Estimating Sharpness and Quality at various print sizes.
2. Although you can expand your files in Photoshop to increase the megapixels, doing so usually does little if any good. The most common way of expanding files is linear interpolation between pixels. Via that method, the interpolated pixels add no detail. If you have an advanced program (e.g., Genuine Fractals) for expanding files, some detail is added. In that case, it is probably better for you to expand the files. If not, it's probably easiest to let our system do that. We expand all files via linear interpolation to 300 ppi for printing.
3. Our system will give you recommendations for how large a print you can make from a given file. However, those recommendations are given with a caveat -- they are valid only if your file is sharp at 100% magnification. If you have expanded your file, it is probably not sharp at 100% magnification.
4. Required sharpness is very subjective. Generally, small prints are viewed at closer distances than larger prints, so small prints generally require a higher PPI. In general, for prints around 8 x 10 in size, a 4 megapixel camera is generally adequate. For prints around 20 x 30, at least 8 megapixels is recommended. For 30 x 40 or larger, 10-18 megapixels or more is advisable.
If you want large prints with incredible sharpness, but don't have an incredible budget, investigate stitching (joining multiple photos to make a single large file). If you learn how to properly take your photos with stitching in mind, and have a good stitching program, you can create incredible prints with moderately priced equipment (e.g., 12-18 megapixels).
5. If you are adding text or line art to your file, then (everything above notwithstanding) you should expand your file to at least 150 PPI before you add the text. That won't help the image, but it will make the text sharper. There are often good reasons for images to be blurry when viewed up close. There are almost never good reasons for text to be anything less than crisp and sharp.
The third image, at right, is what you really will get when you start with an image with too few pixels and blow it up in photoshop. You get more pixels, but each pixel is interpolated (averaged) from the nearest neighbors. It's less pixellated, but it contains no additional detail. The only way to get that detail is to start, in the first place, with a better file (i.e., take the photo with a camera with more megapixels).
and you want to make it sharper, so you go to Photoshop and triple the number of pixels in each direction. You might be hoping to get this:
But you won't. That's not what will happen. This new image, above, has additional detail which the original image never had, and can't be produced out of thin air. What you will really get is the following:
