One of the topics that confuses a lot of photographers is that of image size and resolution. Photographers aren’t really sure of what dimensions to give their photographs when exporting them out of Lightroom. There’s no ‘one size, fits all’ answer to that. The image dimensions you keep will depend largely on what you intend to do with your photographs after export. In this post, I try and explain the concept of image size and resolution. As we understand the terms that surround the concept of image size, you will understand the bigger picture (no pun intended).
The word pixel is comes from ‘picture element’. Every photograph in its digital form is made up of many tiny pixels. A pixel can only be a single color. When a lot of these tiny pixels come together, the image is made.
If you want to see the individual pixels in a photograph, then you can zoom in to it many times over using any photo-editing software. In the image below, one portion has been magnified by quiet a lot to show you the pixels. The little square grid-like structure you see, each square is an individual pixel.
Checking the pixel dimension in Lightroom
The photographs width and height are measured in pixel dimensions. In Lightroom, you can check the pixel dimension on any photo, by going into the Metadata panel in the Library module. Or in the Library module, you can go to the menu bar View > Loupe Info > Info 1 or use it’s shortcut, which is the I key. This will display the pixel dimensions along with some other information on the photograph.
Camera manufacturers advertise this term quiet a lot, even though it shouldn’t be the first consideration when buying a camera. In terms of image size, a megapixel is one million pixels. If you have a 16 megapixel camera, the photo produced by that camera will have 16 million pixels. But, this doesn’t really tell me anything about the pixel dimensions, i.e. the width or the height of my photo. The way that this 16 million number comes about is when you multiply the width (in pixels) with the height (in pixels) of the photograph. So, the camera that I have right now, Nikon D610, is a 24.3 megapixel camera. The full size raw image that it creates is 6016px by 4016px. If you multiply these two numbers, you get 24160256 pixels i.e. around 24.3 megapixels.
Resolution comes into play when you’re printing a photograph. On a screen, resolution is pretty irrelevant as things are measured in pixel dimensions. Resolution is measured in pixels per inch or ppi. It’s a measure of how many pixels there will be in an inch of the printed photograph. The more pixels there are, the better quality it would print. If there aren’t enough pixels per inch, the photo will appear blurry and pixelated (you will be able to see the squares).
Most printers require a resolution of somewhere between 150ppi to 300ppi to print the photo in good quality. 300 being the more standard option. There may be some uses such as a high end magazine, where an even higher resolution would be required.
How does this work exactly? Let’s say I have a photo that I would like to print at 5 inches by 7 inches at 300ppi. That would mean that I would need 5 x 300 = 1500 pixels on the shorter edge and 7 x 300 = 2100 pixels on the longer edge. So, the dimensions of my image need to be at least 2100 x 1500 for it to print in good quality. If the dimensions are lower than that, it will result in a pixelated print.
To print a 12 inches by 18 inches, we can do a similar calculation. 12 x 300 = 3600 pixels and 18 x 300 = 5400. Therefore, for a 12 x 18 print, we would need a pixel dimension of at least 3600 x 5400 for it to print in good quality.
Common Printing Sizes
Whenever printing, check with your printing service what resolution their printer outputs best at. If you’re printing it yourself, you can check this information in your printer’s user manual. I have created a table below, taking 300ppi as the resolution, For the most commonly used print sizes and what pixel dimensions you would need for each one.
|Print Size (Inches)||Image Size (Pixels)|
|3 x 5||900 x 1500|
|4 x 6||1200 x 1800|
|5 x 7||1500 x 2100|
|6 x 8||1800 x 2400|
|8 x 10||2400 x 3000|
|8 x 12||2400 x 3600|
|8.5 x 11||2550 x 3300|
|9 x 16||2700 x 4800|
|11 x 15||3300 x 4200|
|12 x 16||3600 x 4800|
|12 x 18||3600 x 5400|
If the size you want to print on is not on the chart above, or if your printer’s resolution is different, then just do the math in the same way as mentioned above the chart, to get your pixel dimensions.
If you’re printing on one of the larger sizes, you would need to ensure that you have that many pixels in your photo to begin with. If you don’t, then it’s going to have to digitally create pixels that weren’t even there to begin with. This is called Resampling.
When you downsample, i.e. decrease the number of pixels from an image, that’s perfectly fine because you have some excess pixels that you get rid of. In the new smaller size, you have all the pixels you need. But, when you upsample, i.e. add new pixels to the photo that weren’t even captured by the camera, you’re adding pixels that weren’t there to begin with. They were digitally created, usually by replicating nearby pixels. That results in quality loss. When you resize an image to a larger pixel dimension than what was captured, the photo will lose some sharpness and detail.
If this is getting a little difficult to understand, so let me explain it with an example. My camera, the Nikon D610, the largest raw file that it captures is 6016 x 4016 pixels. At 300 ppi, the print that you would be able to take out from that will be 6016 ÷ 300 = 20.05 inches and 4016 ÷ 300 = 13.38 inches. The maximum size that I can print photos from my camera at 300 ppi, without losing quality, will be 20.05 x 13.38 inches.
