file that has the dimensions you want. Once you have that resized copy, you can do what you want with the copy without any risk to your original master image. You can even reimport it back into Lightroom and stack it with the original image if you like, or you can create a special folder or category that holds just your resized images (one for thumbnail sizes, another for email sizes, etc).
Below are practical examples of the Image Sizing options in Lightroom's export feature (select image/s in Library Module » File » Export » Image Sizing). The screenshot shows the setting that were used, and each image's caption details the dimensions of the end result.
What Resolution Setting?
The examples below all use pixels as the measuring system, in which case the Resolution setting (sometimes known as pixel density) doesn't really matter. 2400 x 3000 pixels with a Resolution setting of 72 ppi will result in functionally the same image as 2400 x 3000 pixels with a Resolution setting of 600 ppi. 72 ppi has long been the default standard pixel density for screen display; anything from 240 ppi through 600 ppi or higher us standard for printing. Apple's Retina displays use a higher pixel density--ranging from 220 to 326, depending on the device. But for digital display on a screen, the pixel density really isn't relevant--you should only worry about the number of pixels wide and the number of pixels high. If you get a request from a graphic designer to deliver files that is something like 3000 pixels by 2400 pixels at 300 dpi, the "300 dpi" part is redundant.
The same is not true if you use a physical measure like centimeters or inches. In that case, the Resolution setting matters. 8 x 10 inches at a resolution of 72 ppi isn't the same thing as 8 x 10 inches at 300 ppi. Physical measures are mostly used when you're trying to create prints. The pixel densities used for print are traditionally 240 to 300 ppi (although some printers use higher measurements).
If I'm resizing for making prints, I generally find Lightroom's built-in Print module a better option than using Lightroom's export function. If you need even more flexibility for print resizing, third party software like Qimage, PhotoZoom, or Genuine Fractals might be what you need.
Width & Height
The key difference between using Width & Height as opposed to Dimensions is that orientation comes into play. Using this setting, regardless of the image orientation, the image will not be wider than the width setting you specify and not higher than the height setting you specify. Another way of putting it is that you're specifying the horizontal measurement (width, across) and the vertical (height, up) measurements.
The Dimensions setting is similar to Width & Height but is agnostic of the image orientation. Rather than specifying horizontal and vertical measurements, you're specifying both the long and short edges. Input the maximum image dimensions you'd like to fit within and it will automatically scale, ignoring whether it's actually the horizontal or vertical sides. If you want both portrait and landscape oriented images to all come out the same size, this is the most useful setting to use. Also see below, Dimensions: Square.
Use this setting if you're only concerned about the longest side, ignoring whether it's the horizontal or vertical measurement. The shortest side will not be restricted. This setting can be very useful for resizing panoramas.
Use this setting if you're only concerned about the shortest side, ignoring whether it's the horizontal or vertical measurement. The longest side will not be restricted.
A newish addition to Lightroom's export sizing options, the output to Megapixels has relatively specialized applications, including submitting to some stock agencies that set their royalty free prices by megapixel image size.
This isn't actually a separate Lightroom setting but rather a simple way of using the Dimensions option and specifying identical measurements for the long and short edges. This can be especially useful when creating thumbnails or resizing images for display for an onscreen slideshow or image gallery where the window for the actual image display is square.
Some Sample Image Sizes
There's obviously nearly an infinite number of image size settings that are possible that you can customize according to your own needs and preferences. But here are suggestions for places to start for common scenarios. All are again measured in pixels because I find that it's generally a safer and more flexible way of doing it than constantly cross-calculating Resolution if using physical measures like inches and centimeters.
Sending an Image by Email / Uploading to Facebook
Setting: Width & Height | Width: 1000 | Height: 800 | Resolution: 72 ppi
This is a good compromise between having a large enough image to appreciate the details, will keep the filesize manageable for email even with several attachments, and will display the full image on most modern monitors. The reason I use the Width & Height rather than Dimensions is that monitors aren't square and I know that I have more room to play with horizontally than vertically.
