" When people says they've made 20" x 30" prints you need to ask who their audience was, how closely the prints would be scrutinized and what the subject matter was. For a high-contrast, high-detail image, viewed closely by people who actually know what a sharp image looks like (e.g., a photographer or photo editor) you need 300 PPI of actual image data on the output. In other words for a 2000 x3000 pixel RAW image you could make a maximum of 6.6 x 10 inch print. I work as an image-processing engineer for a medical products company and I've run tests comparing 300 PPI images to the same images at 240 PPI and my colleagues and various amateur photographers CAN tell the difference.
Now, if your intended audience is just nontechnical friends and family you can get away with lower resolution. Likewise if the images are going to be viewed from some distance you can get away with less. And also if the original image is soft or low contrast you can get away with less. That's why people say "it depends".
The other thing to remember is that just because YOU can't see the problems doesn't mean your viewer can't. I have a friend who has some 8x12 prints he made from his Oympus E20 that's he's very proud of. All the high-contrast edges in them have serious "staircasing" but I don't have the heart to tell him.
But the math is inexorable: fine-grain film can resolve about 90 lp/mm. a line-pair takes a minimum of 2 pixels so that requires 180 pixels/mm. The best medium format lenses aren't quite that sharp - maybe 70 lp/mm
(140 pixels/mm). But since you shoot medium format (I shoot 6x7) you can appreciate this: the image area of a Hasselblad frame is 56x56mm, which as you can see above, works out to 7840x7840 pixels - a whopping 61 megabytes!
The other key difference beteen digital and film is the transfer curve. Digital hits a wall at the top and bottom of the transfer curve whereas film falls off gracefully. That's why digital shadows get blocky and digital highlights get blown out. Even when you digitize film you can retain its advantage because scans are 14 bits whereas digital camera images are (usually) 8 bits.
I shoot everything from digital to 35mm to medium format. I love digital: it's very convenient and for many things it's "good enough". But for critical images viewerd critically by critical observers it doesn't come close to film."
One thing that you said I don't understand is that you were
going to send JPEGS to a pro lab for printing. Why JPEGS?
If you want pro results you need to shoot RAW and convert to TIFF for any serious work. JPEGS are strictly for web images and speed. Many thanks F5Foley
If you want to take a digital file and not worry about any post processing or learning how it works, then digital IS NOT for you. Taking a digital photo is only the tip of the iceberg when it comes to producing a digital file.
I know of many photographers, critical judges, that have been shooting digital and find it, with the right post processing, to be as good as film in almost every way. Check out this comparison, this was done when the 3mp D30 was the hot camera to have and you will find that some very critical eyes find it to be every bit as good as slide film.
http://www.luminous-landscape.com/reviews/cameras/d30/d30_vs_film.shtml
Beyond that you have quality photographers every day making enlargements and being quite happy with them. I have 300dpi 10x15s made from a 2.7mp D1h jpeg file that have been looked at by very critical eyes and have been given the thunbs up as just as good as film. DSLRs files handle resizing very well and can produce great images after some work.
But it does take work. You need to use several programs and spend some time with the file if you are really going to make a quality large print. Its not going to happen easily, you will need to learn how to do it, buy some software and have a good computer to work on. But it can certainly be done.
Now I am not going to dispute anything your email writer said. I dont know his qualifications and I dont know what his knowledge is, but I do think that he is either seeing poorly processed images or has an unbelievably critical eye, far beyond what almost anyone can see.
I do not pretend to have the most critical eye, I proabably dont, but I have had prints looked at by some critical eyes, I have also seen other photographers such as Moose, David, and others be completely satisfied and pleased with digital output. These are not guys that are regular every day joes that do not know what a good sharp photo is, these are well established and experienced photographers.
So anyway, I dont know what your looking to get out of your prints, but I would say that with some learning and some dedication to the digital world, you can be very happy with the current crop of digital cameras available. I guess unless you are doing larger than 20x30 prints and selling them exclusively to people who are looking at them with loupes.