A good resolution for 2018 would be to understand resolution.

I encourage people to deal with images by their pixel dimensions rather than arbitrary definitions. For example, you may be asked for a high res file, but not given a size. You need a size and intended use for the image to make a decision.

1200×1800 pixels = 4×6 inches @ 300 ppi

1200×1800 pixels = 16.667×25 inches at 72 ppi


Note that while the size in inches based on the resolution (300 ppi or 72 ppi) will change, the image size in pixels is exactly the same. Resolution is the means by which we determine the output or display size of the image.  Your monitor might be 96 ppi which would render the 1200×1800 pixel image 12.5×18.75 inches on screen. So, the image in pixels does not change but the size of the image changes based on where it will be displayed or printed. The number of pixels in the image used to define an inch is the determining factor, which is why it is called pixels per inch.

The problem in communication is that people would consider the 300 ppi image high res and the 72 ppi image as low res when they are in fact the same size. Given a 72 ppi image you can change the image size without resampling to 300 ppi to determine how large it will reproduce based on your intended output. In other words, the resolution is not fixed, but the dimensions are as long as you do not resample the image.

Play with the Image Size Dialog to get the concept into your head. Note that if the Resample box is not checked the linked bars mean that a change in one of the elements will produce a change in the other two. The key to making sure that you are not modifying your image is that the Image Size at the top and the Dimensions in pixels do not change.

The original image size of 1800 px x 1200 px will remain the same regardless of the resolution. Only the width and height in inches will change based on the desired resolution.

If the Resample box is checked the link will indicate that the width and height are locked to a relationship (aspect ratio) but a change to a different resolution will modify the image lowering the Image Size, and the Dimensions. By Resampling to a lower resolution, you will change the image size and you will have thrown away pixels. Now you actually do have a low res file.

To avoid damaging a file I suggest that the Resample box not be checked unless you know for certain that you need to change the file size.

One of the mistakes made is to assume that you must resize an image when sending the file to a printing service. I ask people who are sending files to me for printing that they do not resize the image in any way, but simply specify the desired longer dimension desired for the print. This is because resizing the file changes the actual pixels. If sizing up pixels are being “invented” and if sizing down pixels are being destroyed. In either case the integrity of the image is being destroyed and the quality will likely suffer, even if only a little.

How then, do I change the size for printing? I use Lightroom for printing. Lightroom was designed for printing by some very smart people. The algorithms used by Lightroom are different than those used by Photoshop. The Lightroom Print module allows setting the final print dimension and handles the resizing of the image and does it very well. In this respect, the image quality handled by Lightroom is as good as possible.

So, it is essential that you understand resolution of your images, but also understand how and when (and why) to modify a file. I typically do not modify my files except to crop for composition, eliminating unwanted pixels, and keeping the remaining pixels intact. That guarantees me the best image quality. And, if a high res file is requested I simply set the resolution to 300 ppi without resampling.  If a client needs a specific size and resolution for their image that is a different story. The file will be modified.

Resolved. Happy New Year!

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