Scanning question - resolution

[svsg]

I know there are previous threads that discussed this topic. I went through all of them,but I still have some doubts.I apologize if it repeated.

I am planning to buy a scanner, before which I would like to gather some advice/opinion. Let me explain my requirements.

My primary aim is to scan the hundreds of printed photos with me and write them into CDs, to preserve them for future.I don't own a printer and don't plan to print most of them. I want to restore/retouch the photos and may be print an occasional one.I do not want to scan film and I would maximum enlarge the photos up to 4 times on each side before printing. For the rare need to print a larger one or from film, I am ready to pay and get it done elsewhere.But I when I do print after some enlargement, I want them to be of photo quality.What (minimum)optical resolution should my scanner be? In one of the threads here I read that 300 dpi should be present at the final printing size, elsewhere I read that upto 600 dpi might be required. What is the number each of you use?

Svsg

[catia]

Hi,

Perhaps thinking about it this way will help.

Assume your maximum print size is 8x10 (just for conversation). Further assume that you always use the maximum dpi for printing of 300dpi. That means you need a 2400 x 3000 pixel image. So, if the photo you are scanning is 4x5 say, and you will NOT be cropping it, you will need to scan at 600dpi to get a 2400 x 3000 pixel file. Get it?

So as a test, what scan resolution would you need if the image you are going to scan is 2 x 2.5?



Catia

[svsg]

Assume your maximum print size is 8x10 (just for conversation). Further assume that you always use the maximum dpi for printing of 300dpi. That means you need a 2400 x 3000 pixel image. So, if the photo you are scanning is 4x5 say, and you will NOT be cropping it, you will need to scan at 600dpi to get a 2400 x 3000 pixel file. Get it?

So as a test, what scan resolution would you need if the image you are going to scan is 2 x 2.5?

I need to scan at 1200 dpi to get 8x10 from 2x2.5. Did I pass the test?

So let me clarify this from both of you photomauler and catia,

The image given to the printer as input should be around 300 dpi(rather 300 pixels per inch).
Going by this logic and given my requirement of maximum enlargement of 4 times, I need a 1200 dpi scanner. This ensures that any image magnified 4 times(by using scanning at 1200dpi , the maximum) can be printed at photo quality.

[catia]

You get a big



This is a rather simplistic way of looking at it and there are a lot of variations that can come in to play; however, this is a good start.

Catia

[G. Couch]

Actually, if you are going to be scanning prints rather than film, you only need a 600dpi scanner...prints do not have much more info than that. If you take the example of a 2"x2.5" print and you want an 8x10, you would probably get the exact same quality image by scanning at 600 dpi and sizing it up 2x in Photoshop...in fact, most prints are only going to have about 300 dpi worth of info in them and scanning at much higher resolutions is not going to result in a better image. Scanning at higher resolutions is really only needed when you are scanning transparencies.

[svsg]

Gouch,
that was really useful info. That helps me find a scanner for much less money
thanks.