Correlation in dpi from scanner to PS to printer?

[Dan Quan]

Hi, and thanks in advance for helping me to get this.

In the past when I was working on an image I made sure the 100% .psd file was 300dpi for whatever the final size was. If i had an 8x10 .psd file which was going to be printed at 8x10 then it was 300dpi. If the 8x10 .psd was to printed at 16x20 then it was either reformatted to 16x20 @ 300dpi or worked 8x10 @ 1200dpi.

Now I have 8x10 negs that I am going to print at 16x20, but the Epson R3880 printer has resolutions of 720x720, 1440x720 and 2880x1440 listed as native.

My questions are:

Are these actual dpi dimensions or am I misunderstanding?

How does final printing resolution relate to scanning resolution?

And, It would seem that printing at 2880x1440 dpi would require an incredibly large file in PS but also make a super nice print. Or is this just marketing hype and magic bullets?

Thanks for taking the time to help.

 

[Dan Quan]

I want to make sure I'm not scanning at a greater dpi than I need for the final size prints I'm planning. That would tax my system unnecessarily.

 

[cabbiinc]

It's more lingo than marketing hype, but I'm sure there is some hype in there too. The software for the printer will convert your RGB file to a CMYK file to a RIP algorithm. Most printers have (C)yan, (M)agenta, (Y)ellow and blac(K), CMYK. Some have multiples of that, but that's the basics of any 4 color process (along with a white substrate to print too anyways, which is why sometimes specifying a paper type can be helpful as the program accounts for the amount of blue or yellow already present in the paper but I'm getting off topic).

So you've got 4 inks producing 3 colors, RGB, that you feed to the program. To produce all those colors you're going to need to mix those 4 inks as well as put the inks down in the proper order on the paper. Most ink jets that I know of can't vary the size of the ink droplets, so the way you vary the color is to vary the number of microscopic droplets of any given color that you lay down.

DPI of a file is one thing, that shows how many pixels you have to work with. DPI of a printer shows how many color combinations you can come up with (sorta) for a given dot size. If you provide a 300 dpi file to print at the size to print it and the printer is capable of much higher you'll see smoother gradations between colors.

For the scanning at a certain size for output you can "get away with" much less than 300 dpi, but virtually no one has seen a problem at 300 dpi but some have seen problems at lower dpi's than that. 300 dpi is just a general rule of thumb that's a good number to try to print at. Even when you print at 300 dpi the printer is actually laying down more than 300 dots per inch, but it's nothing to really get concerned over since that's just the magic that is the printing algorithm.

 

[Dan Quan]

So, no need to scan the 8x10 negs at anything greater than 300dpi regardless of final print size?

 

[cabbiinc]

You will want to scan at at least the size that you want to print at. If you're scanning an 8x10 and plan on printing at 16x20 then minimum would be to scan at 600 dpi. But that's assuming that you're scanner's effective dpi is 100%. The Epson V700 for example only scans at effectively 40% according to this site Test report flatbed-film-scanner Epson Perfection V700 Photo with transparency unit: experiences, image quality, scanning so if you wanted to get an effective dpi of 600 dpi you'd want to scan at 1500 dpi to achieve the sharpness equal to 600 dpi. I have heard that with large format negatives it doesn't do much good, resolution wise, to scan higher than you need.

 

[Dan Quan]

WOW! I bought a $500.00 box of shit! Cool!

 

[cabbiinc]

WOW! I bought a $500.00 box of shit! Cool!

I apologize if I've led you to believe that the V700 is a box of shit. For $500 on an 8x10 scanner you'll not find anything better. But the hardware does have it's limitations. You could buy a used drum scanner, but that takes time and experience to learn to use it and you'll likely need to dedicate a computer running an older OS because drivers weren't made for anything in the last 10 years. I have nothing but the highest regard for the V series Epson scanners. I have an HP Scanjet G4050 myself and that site I linked too earlier won't even review it because it's too crappy (or maybe it's because they don't sell the HP scanner?). If I didn't have Vuescan to use with my HP scanner I would have returned it to the store. As it is it's barely passable but don't bother pixel peeping.

 

[Dan Quan]

no no, i just forgot myself, my apologies. i'm blessed to have it and i'll make due with with that and my 8x10 camera and an r3880, woe is me and all....

enough of that crap, i need to scan at about twice final output dpi you say. thats fine, a little slow on my older mac but doable.

thanks for the input.

 

[chuck94022]

You will want to scan at at least the size that you want to print at. If you're scanning an 8x10 and plan on printing at 16x20 then minimum would be to scan at 600 dpi. But that's assuming that you're scanner's effective dpi is 100%. The Epson V700 for example only scans at effectively 40% according to this site Test report flatbed-film-scanner Epson Perfection V700 Photo with transparency unit: experiences, image quality, scanning so if you wanted to get an effective dpi of 600 dpi you'd want to scan at 1500 dpi to achieve the sharpness equal to 600 dpi. I have heard that with large format negatives it doesn't do much good, resolution wise, to scan higher than you need.

