Optimal DPI for film scans

[viablex1]

wow every day I realize I just don't know virtually anything about what I have been doing.

Just looking for some direction with regard to scanning 6X9 film and 4X5 film mostly iso 100. what is the optimal dpi for the scans? I used to just set to the max and go but after reading some articles that may not be the best. The prints I want to make are small this time but wouldn't mind trying for a 40X50 some day.

thanks

Matto

[rdenney]

There is no easy answer here, and all scanners have different performance curves. I usually scan at the highest useful resolution or a bit higher, and then resize as needed when targeting for a particular display or print size. With my Nikon film scanner, I scan roll film at the scanner's maximum setting, which is 4000 spi. With sheet film, I scan at 2400 spi, which is a bit higher than the scanner's maximum useful resolution. Film scanners often work at their maximum value, but consumer flatbeds usually do not. Scanning too high just wastes file space, it seems to me.

I'm not sure there is a scanner, at least not one most of us are likely to own, that can really scan at too high a resolution for film.

You can sometimes get interactions between grain frequency and scan frequency, which can produce artifacts called aliasing, but this can usually be avoided by scanning at a different resolution--up or down. Don't worry about it unless you see it, though.

If you are sending it out, I suggest choosing a service that can advise you based on experience with their equipment.

Rick "suggesting a little experimentation" Denney

[Preston]

Matto,

What make and model scanner are you using? For example, I scan my 4x5 film at 1800 ppi using my Microtek 1800f, and I like the results. In any event, you might consider scanning the same piece of film at different resolutions to see what works best for you.

Note that the best resolution for 4x5 will likely not be high enough for 6x9cm.

--P

[Ken Lee]

It can be helpful to work backwards from your maximum print size.

Critical print sharpness requires roughly 300 dpi on the paper (there's some controversy there, but more agreement than controversy from what I have observed).

To make an 8x10 inch print at 300 dpi, requires a file whose dimensions are 2400x3000 pixels.

If your film is 8x10 inches, then you can scan it at 300spi (samples per inch) and make a fine 8x10 print.

If your film is 4x5, then you need to enlarge by 2x, so you can scan at 600 dpi.

If your film is 2.25 x 2.75 ("6x7" Medium Format), you need to enlarge by 3.56x, but let's call it 4x for the sake of simplicity - so you need to scan at 4 x 300 or around 1200 dpi.

To make larger or smaller prints, do the arithmetic. For example, to make a 16x20 print from a 6x7 Medium Format negative requires around 8x enlargement - so you need to scan at 8x300 or 2400 spi.

When we say "you need to scan at" some resolution, we mean, you need a scanner that will actually deliver that resolution, not just one that allows you to request that number in the user interface. (For example, an Epson scanner might allow you to request a scan at 6400spi, but it won't give you that much resolution. You'll get a large file, but the data will be redundant low-res information)

[tjshot]

No easy answer to your question, because many factors are involved.

However some general guidelines can be found.

One factor is pixel density requirement for upscaling to desired print size.

300 dpi output is generally accepted as optimal value, it sure is for standard viewing conditions but it's still far from optimal print performance value.
I've done some calculations and empirical tests (you may want to spend some time on the attachment for details), which can be summarized as follows:

- 550 dpi is generally close to top printer performance (175 lp/in MTF 10% resolution, 155 lp/in MTF 50% contrast) for sharpened film scans and Bayer sensor generated images (most actual digital sensors), provided there is enough contrast (MTF) in the image;
- 350 dpi setting is sufficient for Multilayered sensors (Foveon).
So, if you work with film and want highest possible print potential you should aim for at least 550 dpi offset, which generally means using high resolution setting from user-affordable scanners (Nikon Coolscan 9000 or 5000 for MF and 35mm, Epson V700 for LF).

The way to go for top performance with film is scanning to high res, then downsampling with Lansocz algorithm to 550 or 600 dpi and then sharpening to optimal value: the high spatial frequencies attenuation which is typical of the (scanned) medium will generally help avoiding the artifacts and aliasing, which generally occur in downsampling from direct digital acquisition.

