I have a bunch of old negatives (~30 years old) that I recently sent out to a processing shop for scanning to digital. I was told that the scan resolution was slightly in excess of 3000dpi. I don't have any kind of equipment to do this myself, and am dealing with a local company.
However, when the scans came back, I was surprised to discover that many of the images looked more blurry than I remembered. I went back and discovered some old backups I had forgotten about, some almost 20 years old, which contains a few of the same images which had been scanned from prints, mostly on the order of 4x6. To my surprise, some of these images showed more detail in a lower resolution. For instance, one of the negatives scanned to an image around 3100x1900, while the print scan of only 2100x1600 shows detail which is fuzzy or blurry. Yet, it is not this way on all my images. On some of the images, the negative scan show more detail in which I can (for instance) read text which my print scan can't even resolve.
These results surprise me. I expect color changes, dust, scratches, that sort of thing, but not blurriness. I'm not sure if I should complain to the scanning service or ask for a partial refund and try somewhere else (I had a lot of pictures and paid a few hundred dollars up front), or if this is a legitimate possibility that time should degrade the resolution of the images. Many of the blurrish images were scanned from Disc format, and they claim the combination of the small size and the presence of the central hub contribute to the blurriness. However, since this I have discovered that even some of the 35mm scans are blurry. All of the blurry images are color (I suspect C41) negatives - I had B&W and color (positive) slides and even a few 110 negatives that turned out great.
update: I am attaching pictures to show a comparison of the detail in the original print scan and the higher resolution negative scan.
Here is the negative scan:
Here is the print scan, upsized using Cubic interpolation to approximately match the negative scan:
Here is the original print scan:
The grain is clearly visible and the text is verging on readable as compared to the print scan.