What scan resolution is useful for 35mm film when making very large prints?

Question

If I want to make very large exhibition prints from 35mm negatives, how much scan resolution is actually useful?

For example, if the final print is around 1 × 1.5 m, is there a practical maximum scan resolution beyond which no more real image detail is captured from the film? Or will a traditional optical enlargement always retain more detail than a digital scan/print if you look closely enough?

I’m especially interested in the point where extra scanning only records grain rather than more subject detail, and how that relates to large print sizes.

user75526 · Accepted Answer

According to Ken Rockwell:

Fuji Velvia 50 is rated to resolve 160 lines per millimeter. This is the finest level of detail it can resolve, at which point its MTF just about hits zero.

Each line will require one light and one dark pixel, or two pixels. Thus it will take about 320 pixels per millimeter to represent what's on Velvia 50.

320 pixels x 320 pixels is 0.1MP per square millimeter.

35mm film is 24 x 36mm, or 864 square millimeters.

To scan most of the detail on a 35mm photo, you'll need about 864 x 0.1, or 87 Megapixels.

That's about 8128 DPI. But in practice, consumer lenses and higher ISO films don't record that much detail. Scanning more than about 3048 DPI (12.4 MP) usually just increases the visibility of grain. By the time you see grain, you've captured as much detail as that particular piece of film is capable of containing.

The above numbers assume a perfect scanner. However, scanners are limited by the resolution of their lenses. So even though a sensor may be capable of 6400 DPI, the lens may only be capable of 3200 DPI or less.

What scan resolution is useful for 35mm film when making very large prints?

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