Another factor, the one that actually poses limits to the previous approach, is the relationship between the information (contrast attenuation curve) that a film emulsion can register and its typical noise levels (grain).
The previous approach (scan to top setting and downsample) is generally safe until you start scanning into the grain structure of the emulsion: at that point you'll be collecting "noisy" information, having to cope with it later when sharpening.
It's not necessarily a bad thing (printing will attenuate it to some degree), sometimes it adds to the mood and sharpness perception of a shot (Tri-X syndrome!) but it's something worth considering nevertheless.
For example if you set your scanning limit to 2000 dpi you may be wasting some of the film contrast and sharpening potential (the effect of digital sharpening is boosted for higher sampling densities), but on the other side you’ll probably be avoiding grain from most emulsions; 4000 dpi would mean catching almost all the film information and sharpening potential, but also bringing along the grain which will require some smoothing-out later.
Downsampling to 2000 dpi won’t get rid of it because, once acquired , the grain structure remains part of the high frequency details of an image, which will partly be preserved by a good downsampling algorithm: you’ll have to blur the high frequencies, losing some of the benefit from higher scanning densities.
Probably with some wise unsharp masking you could get away with better perceived sharpness, but resolution would likely be close to a direct 2000 dpi for same noise level.
It’s not always easy to find the break-even point: it depends from the film emulsion, the format, the lens quality, the scanner performance and finally personal taste.
I tend to tolerate some grain as I find it distinctive of the medium.
These are general guidelines based on my experience:

a) Films that “tolerate” up to effective 4000 dpi scan (visible but not obstrusive grain)
Color: Velvia 50, Velvia 100, Reala 100 and Ektar 100 neg.
BW: most ISO 25 panchro. and ortho. films (barely visible grain), last gen. ISO 100 (TMax, Acros in FX39 or Rollei RLS or equivalent fine grain developer, otherwise better stopping at 3000 dpi)

b) Films that “tolerate” up to effective 3000 dpi scan (visible but not obstrusive grain)
Color: Provia 100 ( scans well up to 4000 dpi, but not much info there), Provia 400 (even pushed 1 stop), Fuji Pro 160 (both flavors) and Portra 160 NC neg.
BW: Delta 100 in Rollei RLS or FX39 (visible grain but splendid definition down to 60lp/mm, great “mood” and perfectly likeable grain at 3000 dpi scan!), most ISO 400 emulsions in fine grain developers

c) Films that “tolerate” up to effective 2000 dpi scan
Color: most high iso color neg. films
BW: most high speed emulsion and developer combos

I consider previous settings to be the break-even point for scanning with efficient high-end devices (efficient drum scanners, Imacon Flextights, Nikon Coolscan 9000/5000, top Creo flatbeds); for consumer grade scanners like Epson V700/V750, which have “softer” contrast gradation (higher order MTF rolloff), is generally better to use the top res setting as the softer scanner rolloff at high spatial frequencies will attenuate film grain, obstrusive only for very gross high speed emulsions.

Camera lens quality also plays a role.

Good 35mm format primes, used within optimal aperture range and with group a) films will almost always justify a 4000 dpi scan; the same is true for the very best MF primes (most of the outstanding Mamiya 7 primes, the best Contax and Hasselblad primes).
Good to average medium format primes ( Pentax, Bronica etc.) are better served with 3000 dpi scans even for group a) films; you would not find useful information over.
Up to this point dedicated high-end film scanners can make a big difference in performance; however for average MF lense/film combo, results from a perfectly calibrated Epson V700 can be very close.
Group c) films, even with excellent lenses, are hardly worth more than 2000 dpi scans; information is there but noise make it less usable.
For LF lenses film resolution is mostly unrelevant: 2000 dpi is often the sweet spot, with sharpening allowing for a perfect recovery of original film+lens values.
A perfectly calibrated Epson V700 (optimal focus height determined with fraction of mm tolerances) provides excellent performance, very close to higher end scanners.
I’ve run a few analytic models for the previous scenarios, which confirm previous statements.

As a reference for printer performance, I attach a paper with empirical test results and tables, complete with details about modelization procedures, borrowed mostly from Norman Koren’s excellent site; it may be out of the scope here as it is focused on full frame DSLR cameras, nevertheless the printer-related portion is still applicable.

[renes]

tjshot, thanks for sharing with your tests. Great info.

Had you any experience with scanning MF films (6x9) on Epson 4990? Wonder what would be your guidelines to get best film scan from this scanner. Same or close to Epson 700?

[SURF]

Great summary tjshot.

then downsampling with Lansocz algorithm to 550 or 600 dpi

Can add two points from my experience. First is that downscale algorithms like Lanczos (2 and 3) and Catrom are so close to PS Bicubic that I finished playing with them. Second is that the printers produce their top resolution when they print at 600 dpi (Canons) and 720 (Epsons). And this is visible to me. So if you want a 300 dpi nice image to be printed it is better to resample it to 600 dpi (or 720) and then print with 600 (720) dpi. Or you can downscale to that resolution after the scan or scan directly to that.

[tjshot]

tjshot, thanks for sharing with your tests. Great info.

Had you any experience with scanning MF films (6x9) on Epson 4990? Wonder what would be your guidelines to get best film scan from this scanner. Same or close to Epson 700?


I did not test an Epson 4990, but I guess general guidelines would be exactly the same: from what I've read performance should be close.