If I want to print bigger than that, let’s say at a size of 20 x 30, that would mean that at 300ppi, I would need a pixel dimension of 6000 x 9000 pixels. My camera did not capture that many pixels. It would have to upsample i.e. add new pixels to the photo that weren’t even captured. These pixels are digitally created by replicating nearby pixels and hence don’t have the same level of sharpness and detail.
Therefore, whenever resizing your images, be aware of how large of an image your camera is capable of capturing and what’s the maximum size you can print it at. If you don’t pay attention to this, you may end up with prints that aren’t as sharp as you would like them.
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Resizing or Cropping Your Image to Your Size
Resizing Photos for Print
Some of these commonly used print sizes are in the same proportion as the photograph taken by your camera, i.e. 3:2. These sizes include 2 x 3 / 4 x 6 / 8 x 12 / 12 x 18. For these sizes, you don’t need to crop anything. All you would need to do is to resize them to their corresponding pixel dimensions and you’ll be set to print. In Lightroom, you can do that when exporting your photographs outside of Lightroom. I go into the detail of the Export dialog in my post called The Ultimate Guide to Export in Lightroom.
In the Export dialog, under Image Size, switch on the checkbox for Resize to Fit. From the dropdown, I prefer to go with the Long Edge option. In the text field then, I would add the longer pixel dimension of my print size. For example, if I am printing 4 x 6, the corresponding pixel dimensions are 1200 x 1800. In that, the longer side is the 1800 pixel side. So, I would add 1800 pixels in the text field. Choosing Long Edge takes care of differences of width and height in vertical and horizontal shots.
If you haven’t cropped any of your photographs, or cropped them in the same proportion, you can also select Dimensions from the dropdown and then type out both pixel dimensions in the text fields. In the Resolution text field, you can type in 300. Make sure that pixels per inch is selected from the dropdown next to it. Make sure to also check the Don’t Enlarge checkbox. If the photo is enlarged, it means that it’ll have to create pixels that weren’t even there to begin with, which results in quality loss, as we saw above.
Cropping Photos for Print
All sizes other than the ones mentioned under the previous heading, are not in the same 3:2 ratio. As a result, they would need to be cropped before they can fit the size you’re trying to print them on. I am not entirely sure how the print sizes came about, but my guess is that it’s these sizes because of the paper size that’s available. Below, I’ve created a cheat sheet that will assist you in understanding how much crop is needed for the different print sizes. You can save this cheat sheet for future reference, by right-clicking on the image and “Save image as…”
Lightroom provides assistance in cropping them to the right size. When you choose the Crop tool in the Develop module, it shows you some options for it right underneath the tool. There’s a dropdown menu called Aspect. In that dropdown menu, you’ll see some commonly used print sizes (or ratios) to choose from. For example, if I was printing a 5 x 7, I would choose the 5 x 7 option from in there. That will automatically lock my crop rectangle to a 5 x 7 ratio. Any crop that I make will be in that proportion. If the size you want to crop it at is not in the list, you can choose the Custom or Enter Custom… option to add your proportion.
Other than that, you can also change the crop overlay to assist you in cropping your photo to one of the most common sizes. I talk about this in my video called 6 Secrets About Lightroom You Didn’t Know. What you’d need to do is that with the Crop tool selected in the Develop module, press the O key. That’s a shortcut to toggle between the different crop overlays. One of these overlays is the Aspect Ratios. You can also do this from the menu bar, Tools > Crop Guide Overlay > Aspect Ratios. This will show guides with your crop tool, which have the common print sizes marked on them. You can use these guides to crop your photograph to the proportions you need.
Resizing Photos for Web / Social Media
Resizing photos for web or social media is a simpler process, since there’s no conversion between pixels and inches. You wouldn’t really need to crop your photos, unless maybe you’re uploading to Instagram where you have to upload in a 1×1 ratio. You can do that in the same way we saw above, or you can even do that in the social media app itself. When uploading online, you just have to resize your photos and mostly, making them smaller.
In Lightroom, you can do that using the Export dialog in a very similar way to how we did it for print. When you’re in the Export dialog, under Image Size, switch on the checkbox for Resize to Fit. From the dropdown, choose the Long Edge option. In the text field then, I would add my pixel dimension. For online use, you don’t need very large pixel dimensions.
In fact, if you do add bigger images, it involves a couple of risks. It increases load time for the web page you’re going to use it on which results in a bad user experience and your photos will also be at a risk of theft. If the high resolution files are available online, anyone may download them and use them without compensating you for it. You’re better off keeping low pixel dimensions whenever sharing photos online.
When uploading to Facebook, I usually keep my longer edge to 1024px. The dimensions would be different if you’re uploading them on your website and you’ll have to consult your web designer for what they should be.
When resizing images for online use though, the Resolution becomes irrelevant. This setting doesn’t mean anything when exporting photos that you’ll display on the screen. The value in this text field can be anything you like. This field will accept any number between 1 and 65000, so you can add anything over here. It doesn’t make a difference.
Image size and Resolution can be confusing topic if you don’t take the time to understand it. I know that it took me quiet a while to grasp when I was starting out. Does it confuse you too? If you still have any questions, let me know in the comments section below.