Scanning resolution

[Adrian]

Hi!

I have a rather vague recollection of having seen a formula which takes into account an image's type/colour/size and its ultimate output size and allows you to work out its optimum scanning resolution. Am I making this up, or is there such a thing?

Best wishes, Adrian

[ktinkel]

Originally Posted by Adrian

I have a rather vague recollection of having seen a formula which takes into account an image's type/colour/size and its ultimate output size and allows you to work out its optimum scanning resolution.

This is not exactly a nifty formula, but Agfa published a series called Digital Colour Prepress in the mid-1990s including one called “An Introduction to Digital Scanning.”

It offers this for conventional half-tones:

Scan resolution:
Screen ruling X Quality factor (qf) X Sizing factor*

qf = 2 if screen ruling ≤ 133 lpi
qf ≥ 1.5 if screen ruling > 133lpi

* Sizing factor = Desired size / Original size

The discussion lists six items to be considered:
output screen ruling (frequency, aka lpi); sizing factor; correct tonal range; sharpness; colour; grey balance.

Does that help?

[Adrian]

Originally Posted by ktinkel

This is not exactly a nifty formula, but Agfa published a series called Digital Colour Prepress in the mid-1990s including one called “An Introduction to Digital Scanning.”

Kathleen,

Many thanks for that. I'll play around with some numbers and see if it works!

Best wishes, Adrian

[PeterArnel]

Adrian - I always thought the scanning dpi was twice the size of the lpi
ie 200 line screen should be scanned at 400 dpi. and scans should not be enlarged by more than 130% (mark u when needs must they get blown up to as big as required)
Peter

[donmcc]

The actual ratio needed is a minimum of 150% of the line screen. Someone at the Adobe Forum (Len Hewitt?) printed out that math one day, but math makes me sleepy sometimes. Some print shops ask for double to provide some sizing cushion, however.

As for enlargement, you can enlarge a scan any amount, so long as you make sure there are enough pixels. Even 35-mm slides get scanned for National Geographic ... they just scan at a very high resolution.

The 130% figure might be what a printer claims is the maximum you can enlarge when you have to resample ... personally, I would use KTs formula, which takes enlargement into account and makes sure you have enough pixels so that there is no resampling.

Don McCahill

[PeterArnel]

Don - I think you scan the original to the size you want - then that sorts the correct number of dpi. so if you scan a 35 mm to A4 you would still scan to 300dpi. If however u wanted to enlarge your scan to A3 then then you have now only 150.
Peter
Blowing up 35 mm trans is a different matter. If trannies need a lot of enlargement then they have to be oil mounted otherwise you start seeing the grain of the base film.

[Adrian]

Originally Posted by ktinkel

This is not exactly a nifty formula, but Agfa published a series called Digital Colour Prepress in the mid-1990s including one called “An Introduction to Digital Scanning.”

It offers this for conventional half-tones:

Scan resolution:
Screen ruling X Quality factor (qf) X Sizing factor*

qf = 2 if screen ruling ? 133 lpi
qf ? 1.5 if screen ruling > 133lpi

* Sizing factor = Desired size / Original size

The discussion lists six items to be considered:
output screen ruling (frequency, aka lpi); sizing factor; correct tonal range; sharpness; colour; grey balance.

Does that help?

Kathleen,

If I understand your equation correctly, then to take the simplest case of a scanned image being reproduced same size, at a screen ruling of 133, that would yield:

133 x 2 x 1 = 266

so in practical terms I would scan at 300 dpi.

If, however, I wanted to scan say an A5 image and reproduce it at A4 size (and I'm taking the sizing factor as a ratio here, i.e. 2:1), then that would yield:

133 x 2 x 2 = 532

so in practical terms I would scan at 600 dpi.

Does that sound right?

What about line art?

Best wishes, Adrian

[ktinkel]

Originally Posted by Adrian

If I understand your equation correctly, then to take the simplest case of a scanned image being reproduced same size, at a screen ruling of 133, that would yield:

133 x 2 x 1 = 266

so in practical terms I would scan at 300 dpi.

If, however, I wanted to scan say an A5 image and reproduce it at A4 size (and I'm taking the sizing factor as a ratio here, i.e. 2:1), then that would yield:

133 x 2 x 2 = 532

so in practical terms I would scan at 600 dpi.

Does that sound right?

Assuming there are no mitigating circumstances, yes. But photos showing fine detail (of fabrics, say) or ones that need a lot of adjustment (extra sharpening, color correction) might benefit from higher scanning resolutions.

Another issue is flexibility. If you think you might want to use the image in some other way later, you might want to scan at higher res and archive it. Then reduce a copy to the useful size for the job at hand. This will save you from the need to scan it again. This assumes, of course, that you will remember that you have the archived original and can find it when you need it!

Originally Posted by Adrian

What about line art?

Line art is a different proposition. I follow the advice of Bruce Fraser and David Blatner in Real World Scanning & Halftones, which I would have to check for detailed advice. Basically, though, they suggest scanning in greyscale (not line art) mode, at very high resolution. Then sharpen, apply threshold (50%), and convert to line at the desired size. This gives a sharper, cleaner image in most cases.

Theoretically, you never need to end up with more than the resolution of the output device, and no more than 1200 dpi (more than that is barely discernible to the eye).

[Adrian]

Originally Posted by ktinkel

Line art is a different proposition. I follow the advice of Bruce Fraser and David Blatner in Real World Scanning & Halftones.

Kathleen,

Thanks for that. I'll think I'll try to pick up a copy, especially as our local repro house just shut down (owner retiring early!).

Best wishes, Adrian

[ktinkel]

Originally Posted by Adrian

Thanks for that. I'll think I'll try to pick up a copy, especially as our local repro house just shut down (owner retiring early!).

The book may come in handy even if you find another good service. Has a lot of information about working with scanners and images.