This is an hugely inaccurate statement. The review you mention is talking about realistic maximum resolution, not error factor at each resolution. (There is a larger problem with the review, but more on that in a minute.) So, if you want to scan the 8x10 with the scanner set at 600 dpi, just set the scanner to that value, you do not need to scan at 1500 dpi to achieve 600 dpi quality. That's just wrong.

An Epson V700 is a great scanner for 8x10 film. You will get all the resolution you could possibly use in normal printing with very low scanner settings, well within the quality range of the Epson. And with VueScan or ScanFast software, you can scan in multiple passes with different scan exposure settings, allowing you to dig into shadows and get fine highlight detail in difficult negatives. For 8x10 you just don't need any more scanner than the Epson.

The larger problem with the ScanDig review linked above is this: The Epson v700 does not have a focuser, so you are stuck with wherever the focus is as assembled. The depth of field of the plane of sharp focus for the film scanning lenses is very, very narrow, so it is important to move the film into that plane accurately. The standard film holders provided by Epson do not give you sufficient control of film height to assure that the film is precisely in the sharpest focus area, and the focus height of v700 varies widely from unit to unit. Most scanner reviewers have not taken this into account, and their resolution evaluations are not accurate as a result.

At low scan resolutions, like 600dpi, this won't matter. The focus is quite good enough to offer accuracy. But at high resolution, say, above 1500 dpi, focus starts to matter. At 2400 dpi, images will be very soft as you would expect if the film is not in focus. Get the focus plane right and you can get good results up to at least 3200 dpi, perhaps up to 4800 dpi. You can either focus by carefully shimming the OEM holders, or do what most folks who care about this do: get a betterscanning.com film holder, which has infinitely variable adjustment, and holds the film very flat (using their universal mounting station with wet/dry option, mounting under the glass). Flatness matters a lot. With this holder I can scan 35mm up to 4800 dpi and get sharp, well detailed results with very little sharpening required in post.

You won't need this for 8x10 film though. You can scan right on the bed and get great scans for print sizes you care about.

Regarding dealing with very large image files in Photoshop: there is a trick! I learned this from a website that I no longer have the link to, but basically you do this (assumes full Photoshop, not Elements):

1. Load the full size scanned image into Photoshop.
2. Duplicate the image (Image->Duplicate I thnk). You now have two identical images in two tabs.
3. Resize of the duplicate image to something smaller, like 1200 on the long side, keeping proportions. This second image will now be consuming much less memory.
4. Do all your edits on this duplicate, using only Adjustment Layers. Use one adjustment layer per edit. Don't do edits that can't be done in adjustment layers at this stage, such as sharpening.
5. When you have everything to your liking, select and copy the adjustment layers, and paste them onto the full size image. Your edits will instantly be applied to the large image accurately.
6. Sharpen and do any other image-wide tweaks required that can't be done in adjustment layers. You are done!

I've found this to work very well for extremely large images that otherwise would bring my computer to its knees. Good luck!

 

[Dan Quan]

Well thats encouraging.

But what about scanned dpi : output size? Do I scan at my output resolution or ?

 

[chuck94022]

Well thats encouraging.

But what about scanned dpi : output size? Do I scan at my output resolution or ?

Ok. You first have to understand what the Epson spec means. 2880 dpi resolution does not mean 2880 fully toned image pixels. It means an individual monotoned dot. An inkjet uses many dots to make one pixel. For purposes of scanning, you should be thinking in fully toned pixels. Here is a link to a web page that discusses this: Inkjet Resolution (Different printer, same concepts.)

You probably want something around 300 ppi in your final print. Depending on the printer (read the linked page) you might find some odd number like 288 to be optimum considering the printer's internal interpolation. Or something slightly higher, like 360 ppi. More than that won't contribute to image quality, the pixels will just get combined.

So let's say you are going to print at 300 dpi for a 16x20 print. You need to scan your 8x10 at 600 dpi. Pretty simple. If you are going to print it at 8x10, just scan at 300 dpi.

If you are going to print a crop of your image, say, a 4x5 portion, you'll have to scan at 1200 dpi. (16 divided by 4 = 4, which is your enlargement factor, so 300x4).

Make sense?

 

[chuck94022]

By the way, another nice thing about big negatives is you shouldn't find yourself scanning down to the level grain resolution (where an individual ccd sensor's scan spot approaches the size of film grain). The problem with scanning at higher resolutions is not the grain in the film, but the fact that a scanner photosite (individual sensor) sees an area with part of, or a whole grain, or perhaps pieces of two grains. It can only calculate one value for this, so it produces some average tonal value across the whole area of the sensor. Thus, what might have been a tiny grain is now a grain "spot" the size of the whole sensor area. The tone may be an average across the grain and the clear film area around it, or whatever, but it ends up effectively enlarging the grain it saw incorrectly.

This is why optical printing in a darkroom will show less grain for the same size enlargement. Also, for smaller formats where scanning must be at the "grain level", getting the finest resolution scan is best. So for small format negatives that are printed to hang on a gallery wall, especially large, best to either optically print or get the negative drum scanned (a drum scanner's individual "photosite" is extremely small, so there should be much less grain enlargement).

