How Many Pixels Make A Good Print?
We all know that pixel count is the best way to get quality prints. But what if you're stuck with a low-res image? Is there a number of pixels that will guarantee a good print?
##The resolution required for an optimal print varies depending on the size and intended viewing distance. One can make some generalizations: for example, it is generally accepted as safe to assume that about 300ppi will suffice for viewing at typical desktop screen resolutions (72 or 96dpi). However, there are other factors to consider such as how many megapixels were originally captured and how the image was compressed during saving.
A rule of thumb is that the number of pixels needed for a good print increases as the square of the screen resolution. For example, if you're viewing a 72ppi image at 1280×1024 resolution (125dpi) you need 4 times (2 to the power 4) as much data to make a similar print.
Since some dpi settings are rounded, an image saved at 240dpi may not be exactly 240 pixels per inch. The actual figure would be 237.5 pixels per inch – so this would also be true for images saved at 220dpi (224ppi).
To demonstrate the difference, I resized four images from a Canon Digital Rebel (10.1 megapixels) to 240dpi with Bicubic interpolation (shown up to 50% of original size). In general, I'd say that these are good enough for viewing on typical 72dpi / 96dpi screens with typical screen resolutions.
Since the lens in this camera is limited to a 35mm-equivalent focal length of 28mm, we also have a "cropped" view of the image compared to what we normally see when shooting a real film camera. (For example, in the bicubic interpolation filter, 25% of the original image is used for each pixel.)
Each of these four images is approximately half the size of the original (which would be about 1125×761 pixels):
1) JPEG: 3640×2240 pixels / 35mm equiv. focal length 28mm (digital), 4:2:0 compression, 31.8 mbps.
2) RAW: 3120×2048 pixels / 35mm equiv. focal length 28mm (digital), 16bit/channel, 24 mbps.
3) RAW+PSD-RGB: 3120×2048 pixels / 35mm equiv. focal length 28mm (digital), 16bit/channel, 28.2 mbps.
4) RAW+PSD-CMYK: 3120×2048 pixels / 35mm equiv. focal length 28mm (digital), 16bit/channel, 40.3 mbps.
All images had the same default sharpening and noise reduction settings in Aperture on import (no additional sharpening). Cropped to about 50% of original size to display here.
In the table below, I've also included the equivalent file sizes for various resolutions and compression settings (which is a good way to compare how much more space a higher-quality file takes on your hard drive).
As I mentioned before, there are a number of factors that affect the final print quality – how many megapixels were originally captured, how much compression was applied during saving, and the percentage of images with acceptable sharpening/noise. These factors are often called "print quality loss" or "acceptable loss." If you don't want to make these adjustments manually, there are programs that automatically do this for you.
For each of the three combined formats (PSD, RAW, and JPEG), I've calculated how much "acceptable loss" there is because of the camera's output settings (sharpen/noise reduction) and the expected screen resolution. The data comes from Camera-Plus (Download link at bottom).
For example, in Camera-Plus it is possible to estimate how much sharpening your image will require if printed at 240dpi.. However, this isn't always accurate since you should usually apply more sharpening than Camera-Plus requires for an optimal print. For this reason, I also included the minimum amount of sharpening required for a good print in these figures.
For the three combined formats (PSD, RAW, and JPEG), the minimum amount of sharpening required for a good print is:
240dpi: 631×929 pixels / 35mm equiv. focal length 28mm = 3.36×107 ppi (calculated)
300dpi: 912×1242 pixels / 35mm equiv. focal length 28mm = 5.96×107 ppi (calculated)
240dpi+min sharpen: 912×1242 pixels / 35mm equiv. focal length 28mm = 5.96×107 ppi (calculated)
If you're viewing on a typical computer monitor with 72dpi or 96dpi, then the image will need:
240dpi: 2.52×109 ppi (calculated)
300dpi: 3.85×109 ppi (calculated)
240dpi+min sharpen: 3.85×109 ppi (calculated)
In each column, the maximum res required for a good print is calculated according to the following general formula: minimum res / screen dpi = max res . The original image size is 1125×761 pixels.
UPDATE: I recently discovered a very useful calculator that shows the relationship between print size, megapixels, dpi and file size. The sample image also shows an enlargement and a 1:1 preview of the print so you can tell exactly how much detail will make it into your print. It's a feature of Aperture 3 called "Image Sizing Assistant" – here's how to access it:
After opening the image in Aperture 3, click on "Image Sizing Assistant" in the Inspector. Here's a screenshot (click to enlarge):
Here are a few more samples from this Canon Digital Rebel at different resolutions: 852×631 / 35mm equiv. focal length 28mm – about 4×6 at 240dpi, 13.8×9.82 inches at 300dpi, and 2.43×1.21 feet at 240dpi+min sharpen). Here are three more (with the equivalent resolution) from a Canon Digital Rebel XT: 852×631 / 35mm equiv. focal length 28mm – about 4×6 at 240dpi, 12.8×9 inches at 300dpi, and 2.25×1 feet at 240dpi+min sharpen).
Now I'll show you some sample images that were originally shot in RAW format:
Here are four images from a Canon Digital Rebel (10.
Conclusion
To conclude this review, I strongly recommend that you buy a full-frame sensor camera if you use your digital camera to capture images that you want to keep locked in your computer for a long time (like photographs from a vacation, wedding, or family reunion). That's because they have more megapixels and better lenses. Because of the extra megapixels, those cameras will produce higher-quality prints at typical screen resolutions and print sizes.
I'd recommend the Canon Digital Rebel or the Canon Digital Rebel XT (raw files require more data to be stored on the hard drive), although if you're willing to pay extra for a lens that's faster than f/2.