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Scanning: 300 vs 1800 DPI

Jay Staton , Jun 09, 2007; 10:10 a.m.

I recently purchased a Microtek 1800f film scanner. Can anyone give me the best dpi to scan at to produce large prints, 4 to 6 feet, from 8 x 10 negs/slides? I realize final resolution should be about 300 dpi, so why do so many scan at higher dpi, like 1800?

And, if anyone has any experience with this scanner, your suggestions would be greatly appreciated.

Thanks in advance,

Jay

Responses

Leonard Evens , Jun 09, 2007; 12:18 p.m.

If the prints are going to be viewed from normal viewing distance, usually defined as the diagonal of the print, it suffices to set the resolution to produce an acceptable 8 x 10 inch print. That is because, as viewers move progressively further way, their tolerance for loss of fine detail in the print reduces proportionally. If you think scanning at 300 ppi would produce an adequate 8 x 10 inch print to be viewed at about 12 inches, then in principle that would also be adequate for a proportionately larger print viewed proportionately further away.

But there are some problems with that assumption. First, ignoring the printing issue, when you scan a 300 ppi, digital image processing theory says you will get at best 150 line pairs per inch---or about 5.9 lp/mm---resolution. (You need two pixels for each line pair.) But no scanner is going to deliver the theoretical maximum. So in practice you would be better off scanning at a higher scanning rate to achieve what you want. The second issue is that the printing process also degrades resolution. 300 ppi for what you send to the printer assumes a perfect image to start. But when you combine two steps, you are going to get something less than the resolution obtainable at either step. If each step delivers about 6 lp/mm, the combination might deliver something between 3 and 4.2 lp/mm. In this case, if you increase the resolution obtained by scanning, reducing it further to 300 ppi for printing will get you closer to what you could get with a perfect image.

If instead, you assume people are going to get close to the print, you have to take that into account. A 4 x 5 foot print, requires an enlargement factor of about 48/8 = 60/10 = 6 times. If such a print is going to be viewed from about 12 inches, you have to multiply the acceptable scanning resolution by 6, so you need the full 1800 ppi that the scanner is capable of. But again, remember that this would only work for a perfect scanner. Even if the scanner theoretically produced enough pixels, it is unlikely to deliver the theoretical resolution so obtainable.

In terms of the printer, if you want to send the printer 300 pixels per inch, you also have to take into account the print size. If you start with 1800 pixels per inch after scanning, and enlarge 6 times, that reduces you to 300 pixels per inch. As above, taking into acccount the combination, it seems unlikely that you can produce a 4 x 5 foot print from an 1800 ppi scan that would show the same level of fine detail, viewed at 12 inches, that you would see in an 8 x 10 inch contact print.

In connection with this analysis, keep in mind that it assumed a negative with no loss of fine detail. In practice, any lens-film combination is unlikely to produce anything much better than 40 lp/mm. Enlarging that 6 times would produce a bit over 6 lp/mm, which would be barely acceptable if the other steps in producing the print didn't further detract. So in practice, no matter how you go about it, you are probably not going to be able to produce a 4 x 5 foot print from an 8 x 10 negative, in which critical viewers, looking close up, won't be able to discern some loss of quality. The upshot, I think, is that you should operate under the assumption that viewers will be satisfied looking at the print from normal viewing distances and choose your scanning resolution to be as high as the scanner can deliver to minimize its contribution to loss of quality.

Aaron Falkenberg , Jun 09, 2007; 12:24 p.m.

You will be right at the limit of the max optical resolution of the scanner. First, dpi means "dots per inch" and only relates to printing. Digital files are constructed from pixels, so their resolution is actually in "pixels per inch." It can be confusing since scanner manufacturers' talk about their scanner resolution in dpi when they actually mean ppi.

Example: 8"x1800ppi=14400ppi. Divide that by your desired print resolution (300dpi) and you get an image that is 48" or 4 feet. 10"x1800ppi=18000ppi/300dpi=60"

If you were to scan your image at 300ppi, you would get an 8x10" image printed at 300dpi.

You'll want to make sure your focus is spot on for a print this size. Alos,i f it is a color image, you will need a pretty good computer because the resulting file size (if it is 48bit) will be about 1GB.

JÜRGEN LOOB , Jun 09, 2007; 12:40 p.m.

Leonard

Thanks for your very extensive post . I am using the MICROTEK 1800f scanner since ages to scan my 4x5 inch B/W negatives . The supplied SilverFast software is suffient enough and I always scan all negatives , regardless of their size with the maximum resolution of 1800 ppi . Photoshop will then calculatete the maximum print size (without interpolation , which i do not like ) using either 360 dpi or 300 dpi . With that method I obtain good to very good results ( depending on negative size ) . From 6x9 and 6x12 negatives I obtain very good print results up to 20x40 cm . I appriciate what you post , but I dislike interpolation because my tests have never shown satisfying results with any kind of interpolation . You just can't replace not existing image information . Thanks again for your posting .

Regards Jurgen

JÜRGEN LOOB , Jun 09, 2007; 12:52 p.m.

Here an example of a scan/print image from 6x12 B/W negative . Resolution set to 1800 (max).


