Today’s Question: I read on a blog post that they thought 150 ppi is enough to print. Wouldn’t a resolution of at least 300+ produce a better image? Or is the print quality and ppi based solely on the size of the final output of the image – can a smaller print be printed at a lower ppi without sacrificing print quality?
Tim’s Quick Answer: If we use the pixel per inch (ppi) resolution as a form of “shorthand” for describing how much information is in a photo compared to the final print size, then we can use that ppi figure to talk about potential output size. To that end, I would consider 150 ppi to be too low a resolution value for most photographic prints. I consider values of around 200 to 250 ppi to be a good threshold for ideal output, and higher values are generally better. However, in most cases there is no need to go beyond about 360 to 400 ppi for output resolution.
More Detail: Part of the reason I think the general topic of resolution causes so much confusion for photographers is that we use so many different ways of explaining the same basic concepts. In addition, we often mean two different things when we use the term “resolution”.
Resolution can refer to both the total quantity of information as well as the density of information. When printing a photo, both of these concepts intersect.
To produce a print of a given size at optimal quality, you need a specific amount of information. If you’ll excuse analogy, imagine a photo inkjet print of being comprised of individual pixels on the printed page, something along the lines of tiny grains of colored sand you might otherwise glue to the paper to produce an image. It should be noted that photo inkjet printers use ink droplets of variable sizes, so this analogy doesn’t quite fit the reality of the situation. But hopefully it is a helpful analogy in any event.
Based on this analogy, we could say that a photographic print of a given size requires a specific number of grains of colored sand. While there are some added elements of complexity involved with a photo inkjet print, you can think of a print at a given size as requiring a specific amount of information. For a typical photo inkjet printer, that might require 360 pixels per inch. In other words, for a print at 8-inches by 10-inches, you would need an image that is 2,880 pixels by 3,600 pixels.
If you have less information than this, the printer (or the software being used to send the photo to the printer) will need to add information to the image. There is complex software at work behind the scenes to accomplish this task, but the bottom line is that if a photo doesn’t contain enough information to print at a given size, that information needs to be added in some way. Similarly, if a photo contains more information than is needed to produce a print at a given size, the “extra” information will be discarded.
Once you understand that a print at a given size requires (in a general way, at least) a given amount of information, you can choose to describe that information in a variety of ways. One way is to describe the amount of information in the image based on a pixel per inch (ppi) value.
For example, if your image was only 1,440 pixels by 1,800 pixels, you have much less information than you really need to produce the 8×10 print referenced above. If you do the math here, you’ll discover that instead of an image that represents 360 pixels per inch for the intended output size, you only have 180 pixels per inch.
The extent to which you can “get away with” having less information in your photo than is required to print depends on a variety of factors. That includes the degree to which your image is coming up short in terms of the amount of information (the number of pixels), the type of paper you’re printing to, the distance from which the final print will be viewed, the quality of the image you’re starting from, and other factors.
The bottom line is that your results may vary based on a variety of factors. However, in general I aim for having enough information in my image that I will at most need to enlarge by double the width and height (four times the total surface area). You can often produce even larger output, but the more you push the limits, the more likely you’ll be dissatisfied with the quality of the final print.