If you want more specific information on image and graphics dimensions for Facebook, check out this post.
Setting: Dimensions | Measurements: 100 x 100 | Resolution: 72 ppi
Using the Dimensions setting and specifying identical dimensions works well for thumbnail grids. Note that this doesn't create square thumbnails (unless your original images are square, of course) because it's set to fit the entire image without cropping inside the measurements you've specified. Creating square thumbnails isn't possible with the standard Lightroom Export module--you have to use something else like the Print module, the Web module, or an external editor.
Printing an 8 x 10 inch print
Setting: Dimensions | Measurements: 2400 x 3000 | Resolution: 300 ppi
I generally prefer to use the more powerful Print module or external options for resizing for print because you have a lot more control over the output. But if I need to use the Export feature, these are the settings I'd start with. Some printers work best at a much higher print resolution of 600 ppi, 1200 ppi, or even higher, in which case you're probably better off setting the Resolution at the desired output and using inches or centimeters as your measure rather than pixels. If you use the Print module, that's all taken care of in a better way.
More Advanced Alternatives for Lightroom
LR/Mogrify 2: If you're looking for more advanced controls to add to the Lightroom Export module, take a look at the excellent collection of tools offered in the LR/Mogrify 2 plugin by Timothy Armes. It allows much more control over your output, including a wide choice of resizing algorithms, and it can be used in combination with the plugin's other powerful features like fine-grained watermarking control, adding text overlays, and adding borders and keylines.
Lightroom's Print Module: Don't be put off by the name--Lightroom's Print Module can also be used to create jpegs and allows a lot of control over resizing options.
Other Software Options
Resizing for Print
Qimage: (Windows only) This does a lot more than image resizing. Qimage has long been leading the field on Windows systems in preparing images for print, whether you use your own printer or send to a photo lab. It also provides fine-grained control over all aspects of preparing images for print.
Going (Really) Big
Image upsizing / uprezzing is one of the many endlessly debated topics in digital imaging, and there are many "right" answers because so much of it comes down to the look you're going for or how much detail you'll be able to see in the end product. If you're intending to frame a photo for the wall, for instance, will the viewer standing 6 feet away from it really be able to tell the difference between an image upsized with expensive extra software and an image upsized with standard Lightroom functions? Only you can decide what the "right" answer is. After all, it's your work on display.
If you decide that you want more control than Lightroom's built-in functionality offers, then there are many alternatives. Photoshop offers a range of resizing options, and you can also build a Photoshop action to do step resizing which involves incremental stages of only 10 percent for each increment.
If you plan on making very large prints on a regular basis, there are external editors that provide specialized functionality that might be worth investigating. You can use the "external editor" functionality to add a shortcut in Lightroom to these, but they don't technically integrate directly into Lightroom and each involves first creating a TIF version and working on that.
Perfect Resize: (it used to be known as GenuineFractals) (Windows & Mac) By OnOne Software, Genuine Fractals specializes in image upsizing and enlargement.
PhotoZoom: (Windows & Mac) By BenVista Software, PhotoZoom specializes in image upsizing and enlargement.
Neither of these is inexpensive, so they might be overkill for occasional use. Both of these accomplish much the same thing but use different algorithms to get there. The results from both are excellent, but they do end up with a slightly different look if you look very closely. Before buying either of them, I'd recommend downloading the trial versions and seeing which you prefer.
Resizing to a Target File Size
This isn't something that can be done in Lightroom's standard Export module, but I've created a detailed guide on how it can be done on Mac systems here using Automator and Ben Long's excellent Photoshop Actions.
Need Even More Options
If you need even more control over your export sizing than the baked in options offer, check out Rob Cole's Exportant Lightroom Plugin that allows all sorts of other options like exporting to megapixels smaller than Lightroom's 1.0 megapixel limit, exporting to PNG format, or resizing by percentage. It also does a lot more than just resizing. UDPATE: This plugin is not currently available from the developer.
- There are some limited exceptions if you have the preferences set to embed metadata in the image file directly. ↩