That is great information Lenny, thanks.
All my 8x10 negatives are contact printed, but if I need to get really big it is nice to have a sense of what a good drum scan costs.
Dan
I (Professional Drum Scanning - drumscanning.co.uk) do 8x10 scans at 2000dpi for £50 (or $70). These end up as approx 1.8Gb files. There is some more detail beyond this level but you'd better be shooting wider than f/22 on a very good lens and probably be taken in a studio (and be printing at over 48" wide as otherwise those extra pixels will be wasted).
We did extensive tests for a comparison survey a few years back (google 'big camera comparison onlandscape') and 8x10 break point was approximately 620Mp equivalent. This works out as 2200dpi and is a 75" by 93" print at 300dpi (this is for Delta 100 - things will be slightly less for transparency and slightly less still for colour neg)
For 5x4 the break point was 380Mp equivalent which is approximately 4000dpi and gives a 59" by 73" print at 300dpi. Again this is for Delta 100
For medium format we tested on a Mamiya 7 and got 120Mp equivalent which is approximately 5000dpi
A colleague and I have been testing colour transparency film on a good 35mm camera and we think that for transparency we have never seen more detail than 5500dpi worth (note that many scanners say they scan at a resolution more than this but for real world scans they don't. Typically to scan at higher resolutions means a smaller aperture which exacerbates noise to such an extent that it hides detail).
Once you're in the realm of true high end scanners, the results are typically not about resolution (they should all manage 5,500dpi or more which is more than most film has) and it's about the quality of colour, shadow noise, etc. For example, when I had a Howtek 4500 and I had just moved to the Heidelberg Primescan, I worked on a 10x8 from a client in Italy. They asked for a 4000dpi scan which I did on the Howtek but just out of interest I tried a 2000dpi scan on the Primescan. The Primescan result was superior in nearly every way (and yes I checked this multiple times including focussing manually etc).
So you can't completely trust dpi - I would suggest getting scans from few different providers to check everything is as it seems if you're planning on spending the time and effort of getting 1Gb+ scans. Most providers should do a sample scan for free or at least for a substantial discount.
Tim
p.s. If you're scanning grainless microfiche film like Adox CMS20 then you do benefit from higher resolutions (with smaller apertures).
p.p.s. For 35mm negative (colour and black and white) one of the best scans I've seen is from a flatbed scanner. The Screen Cezanne Elite Pro gives pretty much perfect 5500dpi scans with incredibly smooth grain. Sadly it's an awful transparency scanner (built in colour boost in hardware that can't be turned off). I would be interested in hearing from someone with a IQSmart3 to try a side by side test..