Today’s Question: Is there is a difference in quality of image appearing in Flickr exporting from LR and uploading to Flickr using these two different methods:
1) Set file size specifically to 2.3MB
2) Set long dimension to 1920 pixels
I uploaded the same image using the two methods and Flickr shows the uploaded file size as:
1) Exported from Lightroom as a 2.3 megapixel file, then uploaded to Flickr. The image in the Flickr site has the original file size as 1856 x 1239 and 1.0MB.
2) Exported from Lightroom with longest dimension at 1920 pixels, then uploaded to Flickr. The Flickr site has the original file size as 1920 x 1281 and 1.06MB.
Tim’s Quick Answer: There is no difference in terms of the image quality with either approach, assuming that you use settings that result in the same pixel dimensions with the same JPEG Quality setting.
More Detail: The Image Sizing section of the Export dialog in Lightroom provides a variety of ways you can describe the pixel dimensions you’re looking for. All of them essentially vary only in how you describe the final size of the image, not what the final size will be.
For simplicity, let’s assume an example where we’re exporting a square image, and that we want to create a file at a specific size. For our purposes we’ll assume we are exporting the photo to be printed at 10-by-10 inches at 300 pixels per inch.
In this case you can specify, for example, that you want the exported image to be sized at 10-inches by 10-inches at 300 pixels per inch (using the Width & Height option). You could also specify that you want the image to be 3,000 pixels on the long side (using the Long Edge option). You could use the Dimensions option and specify 3,000 pixels for both width and height, with the resolution set to 300 pixels per inch. And you could use the Megapixels option and specify 9 megapixels at 300 pixels per inch.
All of the above options are simply a different way to instruct Lightroom that you want the image sized to 3,000 pixels by 3,000 pixels. The various options are mostly provided simply because different people think of output sizes in different ways. They also provide flexibility in terms of being able to specify all images will be sized to the same size on the long edge regardless of whether the image is a horizontal or a vertical, for example.
The only “catch” with specifying the output size in megapixels is that you aren’t able to specify the pixel dimensions with as much precision, so to speak. This is illustrated by the example provided in today’s question.
If you multiple the pixel dimensions together for the example created with a 2.3 megapixel setting (1856×1239) you find that there are 2,299,584 pixels, which equates to 2.3 megapixels. The image sized based on the Long Edge option has pixel dimensions of 1920×1281, which translates to 2,459,520 pixels, or almost 2.5 megapixels.
So, if you had used the same pixel dimensions for the Long Edge option (1856 pixels) you would have ended up with two images of the exact same pixel dimensions, at the exact same image quality, and with the exact same file size.
Again, the resizing is performed based on the actual pixel dimensions, and there are various ways to describe those dimensions when exporting a photo. But with equal settings, all of the various options will produce the same result in terms of image quality.