How can I determine what kind of dpi is equivalent to the pixel density of a 5D?

Or put another way, if I wanted to compare 100% crops of a film scan and a 5D file, what would be the most similar dpi setting to use for scanning?

Well, the 35mm negative is 24mm x 36mm, or 0.945" x 1.417".

4000 dpi scans would give you 21.4Mpx.
2000 dpi scans would give you a quarter of that.
I'm not sure what the various scanners offer (some might do 3000dpi) but I expect that you should scan at full resolution and scale down from there, taking the noise (grain/scanner aliasing) down with it. Either that or scale the 5D images up. I'm sure that any in-between dpi that does not correspond to native resolution divided by an integer would involve an increase in artifacts and contribute to an unfair comparison.

- Alan

35mm slides, negs or another size of film? I would scan 35mm film at 3200ppi and you should come close. 4000 (if your scanner can actually get there) will go larger. Less res for larger film.
Never heard about in between res settings. I use a lot of res settings and have never ran into one being worse than the other. I try to get the image size and res as close as possible to the final need. Been scanning since the mid 90s. The in between res thing is more for printers, though it may be an urban legend there too.

Tom

I am scanning 120 film. I'm just wondering what is a good dpi for an apples to apples comparison. I'm not trying to pixel peep too much, it's just that as I was using LR to zoom in on some images, it got me wondering what the equivalents are between a film scan and 5D file.

I don't think you can get close to a fair apples-to-apples comparison. Assuming what you are trying to compare is...what? Resolution

Use the best possible film, scan at the highest resolution. If that gives you more pixels than the 5D, then resize the 5D to the same number of pixels. Then you can view them side-by-side at 100% and they will have the same magnification since they have the same sensor size.

what film? Which Scanner? Which scanning software? Which color space are you scanning into? How are you holding the film? Standard holder or fluid mounting? How are you sharpening the scan Which settings on your 5D mk.2? Which software, how are you sharpening? How are you calibrating both the scanner and the camera?

The truth is that while you can approximate the file size the two processes are so very different you only get a very very rough comparison. I know 'cause I've tried it with various high end film scaners from a Nikon Coolscan 5000 all the way up to a drum scanner.

Ideally you never use anything but the maximum optical resolution of the scanner (like 4000dpi o nthe Nikon Collscans) and none of the interpolation settings.

Why not scan a neg and do a print? One thing is certain with film..what you see on a monitor rarely prepares you for the quality of the resulting print...if you know what you are doing!

Also..can't see the rationale behind your question...which should be more along the lines of how to get the best image from an XX mm film...which is where craftsmanship comes in.