Resolution of a Scanner

[Playdo]

I'm looking at getting a scanner. It's mainly for scanning illustrations and linework but could come in handy for the odd photograph. I'm a little confused looking at the specs: http://www.usa.canon.com/consumer/controll...p;modelid=17104. It states it is an A4 scanner with a resolution of 2400dpi x 4800dpi. I presume what they mean is that it can scan A4 at 2400x4800 pixels? How do I know the actual maximum scanning resolution ie. 300 ppi or 600 ppi? (I've calculated it at 345 ppi but I'm not sure if it's right)

The second one: http://www.usa.canon.com/consumer/controll...p;modelid=17103 is an A4 with 4800dpi x 4800dpi. Is this beneficial?

[Mark D Segal]

Firstly, and this confuses many people, in scanner-speak, manufacturers often use the term "dpi" when what they really mean is "PPI". I'm assuming that's the case here, because it's a conventional error.

The number is the maximum hardware-based input resolution of the scanner. So 4800*2400 PPI means it can scan, optically, 4800 PPI for the long dimension and 2400 for the short. A scanner specified at 4800*4800 would scan the same 4800 maximum PPI on both dimensions. Depending on the originals being scanned, you may or may not ever need such high resolution. Examples: (i) you are scanning a 35mm slide which has input dimensions of about 1.5 by 1 inches, without the slide mount. Set at 4800 PPI, the scanner will deliver a total of 7200 pixels on that dimension (4800*1.5). Let us say the maximum size of the output you will make from that scan is 20 inches on that dimension. So you have 7200 pixels to fill 20 inches. 7200/20 = 360 OUTPUT PPI, which would print very nicely at that size. (ii) Your orignal is a sheet of paper 11 inches long and you will output an image 11 inches long. You only need to print it at 240PPI. You can select an INPUT scan resolution of 240 PPI, because there is no magnification between input and output. If you were to select something like 600 PPI you would be generating a huge amout of needless data. So selecting the appropriate INPUT resolution means knowing something about the relative size of the OUTPUT document. As you can see from these examples, 4800 PPI gives you quite a substantial playing field.

[Playdo]

So according to that. For the scanner with a 2400ppi x 4800ppi resolution: If it scans a whole A4 page (8.3" x 11.7") then it will produce (8.3 x 2400) x (11.7 x 4800) = 19920 pixels (W) x 56160 pixels (H).

I'm looking to use it to scan lineart. I've read in several places that it should be scanned at a minimum of 900 ppi and is typically scanned at 1200ppi. How do I calculate if I can get 1200ppi out of the pixels given above?

[Mark D Segal]

Those instructions are probably talking about INPUT resolution; if so, you select that resolution for the scan - far below the maximum the scanner is capable of generating. But not being clear about the dimensions of the input or the outpu tor what kind of output printing process will be used, it's hard to say whether those numbers make sense.

[Playdo]

Re: Scanner - 2400ppi x 4800ppi

Yes it's for input so the A4 lineart will be scanned in at 1200 ppi. Then the output will probably be 300dpi at A3 size. Is it more than adequate?

And if I was going to scan in 5x7 photographs and blow them up to A3?

[ErikKaffehr]

Hi!

It's not 300 DPI, it's still PPI ;-), forget about the DPI-stuff, it's totally irrelevant.

A couple of ways to see it:

A3 is twice the area compared to A4, so you would need sqrt(2) times the resolution, something like 300 PPI * 1.414 about 424 PPI. So the 1200 PPI gives you pretty good margin. You may consider using 480 PPI for output if you have an Epson, that's the internal resolution they seem to have. That would be 678 PPI needed on input, 1200 PPI is still safe.

I'd be a bit cautious about scanning photographs. You cannot blow up what 'ain't there'. So if you have 5x7 prints, your mileage may vary. If you are lucky enough to have 5x7 negatives or slides I'd consider going to 2400 PPI.

