Printing from 4000dpi vs 5400dpi scans - at which print size do you usually begin to see a noteworthy difference?

[albireo]

A question for the digital printing gurus here.

I've recently started sending some of my scanned negatives to a lab to have them wet printed (see this thread for context).

For my scans from 35mm, I've been using my main film scanner, which operates at 4000dpi.

I also have another dedicated film scanner operating at 5400 dpi (according to tests around the web, it's more like 5000 real dpi) which has been sitting in storage for a while.

I was wondering if someone could refresh the theory for me and also contribute their personal experience on the ballpark print size one would have to aim for to start discerning a difference between a 4000dpi and 5600 dpi scan of a full 24x36mm negative.

Or - looking at it from the other side - is it worth unpacking and setting up the 5400dpi scanner if all I'm aiming for is at most A3 sized (approx. 11 x 16 inches) prints from my 35mm negatives?

 

[albireo]

To start answering my own question, and based on memory..

I seem to remember the magic number '300dpi' as a target for a good quality prints.

If that's the case (please correct me if not)

Let's say I'm scanning the full 35mm frame at 4000dpi. If that happens I'm getting approximately (36mm * 4000/25.4 ≈ 5600, and 24mm * 4000/25.4 ≈ 3780) so 5600x3780 pixels.

So then the max print size would be

• 5600 / 300 ~ 18.6 inches (long side)
• 3780 / 300 ~ 12.6 inches (short side)

Which translates to ~32cm x 47cm so a little more than my 'A3 target' above.

I wonder how viewing distance factors into the above though.

 

[bernard_L]

Three replies. [edit: you started answering your own question while I was typing]

1 - A quick calculation you have certainly done yourself. A3 is 12 inches on the short side. At the consensus resolution of 300dpi, that is 3600 pixels. Your 35mm frame is ~1inch on the short side, so you need to scan at ... 3600dpi.

2 - Use your eyes, or those of a good-natured guinea pig. Small test prints; change resolutions, (double-??)blind test. I would say, with A3 prints, the subject is not allowed to stick his/her nose on the print.

3 - "Resolution" in scanner specs or tests usually means the highest lpi (2x, actually) where the stripes are still discernible. In technical terms, this means scanner resolution is defined at vanishing MTF. Which is not very meaningful. Below some MTF plots from tests of my V700 10 years ago. Not in the 4000dpi ball park, but just to illustrate the principle.

Straight from scanner:

With the "best" parameters for MTF restoration using Gaussian USM:

In both cases, the MTF drops to zero just beyond 0.4 cycle/pixel. But the boost in MTF at 0.3 cycle/pixel, from ~0.17 to ~0.55, makes a large difference in perceived (sharpness, resolution, whatever, I'm not here for semantics).

Please note that the kind of MTF restoration I'm proposing is much less than what I sometimes see online or in shows, that just hurt the eyes. On occasion, I've brought this up to the photographer, that either admits sheepishly having sharpened maybe a little too much, or takes a stand and claims that's how he likes his pics.

 

[albireo]

@bernard_L thanks for the insightful reply.

Out of interest, what is the reason for the dip at .6 cycle/pixel in the sharpened example?

 

[bernard_L]

@bernard_L thanks for the insightful reply.

Out of interest, what is the reason for the dip at .6 cycle/pixel in the sharpened example?

1. Spatial frequencies beyond 0.5 cycle/pixel, in actual use, "do not exist". More precisely, they are aliased onto the frequency that is symmetrical wrt 0.5 cycle/pixel. Remember grain aliasing, buzz word occasionally found on internet discussions.
2. How then will you ask, can the plots display frequencies beyond 0.5 cycle/pixel. Because of the trick of slant edge MTF measurement, not applicable in general image scanning. Google it...
3. The actual V700 hardware+firmware is a bit complex (staggered rows of sensors) and must be treated as a black box. Short answer: I don't know.

 

[loccdor]

Or - looking at it from the other side - is it worth unpacking and setting up the 5400dpi scanner if all I'm aiming for is at most A3 sized (approx. 11 x 16 inches) prints from my 35mm negatives?

From my viewpoint the 4000dpi (if that is its true resolution) will be sufficient to at least 20 inches. Maybe 24. Since you need less than that I wouldn't set up the 5000dpi scanner for this purpose.

5000dpi will be sufficient to at least 25 inches. Maybe 30.

Just my personal experience and taste. Yours may vary.