I recently answered a question for someone on Twitter about using the Crop Tool in Lightroom so I thought I’d embellish that a little bit and post it here as well…
There are two related issues here, so let’s forget about computers for a moment and deal with paper. If you have a sheet of paper that 1″x1″, that’s a specific size. A sheet of paper that’s 4″x4″ is also a specific size, but they both have the same aspect ratio (1:1). Similarly, a sheet that’s 4″x5″ and a sheet that’s 8″x10″ have the same aspect ratio but obviously one is four times the size of the other. Now, if you have a print that’s 8″x12″ and you cut two inches off the long edge to create an 8×10 you’ve both created a specific size and cropped it to a specific aspect ratio.
With digital images, however, things aren’t quite so simple. First of all, as I’ve mentioned before, we need to consider that a digital image is not really an ‘image’ as we think of it. A digital file, every digital file is a string of 1s and 0s put together to represent information. For digital images, part of that information is a grid of pixels, and each pixel has colour information attached to it. You can read more about this in our other photography tutorials or elsewhere online. Now, every digital image has a specific ‘size’ in terms of pixels (say 3000 x 2000 pixels for example), but if you want to translate that binary information into an image that can be viewed on a screen or you want to create a print you need to consider a third factor: pixel density or resolution. Images viewed on a monitor for example don’t need to be much more than 72 pixels/inch (ppi); although some monitors can display higher resolutions, anything much over 100ppi is wasted resolution and therefore wasted disk space. My laptop screen is 13.5″ x 7.6″, which works out to a 16:9 aspect ratio. The screen resolution is 1366×768 pixels, so 1366/13.5″ works out to 101ppi. If you want to make an inkjet print that’s 16″x20″ at a resolution of 300 dots/inch (dpi), then you need an image that has a pixel dimension of 4800×6000 pixels. There’s also bit-depth to consider (8-bit vs. 16-bit or 32-bit, but you can read more about that in our Photography and Colour Management post).
(UPDATE: This post was written in 2012, and since then we’ve developed much higher resolution screens, 4K monitors, etc. A retina display is an Apple term that relates to a display that has a resolution of about 300ppi. My current laptop has a 4K screen, with 3840×2160 pixels on a screen that is 13.56″x7.63″. That works out to a screen resolution of 283 ppi. The important thing to note with digital images is that resolution isn’t really as important as number of pixels. On an HD monitor (1920×1080 pixels), a 1920×1080 pixel image will display full screen whether it’s a smart phone screen or a larger monitor. That same image will only cover 1/4 of my 4K screen. Pixel density becomes important in terms of the quality of the image displayed at a certain distance; with higher resolution it becomes difficult to impossible to separate individual pixels at normal viewing distances. When we get to physical prints, resolution becomes much more important because we’re transferring the number of pixels into a specific physical image size.)
Now in a pixel-editing program like Photoshop, when you edit an image you’re actually making changes to the pixels themselves. To get around this the folks at Adobe introduced layers to make the editing somewhat non-destructive, but when you save the file you’re overwriting the existing information (unless you use ‘Save As’, of course). Lightroom, however, uses a non-destructive workflow. No matter what changes you make to a file in Lightroom, the original image file is not altered. Lightroom is essentially a database management program, and what it does is keep a list of the changes you make in the Develop panel for example as a History file. Therefore, when you crop in Lightroom it’s not throwing away any of the pixels, it’s just writing a note in the History file that says, ‘Change to X:Y aspect ratio’. The original file is untouched.
When you export an image from Lightroom or when you create a print it takes that History information and applies the changes to the output. It still doesn’t alter the original file.
There’s one other factor that comes into play here and that’s what’s known as ‘up-rezzing’ or ‘down-rezzing’. If you have an image that has 4000×5000 pixels and you want to create an output file that is 8000×10000 pixels or a print of 8″x10″ at 720dpi then the original file doesn’t have enough information to cover that. What the software does in up-rezzing the image is ‘invent’ pixels based on the surrounding information to fill the gaps. This artificially increases the dimensions of the original image. How far you can ‘stretch’ an image in this way depends on a number of factors and there are several different algorithms/software packages for making an image larger or smaller.
