Uprezing Or How Big Can You Print Digital Images

There are some very nice people who tell you they can make 16X24 prints from 6 MP digital slrs or even from 8 MP consumer cameras. They do this by either allowing the print driver to magnify the image or more often by using one of a large assortment of uprezing tricks and formulae - from genuine fractals to photoshop bicubic to stepwise upsizing, etc. They then claim to produce great results. As a landscape photographer photographing fine detail, I know this is a lot of hooey - but their prints do look good - so whats going on? The ideal image to upres would be a black rectangular box against a white backdrop. Theoretically and with a good uprezing programme there would be absolutely no limit to the amount you could enlarge- billboards that accept nose prints would be the norm. If you look at the images that upsize well, they tend to be along that vein - large blocks of colour, little fine detail. I find that faces upres very well - hiding fine detail like pores can be a blessing. Sports images and buildings upres nicely too.Grass and trees in the distance on the other hand uprez very littlewithout turning to much or jaggies of over sharpening. Better sharpening algorithms have allowed me to make bigger prints than previously, but they definitely aren't nose on print sharp. I still find that for close inspection (8 inches away) of any size print, you still need 300 dpi of real information in a highly detailed image. That means for a print that is 15 inches across by 10 high, you are looking at 300X15X300X10 = 13.5 megapixels - so for a detailed image like many landscapes, you need to stitch with a 6 MP camera, or use at least a Canon 5D, 1Ds2 or Nikon D2x.