Attachment: @.RUBI.WATER.jpg

Keith Lubow , Jun 12, 2007; 05:37 a.m.

The best way to scan is to know all the details about your final product beforehand, so you can tailor the scan directly to it without having to resample up or down. You need to know the final image size and the number of dots per inch at which it will be printed. First find a place to get the stuff printed, and find out what their specific requirements are for their printer, then we can walk you through exactly how to perform the scan.

When in doubt about what your final product's size and dpi will be, and if you have the time and disk space, always scan at max. ppis, at not more than 600% of original size.

Kelly Flanigan , Jun 12, 2007; 09:03 p.m.

Many of us just scan at what the lessor of a scanners real resolution in dpi or abit lower if the original is poor or out of focus a tad. Thus is because we have many applications for the scan; and really dont want to have to scan it again. Its easy to downsize; way easier than rescanning.

Its not improper to use dpi for scanning. Its whats been used for decades; its in formal government specs. Usage of "dpi" with scanners is older than Photoshop. There is no reason to redefine ancient terms. Some of us that have been around awhile will still use dpi for scanners; hypo for fixer; soup for developer; "film" for There is no reason to devolve into the marketing world and trends. Book writers who preach dpi is wrong are just ignorant. They are ignoring past scanner terms used before Photoshop, before many of them were even born. Its proper to use dpi for scanning. My 15 grand 36" wide color scanner uses it thats 2 months old, and ones older than photoshop do to. All of our scanners except one use "dpi"; even ones that were pure DOS; pre Photoshop.

Dpi is the scan setting one uses when scanning; its the proper term to communicate with. The hell bent dpi crowd and book writers are the ones in the foul box.

An image thats 4x6 feet a 300 pixels per inch usually is for a wall map; not a photo; unless its a stitched together super res aerial image. This is a 900meg image with 8 bit rgb. 8x10 inches enlarged 6 times would be a 48x 60 inch image. Thus an 1800 dpi scan enlarged 6x convolves to the 300 number maybe on the print. In practice many of us printers get a mess of inputs that are really bloaded. Its a major problem today. What matters is if the results you get on the print look ok.

Ok this probably is simpleton; but maybe will help. Lets say you are going to enlarge digitally a 8x10" negative 4 times to about 32x40 inches. If you scan the original at the 300 dpi setting; you get an image thats 300 x8 by 300 by 10 pixels; ie 2400 by 3000 pixels. Without changing the number of pixels; ie not upsizing this image becomes a 32 x40 inch image at 75 pixels per inch. This actually is fine for a poster several feet away; but will be soft/lacking if looked at close. Scanning at a 600 dpi setting and "enlarging/morphing" the image 4 times results in a 150 ppi image on the print; IF the negative is sharp you see more detail. Scanning at a 1200 dpi setting with a flatbed pulls out the majority of info from most flatbeds when a sharp orignal is used. Beyond 1200 you are in the downhill return area. Most modern flatbeds DO pickup more info beyond 1200. The pickle is finding what number above this really is your working area. For my old 600 dpi we used 600; for our 1200 dpi; we used 1200. For our 1600dpi; we used 1200 and 1600. For our 2400 and 3200 dpi and 4800 dpi flatbeds; we use between 1600 to 2400; on ours 3200 and the 4800 settings are bs.

With higher dpi settings above 1200 on a flatbed; most flatbeds you build up file size way quicker than actually pulling out more info. With our sharpest flatbed; its an older 2400 unit thats sharper than another 2400 unit . The 3200 and 4800 units were no better.

If your original is less sharp; it can be the limit and not the lowly flatbed.

Get a favorite 4x5" SHARP original or other size and have one high end drum scanned. USE this as a reference to see what your scanner is missing; or not missing. Having a reference removes the emotion from the equation.

Flatbeds like saws and shop vac are tools. Finding out the limits of your shop vac or saw is what matters; not he marketers decals. In electic chains saws a Stihl here draws 15 amps and cuts the best; it doesnt have any horsepower decal. The home electric chain saws I have are marked with 4, 3.5, 3, 1.5 etc Horsepower decals and have nameplates of 13 thru 8 amps.

Think of "dpi" of a flatbed like the flashy Horsepower decals on chainsaws; they are peak; best case marketing fluff aimed at the home user.

With the 36" wide color 8 bit scanner here; we often do the reverse IF REDUCING. Thus a 36x48 color map thats to be shrunk down and printed 11x17" dosnt have to scanned at a 600 dpi setting. The 11x17 printer here will support a 600ppi image; but its really not much batter than one at 400 to 450. Thus we often scan at say the lowest setting of 200 dpi; when a dinky output is to be printed. A 200 dpi setting for a 36x48" scan becomes a 800 ppi image when dropped 4x to 9x12 inches. The printer high pass filters the info above 450ppi on the 11x17" printer.

Most of us know that when a person says they scanned at a 1200 dpi setting on a scanner what they mean. Most folks know also what a person means when they say they bought six treated 2x4's. Most folks know that the useable area of a 4x5 negative is not 4x5 inches. Alot of folks know that "bucks" is slcang got a dollar.

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