ALB: "...This is not necessarily true, as the marketing tendency of the manufacturers theses days seems to be to pack more and more pixel power into smaller and smaller processors. This does not necessarily mean your going to get the sharpest and best output because of this...."
That's absolutely correct, Larry. For several years, there was what amounted to a "pixel war", where each mfgr tried to pack more pixels into their digital cameras. The presumption was that pixel count was something that John Q Public would likely use when thinking about buying a new camera. Around a couple of years ago, people started to realize that more and more pixels were not necessarily the best approach. It is better to have a reasonable number (say, 10 Megs) of high quality pixels than a huge number of lower quality pixels.
There are several problems with packing too many pixels into a sensor of fixed size. The first is that the area of each pixel goes down. This means that the number of photons it will receive at a given exposure (ie, f-stop and duration) is decreased. This means a decreased signal to noise ratio. The 2nd problem is that the fraction of dead space on the array increases as you pack more pixels in, so again, the photo capturing ability decreases. A 3rd problem (little discussed) is that as pixels approach the size of the wavelength of light, there is more spill-over into neighboring pixels, so you get an effect not that dissimilar to halation in old film emulsions.
ALB: "...I have read that a good rule of thumb is to figure that you need 200 pixels per inch of print dimension. In other words, to get a good 8 x 10 you would need 1600 x 2000 = 3,200,000 pixels or a 3.2 Megapixel camera to produce it. So, my 6.2 MP would work for that just fine. OK, so now I want to produce a print that is 11 x 17 to print on my 13 x 19 available paper on my Pro 100. That calculates to a requirement of about 7.5 megapixels...oops I'm over my capability..but shouldn't I still get a pretty sharp print from the 6.2 MP camera? What equipment do y'all use? ..."
The calculation of number of pixels that you did is numerically correct, but there are a couple of factors that modify such estimates.
1. As prints get larger, people tend to view them from further away, so one can often get away with lower ppi than you might otherwise expect. For example, I developed many 4 foot by 6 foot theatre posters assuming 100 ppi (before final up-rez'ing by the printer's RIP software), and had numerous compliments on their sharpness.
2. Almost every geometric transformation you make in post processing degrades resolution, so, one often needs more resolution than you think. For example, do something as simple as a series of nine 10 degree CCW rotations of a good sharp test image, then rotate it back by a single 90 degree CW rotation and compare the sharpness before and after. Assuming you are looking at the level of individual pixels, I think you'll be surprised at the degree of softening. Similarly, use the automatic lens distortion correction in ACR and watch the sharpness become slightly degraded.
A 2nd factor that suggests you should have more pixels than a simple numerical estimate would suggest is that many operations benefit from more pixels. For example, compare a Lanczos down-rez'ing from, say, a 1000 x 750 pixel starting image down to 700 pixels (long dimension) for this forum to using exactly the same algorithm, but starting with a 4000 x 3000 pixel image. In the latter case, the result of down-rez'ing will be sharper and more noise free, even for something as seemingly non-taxing as web posting.
ALB: "...How many of you actually print your photos these days? ... I think I may have to upgrade my camera ..."
I don't do any serious photographic printing at home any more. I find that maintaining a printer is just too much work and is not cheap. In addition, since I only need poster prints a few times per year, keeping such a device at home simply takes up too much space.
My work photos are regularly printed by my employer in high quality glossy publications, but at small size and by offset press. I'm lucky to get a half page (ie, roughly 4"x6") for a photo, so the resolution requirements are modest. For medium sized family prints (eg, 4x6 inches up to 8x10"), I have them done at either a local camera store (who do a very good job), or by Mpix, Snapfish, or someplace similar.
For large prints (eg, poster size and up), as well as the occasional wedding that I shoot, I have such prints done at a local specialty pro printer that I have been using for at least 30 years. He is utterly reliable and a god-send. He prints for many of the local wedding pros including some internationally known, high society wedding photographers. He does no work for the general public.
As an example of his reliability, I once was called in at the very last moment to produce a series of photo quality posters for the opening of a play. There was so little time available, I didn't even have time to inspect the posters before they went to the customer. After working all night on them in PS, I FTP'ed them to my printer, and he produced them and hand-carried them to the theatre early in the afternoon of the opening day. I walked into the theater that night, and breathed a big sigh of relief when I saw that they were in place, looked exactly like I expected, and the producer and director were utterly delighted.
With respect to your question about upgrading your camera, from your comments, it sounds like you probably have either a d40, 50 or 70. The newer cameras are vastly better in all respects, not just pixel count. Even a d300 (which is now a several year old model) would be substantially better, but something like a d7100 would probably take care of you for years to come, and it's not just the pixel count - it's the vastly improved signal to noise, high ISO capability, fast focusing, better colors and AWB, etc.
Personally, I have been using a pair of d700 bodies for the last several years and am very happy with them. Eventually, I'll probably upgrade to a d800, but I'm in no rush.
I hope this answers some of your questions, Larry.
Best regards,
Tom
Again thanks for the informative reply.