Today’s Question: When I export from Lightroom trying to use a size that will look good on an iPad I understand that 2048 pixels on the long dimension is good to use. I am still unsure what dimension I should put in the resolution box. Say I want to use every available pixel in the image so it will look really good. It seems that there should be a difference between 50 and 500? I usually use 200, but it is only a guess.
Tim’s Answer: This is obviously a bit of a follow-up to Wednesday’s question. I thought that since resolution tends to be a subject that many photographers struggle with, it made sense to amplify the issues raised in yesterday’s email by addressing this question today.
In short, the pixel per inch (ppi) resolution for an image does not matter unless that image is being printed. What that means is that for an image you intend to share electronically, it doesn’t matter what you set the ppi resolution to. All that matters is how many pixels are in the image.
So, for example, if you are preparing an image to be shared on an iPad, it makes sense to size the image to match the pixel dimensions of the display. In the case of the latest iPad tablets, the resolution is 2048×1536 pixels. So you can size your images to 2048 pixels on the long edge to get great results, and you can set the ppi resolution to anything you like. Regardless of the ppi resolution you set, the image will still be 2048 pixels on the long side, and so will be sized to match the number of pixels (at least on the long edge) for the display.
In other words, for a digital display you’ll get the best results when there is one pixel in the image for each pixel on the display. That way, you have an image that matches the “size” of the display in terms of the amount of information, and you can expect the best display quality for the image.
I should add that some software does actually look at the ppi resolution value when you add an image to a document, even if the aim is not printing. As far as I’m concerned that shouldn’t be the case, but in some cases software will adjust the apparent size of the image based on the ppi resolution value. But that doesn’t change the number of pixels in the image, and thus has no impact on the quality of the image if it is sized based on the actual pixel dimensions.
So, for printing you should absolutely set the ppi resolution based on the type of printed output you’re producing. But for sharing images electronically, you don’t have to worry about the ppi value at all. In those cases, I recommend setting the ppi resolution value to your lucky number, just for fun.