So, in Lightroom, when you crop an image in the Develop panel you’re simply setting the aspect ratio. When you export an image or when you create a print you tell Lightroom the output dimensions and the resolution. Now, the OP mentioned that s/he wanted to create a banner for the ‘net of 359×733 pixels. My answer was that to do so in Lightroom would take a couple of steps, depending on the aspect ratio of the images the camera captures and the output. Using this example, 359×733 pixels is an aspect ratio of 1:2.04178. It’s unlikely that any camera shoots in that aspect ratio (more likely 3:2 or 4:3) so in the Develop module it will be necessary to define a custom crop ratio. We’ll use an image I made recently at Victoria’s Butterfly Garden as an example. I’m using Lightroom 4, but it works the same in Lightroom 3.
1) Open the image in Develop and either click on the Crop icon in the top of the right panel or press ‘R’ to enter the crop module.
2) Between ‘Aspect’ and the little lock icon it says ‘Original’, with a drop arrow beside it. Click on that and while Lightroom provides several standard crop aspects, below that it says it says ‘Enter Custom’. Under Aspect Ratio type 359×733 and click Okay. Lightroom will create a crop overlay on your image, and since it’s a banner it will max out to the long side of the image. Now, if you put your mouse inside that crop window anywhere and click and hold the mouse button while dragging the mouse you can move the image up and down within the crop window to select the exact image crop you want. Various overlays are available to help with composition. You can turn the Tool overlay on/off at the bottom left of the screen and pressing ‘O’ (oh, not zero) will toggle through the options.
If you click and drag on one of the bounding boxes (the corners or middle-top/bottom/side buttons) of the crop box you can make the box smaller or larger. If you press the ‘X’ key you can rotate the box from horizontal to vertical or vice versa.
If you place the mouse outside of the image area in the gray surround and click and drag you can rotate the image. You can also rotate by using the angle slider (or typing in an angle) or by using the Straighten tool – if you hold down the Ctrl/Cmd key, click a point in the image and (still holding down the Ctrl key) click on a second point in the image LR will rotate the image to make the line between those two points either horizontal or vertical. It’s handy for horizons in landscape shots for example. Remember Ctrl/Cmd-Z will back you up one step just about anywhere in LR
As long as you don’t click on the little lock icon to unlock it, moving the image within the crop box, rotating it or reducing/enlarging the bounding box won’t change the aspect ratio of the crop. When you’re done, click the Crop icon again or click Done at the bottom. You now have a cropped view of your image.
If you want to apply the same crop to more than one image you can select them all in the Library module, apply the crop to the ‘most selected’ image in the Develop module, and then press the ‘Sync’ button in the bottom right panel of the Develop module. Uncheck all, then just select the Crop box and click Synchronize.
3) Now to create the output image(s). With the file(s) selected in either the Library or Develop module, go to File/Export. You’ll have to tell LR where you want to put the new image(s) (folder on the hard drive, upload site or whatever), you can rename them if you want, and since (for this example) you’re uploading them to the web, under Image settings you’ll want to set JPG, sRGB, and choose a quality setting: 75 will probably be sufficient, maximizing image quality without creating overly large files, but you can experiment.
Under that, in Image Resizing, select Resize to Fit, Width and Height, 359×733 pixels. Resolution mostly comes into play when your dimensions are in a fixed size, like inches or mm. As I mentioned, for the ‘net anything over 100ppi is a waste of file space; most people use 72 ppi. The other options for sharpening, copyright, etc. I leave to you. If you’re going to be using these settings frequently you can create an Export preset by clicking ‘Add’ on the left side of the export window. Make sure the name you give the preset makes sense to you – call it Banner or 359×733 or whatever you want.
Click Export and LR will create a new file(s) in the location you’ve specified, applying all of the Develop and other settings you’ve stipulated, at the crop ratio and pixel dimensions you’ve specified.
NB: you may wish to crop an image but not reduce the image size, i.e. you may have an image that’s 4000 pixels x 5000 pixels, and want to create an image that’s 2500 pixels x 5000 pixels instead (1:2 aspect ratio). This preserves the full dimensions of the image but changes the aspect ratio. Creating the crop is the same as above, but when you go to File/Export under Image Sizing you can enter 5000×5000 pixels as the width and height. Doing so will not change the aspect ratio as these are the ‘maximum’ dimensions Lightroom will use for the export. You could even enter 10,000×10,000 pixels, and as long as you click the ‘Don’t Enlarge’ box, Lightroom will create an output file that’s as large as possible without uprezzing or ‘inventing’ pixels.
Now go out and make some photographs!