Today’s Question: The same image exported twice from Lightroom with same pixel dimensions but one at 72 ppi and one at 300 ppi are the same file size. I would have thought that the 300 ppi file would have been larger. Why isn’t it?
Tim’s Answer: The issue of resolution continues to be one of the more common sources of confusion in photography, in part because information about an image is often presented in a way that can be a little confusing.
When it comes to the pixel per inch (ppi) resolution for an image, I think it is best to think about this as simply being a metadata value. It has absolutely no bearing whatsoever on the “real” information in your photographic image.
The overall size of an image is determined by how many pixels are in that image. Adding confusion, that total volume of pixel information is often referred to as resolution as well. In other words, sometimes the term “resolution” refers to the total volume of information (pixel dimensions, megapixels, etc.), and sometimes it refers to the density of information (how many pixels per inch, for example).
Forget about printing for the moment, and think about the image size as it appears at a 100% scale in Photoshop (for example) as well as the size of the file saved on hour hard drive. The number of pixels is the primary factor here.
This makes sense when you consider how the appearance of an image in Photoshop at a 100% zoom setting changes based on how many pixels are in the image. If we have a square image that is 10 pixels on each side, that image will look very small in Photoshop even at a 100% zoom setting. A square image that is 10,000 pixels on each side will look very different, with the image being so big that we can only see a small portion of the image when viewed at a 100% zoom setting.
If the pixel dimensions remain the same, with only the pixel-per-inch resolution changing, the file size will not change. The ppi resolution only affects (for the most part) how the image is printed. In other words, when you send that 10,000 pixel-per-side image to the printer, how do you want the pixels spread out on the page? If you spread them out really far (perhaps only 72 pixels per inch) you’ll be able to make a very big print, but the quality won’t be very good. If you keep the pixels pretty close together (perhaps 360 ppi) you’ll have a smaller print, but that print will have great image quality.
In both examples above, the number of pixels didn’t change, so the file size would be the same (all other things being equal). All that changed is a simple metadata value that provided information on how the pixels should be distributed on the page when printed.
There are, of course, other factors that impact file size. These include (among other things) the bit depth of the image, compression applied to the image, and whether layers are included with the image. However, all other things being equal, changing the ppi resolution for an image will have absolutely no impact on file size. The number of pixels in an image is the key factor in overall file size, as well as for the potential output size for the photo.
It is worth noting, by the way, that if you had specified the output size in inches (for example) instead of pixels, this would have made a difference. For example, to create a square image that is one inch on each side at 300 pixels per inch, the resulting pixel dimensions will be 300×300 pixels. At ten inches on a side at 300 pixels per inch, the resulting pixel dimensions would be 3,000×3,000. And the ten inch image at 72 pixels per inch would be 720×720 pixels. So describing image dimensions in inches at a given pixel per inch resolution may result in different pixel dimensions. But if pixel dimensions are fixed, the file size is fixed (all other things being equal, of course).