I' also enlarged an agfacolor 200 to 40x60 and it too came out razor sharp. The grain was there but didn't interfere with the image. The shot was taken with an Alpa Macro Switar in close-up and could have been enlarged more if my set-up had been able. This was 30 years ago. With the quality of modern films now, the sky is almost the limit
One hypothesis is that the grain enhances the perception of sharpness due to local contrast, when viewed at the correct distance. A simple test of this for me is comparing enlargements from TMAX and HP5+. My 10 year old son says that he prefers HP5+ shots.
These are awesome. I had in the past an Edmund optics resolution chart and saw my Pentax 24mm achieve over 100lpmm, using TMAX 100 and XTOL, and reading directly on the negative using a 20x scope and raising the enlarger head up. It would enable reading printed letters at 4mm as small as maybe 4/100mm on the film. This equates to shrinking the text by 100x to get it on the film. By the same token, this means being able to record 4mm text from say 5m with a 50mm lens. Then when you enlarge it, maybe you enlarge it by 10x. Now the lettering is there, in the print, but only measuring 0.4mm, so you cannot read it! I believe I am right in saying paper like MGIV can also do similar resolutions to film, around 50lpmm. This means that the text is there on the paper, but you cannot read it without a loupe!
I have a shot with flyer in the frame, in the plane of focus, where you can read a phone number on the bill, but you have to use a loupe, on a 8 x 10 print. Shot was with a 100mm Pentax M lens, f8 sunny day, 100 speed film, 1/500 shutter speed on a monopod, with a hood. Using a loupe to see the detail in a print is really sniffing the print. I've just processed a roll of Rollei 100, which although grainy, has done the same again, with text in the frame of BBQ stuff.
If you want to get really geeky, you will start wondering about differences in resolution limits by using tri-colour filters, to remove chromatic aberration.