Something you should consider is output sharpening. When processing sharpening is needed in a couple of steps:

1) Capture sharpening compensates for loss of contrast in camera-lens, film, scanner etc.
2) Output sharpening correct for loss of contrast in the print process

Real World Image Sharpening by Bruce Frazer and Jeff Schewe is a good source of information (or just use Lightroom for printing, it has all built in ;-).

Best regards
Erik

Re: Scanner - 2400ppi x 4800ppi

Yes it's for input so the A4 lineart will be scanned in at 1200 ppi. Then the output will probably be 300dpi at A3 size. Is it more than adequate?

And if I was going to scan in 5x7 photographs and blow them up to A3?

[Mark D Segal]

Re: Scanner - 2400ppi x 4800ppi

Yes it's for input so the A4 lineart will be scanned in at 1200 ppi. Then the output will probably be 300dpi at A3 size. Is it more than adequate?

And if I was going to scan in 5x7 photographs and blow them up to A3?

I don't know what size your "A4" line art is,. but I'll suppose for this example the long dimension of image material is 10 inches. I don't know what size the image you will want on the A3 sheet, but let us say the long dimension is 15 inches. (Both of these dimensions are image dimensions exclusing borders on input and output.) You will scan at 1200 PPI INPUT resolution, giving you a total of 12000 pixels (10 inches * 1200 PPI). You are spreading those 12,000 pixels accross 15 inches of output. The OUTPUT resolution in PPI will therefore be 12,000 pixels/15 inches = 800 PPI OUTPUT resolution. But you only need 300 PPI - so yes, massive overkill - you would never send 800 PPI to a printer.

Now, turning to your 5*7 original. Firstly, if you have the negative and your scanner has a film scanning capability, you'll get much results scanning the film than scanning the photograph. Wet darkroom photographs may have at most about 200 PPI worth of output resolution if that much, and fewer distinguishable levels of tonality than a scanned negative if scanned at high resolution.

But let us assume you don't have either the negative or film-capable scanner, so you will scan the 5*7 inch photograph. I'll assume your long dimension of the photo is 7 inches excluding borders, and the long dimension of the A3 image without borders is 15 inches. I'll assume you want to print the scan at 360 PPI, which would be ideal for an Epson printer, but you could use anything in the range of 240 to 480 PPI. At 360 PPI for 15 inches of long dimension you need 15 inches * 360 PPI, or 5400 total OUTPUT pixels. Your original is 7 inches long. Therefore it needs to be scanned at an INPUT resolution of 5400 pixels/ 7 inches, or 771 (say 800) PPI.  

Remember the formulas I'm showing you here and you can select any scan resolution with confidence to get coherence between the size of the original and the intended size of the output. Go for more than less (give yourself a margin on the up-side) because you may want to crop the image later, or you may want to make yet a bigger enlargement of the whole image at some time. But resist the temptation to go wildly over-board on input resolution.

Hope this helps.

[Playdo]

Excellent information. I didn't realise that it is calculated off of the longest dimension. BTW the lineart is scanned in at a high res then immediately downsized to 300ppi. It keeps detail and smooth edges.

[Mark D Segal]

Thanks.

You can use either dimension as long as you keep yourself consistent between input and output and the scanner's resolution capability for the chosen dimension.

Scanning at much higher resolution than you need and then downsizing will not necessarily produce a better scan than scanning at the right resolution. I've experimented with a number of such options and can't tell the difference (or see really minimal differences), provided the eventual output resolution is high enough for a good print (but not excessively high relative to what the printer is designed to handle). This advice is also confirmed in the SilverFast manual for Ai Studio 6.x, and they have of course tested this issue quite extensively.

[Eric Myrvaagnes]

Thanks.

You can use either dimension as long as you keep yourself consistent between input and output and the scanner's resolution capability for the chosen dimension.