For an 8x10 negative, you're likely to never scan it at a scanner resolution that will see and magnify individual grains, unless you're making billboards, and even then, the viewer won't see the grain as they rush by in their car, 50 meters away.

 

[chuck94022]

... And with VueScan or ScanFast software, you can scan in multiple passes with different scan exposure settings, allowing you to dig into shadows and get fine highlight detail in difficult negatives. ...

ScanFast? Sorry, I meant SilverFast!

 

[Dan Quan]

Well this is encouraging. I'll check out the Silverfast demos first.

 

[artobest]

First of all, try not to get confused with terminology: dpi is properly a printing term (referring literally to dots of ink on paper), but image file resolution should be expressed in ppi (pixels per inch). However, even scanner manufacturers get this distinction wrong! Those big numbers you used earlier refer to dithering resolution - how accurately the machine can lay down its droplets ink. But Epson printers have an actual printing resolution of 360 dpi (720 in fine mode); for HP and Canon printers, it's 300/600 dpi. (Incidentally, Epson machines can produce variable droplet sizes - I think there are two sizes, from memory.) Some people believe that sending a file to the printer in its native resolution or a multiple thereof (but not too big a multiple) may produce slightly better results. It costs nothing to follow this method, which, if nothing else, does remove at least one resampling step from the process.

Me, I keep things simple - I print through Qimage, and let it take care of all the sizing issues automatically. Its algorithms are sufficiently robust that I'm never disappointed.

Lastly, Chuck is right - the Epson is a decent machine let down by tricky focus issues that have defeated many lazy or ill-informed reviewers - but when you get those sorted (through the betterscanning holders) you can achieve nice scans that translate into terrific prints.

 

[cabbiinc]

I don't get it. Chuck says I'm inaccurate yet then goes on to explain sharpness and how you've got to go the extra mile to get things sharp because the scanner has no focusing and isn't all that sharp. I guess I should have said specifically that there's not focusing. Oh well, at least we all agree that the Epson is NOT a bad scanner.

Regards

 

[chuck94022]

I don't get it. Chuck says I'm inaccurate yet then goes on to explain sharpness and how you've got to go the extra mile to get things sharp because the scanner has no focusing and isn't all that sharp. I guess I should have said specifically that there's not focusing. Oh well, at least we all agree that the Epson is NOT a bad scanner.

Regards

Cabbiinc, here is the part that I was referring to when I said you were innaccurate:

The Epson V700 for example only scans at effectively 40% according to this site <link> so if you wanted to get an effective dpi of 600 dpi you'd want to scan at 1500 dpi to achieve the sharpness equal to 600 dpi.

This is not correct The Epson does not "scan at efectively 40%". That is not the source of the softness problem. With an epson, if you scan at 600 dpi you will get a 600 dpi image. You do not have to scan at 1500 dpi to get that. The issue with the Epson (and most scanners) is that their manufacturer overstates the maximum resolution possible. In the case of the Epson, they claim 6400 dpi resolution. They claim this because their mechanical stepping motor can move the scanner head in 6400 dpi increments. But the optical system doesn't resolve that fine, even when perfectly focused, so after about 3200 dpi, you won't get much improvement. But that doesn't mean that scans at lower resolutions will suffer by the same factor. It is just that the highest resolutions don't produce much more detail, if any. If the manufacturer claimed their maximum resolution was 2400, the reviewer would have said it scans at 100% of the manufacturer's claimed resolution, not 40%. But because of the need to focus, the results would still be soft.

So I took specific issue with your statement that to get a sharp 600 dpi image you had to scan at 1500 dpi, due to this "40%" statement by the reviewer.

However, there is a completely separate issue with the Epson. It has no built in focuser (unlike the Nikon Coolscan, for example). And further, Epson doesn't focus their equipment well at the factory. So when you get your v700, is is most likely to be way out of focus for the film holders. Epson gives you a small amount of adjustment with the OEM holders, but that adjustment has been found to be ineffective. What you need with the Epson is a film holder that provides fully variable height adjustment, and usually (but not always) that adjustment has to lift the holder significantly higher than the OEM holders can reach.

Without doing that, all your scans, at all resolutions, will be softer than they should be, and require lots of sharpening after scanning. But that's a different issue from the issue of scanner resolution. Your approach mixes apples and oranges. I suppose you could scan at higher resolution and reduce in Photoshop, because a side effect of image reduction is naturally a sharpening. But that's not a great way to deal with image softness. It is leveraging a side effect, which doesn't give you much (if any) control. Better to fix the real problem, which is scanner focus.

 

[Dan Quan]

I just wat you to know I am not ignoring this thread, I am scanning some old 8x10 negs and becoming a little more experienced with the scanner and software. But on my monitor at least, I see an enormous gain in detail between 600 and 4800 res.

 

[chuck94022]

I just wat you to know I am not ignoring this thread, I am scanning some old 8x10 negs and becoming a little more experienced with the scanner and software. But on my monitor at least, I see an enormous gain in detail between 600 and 4800 res.

As you should! But unless you are printing a huge print, that detail will be lost when the image is printed at any normal size. And if optically printed and viewed at normal distances your eye can't see the detail either.