I have the Epson 4490 scanner. I have some slides to scan that will be printed as 4x6 photos. Any ideas as to what the best resolution would be for scanning? Thanks!

I would scan at 1200 pixels per inch unless you want to crop in which case you could go to 2400 ppi.

This is adequate for 4x6 prints and higher resolution will increase scan times without a major quality improvement.

Note: your scan software probably call it dpi. They do mean ppi. 1200 ppi in an RGB scan would end up printing as 4800 dpi (dots per inch) as each pixel will be represented by four dots (Cyan, Magenta, Yellow and Black) in the print.

The 1200 dpi is technically accurate as in an RGB display each pixel is a single dot with Red, Green and Blue components but gets split into 4 dots for printing. It can be a bit confusing. Understanding how a pixel morphs when going from a screen display to a print should help a bit.

What are you planning on doing with the scans?

We scan slides for major publishers all the time, and it's very rare to see them want higher than 400dpi anymore.

Thanks EMacMan. I'll give it a try. I've been printing samples on my HP laser printer and they all look pretty bad. I am assuming it is the printer and that professionally printed photos will look best.

What are you planning on doing with the scans?

We scan slides for major publishers all the time, and it's very rare to see them want higher than 400dpi anymore.

The slides will be printed as 4x6 photos. Is 400 dpi adequate?And should the unsharp mask be off?

An hour here might help you.

Scanning Basics 101 - All about digital images (http://www.scantips.com/)

The 1200 ppi scan will produce a roughly 1200x1800 pixel image. This works out to about 300 pixels per inch which is about the maximum that most photolabs can take advantage of.

If I was scanning for an Epson inkjet printer I would probably go with a 1440 ppi scan. But all other inkjets should be fine with a final image of 1200x1800 pixels.

Unsharp mask should not be applied at the time of scanning. Do all other colour and density corrections THEN apply any sharpening filters, as lightly as possible, being sure you are viewing image at 100% or 50%. Odd viewing ratios will give you a false impression of whether or not sharpening is required.

Also be sure to calibrate your monitor. Set your back ground to neutral gray. Go to SytemPreferences>Display>Color>Calibrate. Be sure to set Yellow point to "Native" and Gamma to PC setting. The gray desktop will make it much easier when calibrating the density and colour.

The Gamma setting is very important as the photolab and even your personal printer assume you will be using PC settings.

Thank you EMacMan. My next question would be about resizing. I am scanning at 1200 dpi and changing the target size to 4x6 before I scan. Should I be changing the size after I scan?

^^^
When I do such things - I scan at high dpi settings, usually 2400 dpi. I then downconvert them to whatever I need; by first doing any cropping or adjusting I need to do, then by changing the size to what I want, then by downconverting the dpi. I keep the "original scans" just in case I want them later. I use Graphic Converter X for such work - the scanner software is rarely very good for small edits, cropping, or adjusting colours, and generally have only a small number of file formats they can export to.

As for final size, that depends on what size you want, like 4x6 or 3x5 or whatever. DPI is set by the device I want to dump to. For my printers, I stick to 300dpi; though if it something for rough work for my laser, I might go 150dpi; and if it is just for the screen or a photo frame or web site, then I would go 75dpi.

I would scan at original size first, at high DPIs - then resize the scan - then change the dpi last.

Thank you EMacMan. My next question would be about resizing. I am scanning at 1200 dpi and changing the target size to 4x6 before I scan. Should I be changing the size after I scan?

I believe if target size is 4x6 then scan should be 300 dpi. Check the scanned image, you want to see something around 1200x1800 pixels to deliver a good 4x6 print.

As Evan said you can go to say 2400x3600 pixels. With my Epson scanner I have never found it worth quadrupling the scan time, but to each their own. However it is well worth the time if you wish to crop down from the original image. Again Evan is right that the place to do your cropping is with an app such as Graphic Converter or PhotoShop Elements not in the original scanning.

Thanks Evan and EMacMan.
Here's what I did:
I scanned the slides at 2400 dpi and retained their original size.
I then opened them up in photoshop and resized, repaired, sharpned, etc...
I then saved the original and the edit in separate files, ran over to the local walmart and printed some samples of each. I asked someone who had no idea what I'd done to tell me of they saw any difference in the photos. To the untrained eye, there was only a slightly discernable difference in brightness. Of course, I noticed the other differences. :)

Thanks for all your help!