Scanning at much higher resolution than you need and then downsizing will not necessarily produce a better scan than scanning at the right resolution. I've experimented with a number of such options and can't tell the difference (or see really minimal differences), provided the eventual output resolution is high enough for a good print (but not excessively high relative to what the printer is designed to handle). This advice is also confirmed in the SilverFast manual for Ai Studio 6.x, and they have of course tested this issue quite extensively.

I would simply add that most good scanning software that I'm aware of (such as Silverfast) asks the user to specify magnification and output resolution, so it does Mark's calculations for you internally, and you don't have to worry (as long as your sacanner has sufficient resolution available -- and the posts above suggest that you have plenty).

Eric

[Mark D Segal]

Eric, yes you are correct - SilverFast and other good scanner software does the math for you, but it's always good for users to understand the basics of what they are doing/reading - and indeed what their software is doing/saying.

[michaelnotar]

Firstly, and this confuses many people, in scanner-speak, manufacturers often use the term "dpi" when what they really mean is "PPI". I'm assuming that's the case here, because it's a conventional error.

The number is the maximum hardware-based input resolution of the scanner. So 4800*2400 PPI means it can scan, optically, 4800 PPI for the long dimension and 2400 for the short. A scanner specified at 4800*4800 would scan the same 4800 maximum PPI on both dimensions. Depending on the originals being scanned, you may or may not ever need such high resolution. Examples: (i) you are scanning a 35mm slide which has input dimensions of about 1.5 by 1 inches, without the slide mount. Set at 4800 PPI, the scanner will deliver a total of 7200 pixels on that dimension (4800*1.5). Let us say the maximum size of the output you will make from that scan is 20 inches on that dimension. So you have 7200 pixels to fill 20 inches. 7200/20 = 360 OUTPUT PPI, which would print very nicely at that size. (ii) Your orignal is a sheet of paper 11 inches long and you will output an image 11 inches long. You only need to print it at 240PPI. You can select an INPUT scan resolution of 240 PPI, because there is no magnification between input and output. If you were to select something like 600 PPI you would be generating a huge amout of needless data. So selecting the appropriate INPUT resolution means knowing something about the relative size of the OUTPUT document. As you can see from these examples, 4800 PPI gives you quite a substantial playing field.

politely, i heard, and its been along time, from the director of the digi labs at our school (A reputable school) that in such case the true resolution of the scanner is 2400 PPI and to get the 4800 number its just 2 steps, cuz the scan bar moves across the image. so the 2400 number is the limiting factor of a scanner. so a 35mm slide was scanned at 2400 ppi which made an 8x12 at 300. but i havent scanned a whole lot and perhaps you can use the 2400x4800 resolution with good results.

[Mark D Segal]

politely, i heard, and its been along time, from the director of the digi labs at our school (A reputable school) that in such case the true resolution of the scanner is 2400 PPI and to get the 4800 number its just 2 steps, cuz the scan bar moves across the image. so the 2400 number is the limiting factor of a scanner. so a 35mm slide was scanned at 2400 ppi which made an 8x12 at 300. but i havent scanned a whole lot and perhaps you can use the 2400x4800 resolution with good results.

Well, that may or may not have been the case at the time you received this advice - hard to tell exactly what was going on under the hood without knowing more specifics about the scanner model and how it was designed. But in a way, it doesn't matter. At least two things do matter: (i) whether there is truth in advertising, and (ii) whether the stated resolution is said to be OPTICAL resolution or INTERPOLATED resolution. Truth in advertising of course simply means that if the manufacturer advertises the optical resolution to be 4800*2400, then it should be that. The distinction between optical resolution and interpolated resolution is that optical resolution is supposed to be the PPI which is created directly by what the lens captures from the scanned media. Interpolated resolution is the additional PPI which get generated by mathematical expansion of the optically scanned data. The highest quality data will be that which is optically scanned without mathematical expansion, hence our reliance on optical resolution as one important indicator of scanner capability, amongst others.

You can definitely use 2400*4800 with VERY good results depending on the size of the media being scanned and the magnification ratio.