While it is clear that most current digital cameras do not offer resolution that matches film quality, it is important to note that you can get photo quality results from current digital cameras. Amazingly enough, it takes less resolution than most people think to create a photo quality image. In reality, you only need about 300 pixels per inch (ppi) to create a true photo quality image (though some will be satisfied by a lower resolution of 200 ppi). This means that you need 300 pixels of information for each line of a print. When you consider that many printers today start at 600 dpi (dots per inch) and go up to 1440 or more dpi, it seems as if you should be able to get photo quality results from almost any printer made today. So why isn't this necessarily the case?
Once again, the answer is in the numbers. While 300 ppi sounds like a low bar to reach, in reality it can be a daunting level to reach. You have to have an image file that is capable of providing 300 ppi of information to the printer, and it takes more resolution than you'd expect. First, you must have 300 ppi of information in your original image file, and next you must have a printer that can create a 300 ppi image (unfortunately, ppi and dpi aren't the same thing, as I will explain below). Since you need 300 ppi both vertically and horizontally, the formula to find the required resolution in MP is quite simple:
(where x and y are the dimentions of the print in inches)
Once you have the pixel resolution figured out, convert it to megapixels. What you'll find is that it takes quite a bit more resolution than you think to get a good 300 ppi print! For every square inch of print, you need 90,000 pixels of information. Here's how it works out:
| Print Size 4x6 5x7 8x10 11x14 16x20 |
MP Required 2.16 3.15 7.2 13.86 28.8 |
Also keep in mind that this is the minimum resolution required for the print, and to get a full 300 ppi print from a digital file, you will likely need an extra 10-15% to allow for cropping. For heavy cropping, you may need as much as twice as many MP to maintain image quality. Even if you don't worry about the cropping percentage, you still run into a brick wall of minimum MP for a given print size. For me, this table is the most important one when considering digital versus film. It tells you exactly how much resolution you need to get a good print, and what digital camera would be required.
This is where everything gets interesting. If you plan only to use your digital camera for snapshots and nothing larger than the occasional 5x7, then any of today's 3 MP cameras will to a terrific job and provide film quality results. Even a 2 MP camera would likely suffice. But if you buy digital to make 8x10 enlargements and save a little money on lab processing fees, you run into resolution problems (and cost problems, as I will explain in the next section). This table also spells out clearly how much digital photography must advance before film will truly be replaced, but it also shows an interesting truth... At 16x20, most top 35mm systems are not capable of a 300 dpi print, as most top out at 22 MP resolution! Ever wondered why so many people consider medium and large format when making large prints? Here's a big clue (especially when you consider that a 4x5 neg can hold about 180 MP of information given a 60 lpmm lens...). But that's another topic...
Now back to the ppi vs. dpi problem. Theoretically, you want to put down 300 pixels of information on the paper. And since a pixel is nothing more than a dot of information, they should be interchangable as McBroom suggests in his book. The problem comes in when you find out how printers print. The pixel records a color, say turquoise with a hint of aquamarine. When you ask your computer to make a print, it sends that pixel color to the printer. The problem is that the printer often does not have an ink color that is a perfect match for the original turquoise with a hint of aquamarine. So what it has to do is mix the inks (which is very difficult) or put down finer resolution dots on the paper so that when viewed at a distance, the different color dots merge to appear as the proper turquoise with a hint of aquamarine. As a result, inkjet printers may need to have resolutions 2, 3, or 4 times higher than the desired output of 300 ppi just to make that photo quality print! But to confuse things further, dye sublimation printers (which often have a maximum resolution of 200 dpi) have been creating photo quality prints for years because of the way the print head is designed! So even having a high dpi rating isn't always necessary. In the end, to get a 300 ppi image, you need a combination of both a high resolution printer and one that can either mix the inks or lay enough finer resolution dots to give the accurate 300 ppi result. For today's inkjet printers, however, this generally means you need a resolution of at least 600 dpi (and higher is often better).
When you get right down to it, digital cameras can do well if you work within their limitations, or choose a camera based on your expected use. A 3 MP camera may do well as a film replacement if you only snapshoot, and don't really want or need enlargements. Likewise, for web page work, you don't need resolution higher than 600x800 in most cases (and in many cases, it's best to keep resolution lower to keep file size and download times in line). But if you expect any of the current digital cameras to be an honest replacement for film, please keep the above table in mind. Digital technology just isn't there yet, and it shouldn't be considered a truly viable replacement until the 10 MP barrier is broken at reasonable cost (to allow for creative cropping for an 8x10).
However, when considering the switch to digital, many people are willing to give up a bit of image quality in exchange for the convenience and cost savings. You point and click, and the image is instantly downloadable to your computer for printing or e-mailing to friends and family. Likewise, you don't have to spend any money on film or processing. Unfortunately, when taking a closer look, you will find that the cost of digital photography is higher, and the hassle significantly greater. This and more, in the next section:
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