Right dpi value for photo scanning ? 1

[TSch]

Hi folks,

I'd like to scan a lot of old 10x15 cm photos and now I'm facing the question which dpi setting to use for scanning.

It is important for me - if possible - to be able to print the photos with at least 30x40 cm or more after scanning them and to be able to watch them on my screen with a resolution of 1280x1024 pixel or better 1600x1400 and still have good quality ...

But what would be the right dpi setting for this ?

300 ?
400 ?
600 ?
1200 ?
2400 ?
more ?

Regards
Thomas

 

[eugenetyson]

The thing with photographs is that they are "originals". They have no "screen" effect in them that you get with a lithographic print. A photograph, unlike a printed magazine or newspaper, can be scanned to a size larger than the original photo.

For printing from your own printer or even for a lithographic printing you would need to have 300 dpi, this is the standard size of dots per inch you need.

However, you scan the photo at 10cm x 15cm by 300 dpi and you can only print it at 300 dpi.

So if you wanted to print it 20 x 30 cm at 300 dpi:

You either need to double the Finsihed size to: 20 cm x 30 cm by 300 dpi, or

You could double the DPI to 600 and the Finished size 10cm x 15cm. You can still print it at 20cm x 30 cm at 300 dpi with this.

So if you wanted to have these images for printing at 30 x 40 cm from a 10 x 15 cm photo then

You need to triple the size of your finished size to 10x3 = 30, and 15x3 = 45.

You can't, by ratio, reach a Finished Size of 30 x 40, without squashing the image, which you don't want to do. You can get it to 40 in eight if you crop 2.5 cm from the top and bottom.

If you scan your image at the Original Size (10 x 15 cm) at at 900 dpi it will print at 30 x 45 cm at 300 dpi.

It's different if you are scanning from a book or magazine, or any other preprinted material other than a photograph. The images are made up of CMYK dots that run at different angles. So if you have a descreening option in your menu then use it.

A newspaper descreens at around 72-100 lpi (not the same as dpi), A book or magazine at 150 lpi and a Art type book at 175 dpi. You do not need to descreen an original photograph.

 

[eugenetyson]

I meant to say in the first paragraph:

The thing with photographs is that they are "originals". They have no "screen" effect in them that you get with a lithographic print. A photograph, unlike a printed magazine or newspaper, can be scanned to a size larger than the original photo without too many problems.

 

[TSch]

Hi,

thanks a lot for the help !

Regards
Thomas

 

[stduc]

A few points you may want to bear in mind.

Scan at the highest resolution you can that you can tolerate with regard to the final file size and scanning time as printers are constantly improving.

Pay some attention to the format you decide to save the images in. If you, say, use TIFF format the file sizes will be huge, but the image will not suffer. On the other hand if you decide to use jpg format then remember this is a "lossy" format and so each time you open, edit and save an image it deteriorates, how much depends on the compression level you set when saving. The files will be much smaller though.

There is no point in you scanning using a resolution higher than the scanners 'native' resolution.

[navy]When I married "Miss Right" I didn't realise her first name was 'always'. LOL[/navy]

 

[eugenetyson]

I would only scan to the size that I want to output at. Rule of thumb with good scanning is to scan exactly what you intend to output. If you scan at 1200 dpi and you need to reduce it to 300 for some reason then you will lose data from the image, as small and as little as it may be, it damages the photo's data. As well scanning it in the right way up. If you rotate the photo after you scan it in it loses data. So scan as you intend to output.

I would scan it to a TIFF first, and then open it to a Photo editing package and then Save As a JPG. At least that way you have one very good file that is an excellent backup of what you scanned so you don't have to scan again. I wouldn't ever recommend scanning to JPG.

You're best bet is to set the scan size to 300% and select your DPI as 300 dpi. 300% will give you the correct size for the photo you want. Again, there is no point in scanning past your printers "native resolution".

What way are you going to output this? What software do you have for printing the image?

 

[TSch]

I usually use an old Paint Shop Pro Version 5 or Irfan View.

I tried a test scan at 600 dpi (There were no other settings available and it created me a 100 mb tif File with a 6000x8000 resolution .... Yikes ...

 

[eugenetyson]

A 10 x 15 cm scanned at 900 dpi should result in a 71.8mb file. When you save the file the file size should reduce.

It should also be 3543 w x 5316 h pixels

Not surprisingly if you scan it at 30 x 45 cm at 300 dpi. The file size is 71.8mb.

And it is also 3543 w x 5315 h pixels.

This is the physical file size it's going to be for the size scan you want.

Again when you save it to TIFF with LZW compression it will reduce the final file size. And you can save it as a JPEG which will reduce the file size significantly, but it will also degrade the image slightly.

 

[stduc]

IMHO you need to upgrade to at least PaintShop Pro 9. You then need to spend some time learning how to use it. Especially in respect to colour correction etc.

Check for an updated driver for your scanner, you should be able change every feature of the scan - from DPI to print size, colour balance to brightness.

Unless you are a real expert, or a magician, you will not be able to place your images squarely on the scanner. Paintshop Pro 9 has a handy 'straighten' feature. Scan a small margin around the photo so as not to lose any image edges when straightening. You can then crop the image when it is straight.

I would only scan to the size that I want to output at. Rule of thumb with good scanning is to scan exactly what you intend to output. If you scan at 1200 dpi and you need to reduce it to 300 for some reason then you will lose data from the image, as small and as little as it may be, it damages the photo's data. As well scanning it in the right way up. If you rotate the photo after you scan it in it loses data. So scan as you intend to output.

I beg to differ. Rotation causes no loss or damage if you use the right software. Don't use XP to rotate the image though - that will damage it!
If you scan and keep at 1200 then use that image. When you get a 1200 dpi printer - great! Meanwhile if you print it at 300 dpi it will be just as good as if you scanned it at 300spi. The only good reason to reduce the image is for emailing purposes.
If you have the disk space - scan and save the images in TIFF format in a folder called "originals". Work on the images and save in another folder. Then, at any later time you can process them again without needing to re-scan them. Why would you do that? Because you found a better way of doing it perhaps?


[navy]When I married "Miss Right" I didn't realise her first name was 'always'. LOL[/navy]

 

[eugenetyson]

What's the point in scanning a 1200 dpi image if you don't have the means to print it out. 300 dpi is more than enough for printing. The only reason you would need to scan 1200 is for Line Art or drawings, to prevent jaggies. For photographs 300 dpi is really perfect for the job. 1200 dpi will just increase the size of the file and not really add anything to the quality. Infact it could blur the image because the CCD of most scanners can't really scan at 1200, the scanner interopolates the image, which can be done in a good photo editing program.

Trust me when I say that 300 dpi is the right dpi for printing photographs. Consult any books, consult any website, consult any printing company with a prepress department. It is a complete waste of time to scan a photograph at the maximum resolution allowed by the scanner. Scan for either 300 dpi with the Target Size being what you want, or scan at the Original Size, but multiply the dpi by what you want the dimensions to be, see above.