Today’s Question: I know this is a bit of an old subject, and perhaps the question is no longer really valid or important. But I wonder how many megapixels do we really need? Can you provide some perspective on this?
Tim’s Quick Answer: This is a perfectly reasonable question, and one that I think tends to get the “wrong” kind of attention. In short, while most photographers probably only need a camera with a resolution on the order of no more than 20 megapixels, there are certainly photographers who need more.
More Detail: Ultimately, how many megapixels you need is determined by how large you need to print a photo. Complicating matters slightly is the need to have adequate resolution to crop a photo and still print at a reasonable size, but that is in my mind a secondary concern.
First, let’s consider some extreme scenarios. The first thing a photographer needs to consider when determining their digital camera resolution requirement is how large they want to be able to print their photos. For now, let’s assume the simple resolution numbers required for actually producing a print.
If you need to be able to print up to 40-inches by 60-inches, assuming a photo inkjet print with a 360 pixel per inch (ppi) resolution, you need over 310 megapixels to be represented in the final image. That translates to 14,400 pixels on the “short” side, and 21,600 pixels on the “long” side. For a 20×30-inch print you would need just over 78 megapixels (7,200 pixels by 10,800 pixels).
Of course, this isn’t entirely fair, since you can enlarge (via interpolation) an image with excellent results. As a very general guideline, I recommend limiting your enlargements to no more than about double the width and height of the original capture. So, for a 20×30-inch print you could actually start with a file that represents 10×15-inches of information. Based on the specifications provided above, that translates to a little over 19 megapixels (3,600 pixels by 5,400 pixels).
I think the reason many feel that the megapixel subject is no longer worth discussing is that we’ve reached a point where most photographers will be adequately served by the resolution offered by most digital SLR cameras today. A somewhat “typical” digital SLR with an 18-megapixel sensor enables the production of prints up to about 20×30 inches without much difficulty.
Of course, some photographers need to print much larger than 20×30-inches, and for them a higher resolution camera makes perfect sense. But if you consider 20×30-inches to be a very large print, then around 18 megapixels will be perfectly fine. Beyond that, it is worth considering your needs in terms of output size (and cropping) to determine how much resolution you really need.
But as you can see, with a 40×60-inch print calling for over 300 megapixels worth of information, it isn’t outrageous for photographers to still want ever-greater resolution in their digital cameras. Extremely high resolution cameras may still be a bit expensive, but they do serve a valuable purpose for those who need to produce large output.