jeryh Loc: Oxfordshire UK

For trannies, I found 2400 was really the best for 6X6' . For 35mm, between 2700-4000.And yes, it is slow, but bearing in mind you are using a flat bed scanner ! I also found that using some pre cut plate glass to keep the 6x6 trannies perfectly flat made a world of difference.

selmslie Loc: Fernandina Beach, FL, USA

Confusion reigns. That's not the optical resolution, it's the scan resolution. A lens limits the optical resolution just like it does with a digital camera. The review did not measure that. They just took Epson's word for it.

You can see at ScanDig that the Epson flatbed optical resolution is about 2300 dpi even though the scan resolution can be set much higher. ScanDig actually measured the optical resolution. They did not just read the spec sheet.

After a point (about 2x the optical resolution) any increase in scan resolution will be limited by the optics and you will not see any improvement, just larger files.

The same thing happens when you use an inferior lens with a 36-50 MP sensor. The images don't get any better but the files get a lot larger.

selmslie Loc: Fernandina Beach, FL, USA

That may be even more than you need for most purposes. See the second tab in the attached spreadsheet.

For 6x6 cm, about 1200 dpi is enough but there is no reason not to use 2400.

It gets even better with 4x5 film (third tab).

Scan resolution needed.xlsx
Attached file:
(Download)

amfoto1 Loc: San Jose, Calif. USA

Use the highest possible resolution for your best slides... anything you might want to work on with editing software, in particular.

Scanner resolution (input) IS NOT the same as print resolution (output).... 300 ppi (not dpi) is plenty for most printing processes, but way, way too little for scanning.

The actual size of the image on a slide is roughly 1" by 1.5".... scanner resolution of 4000 ppi gives an image 4000 x 6000 pixels. Does that sound familiar? It's what a 24MP DSLR captures. 4000 x 6000 is a common resolution used in a lot of DSLRs now.

What takes more time and really increases file size in scanning is over-sampling. When I scan slides with my Nikon 4000, I set it to 4000 ppi and 16X over-sampling. Each time the image is re-scanned (ovesampled), a little more detail is found and added to the image. It takes 10 or 15 minutes per image to scan. It also generates a 130MP 16-bit TIFF file! For a quality image that I want to do some work on and print, I wouldn't scan at any lower resolution. Sometimes I wish I had even more. There are 5400 ppi scanners... and even 9000 ppi and higher. Rather than buy one of the latter (expensive), for a really special image I might have a 9000 ppi scan done. That will probably cost about $100 per image.

Note: "dpi" is a printing term... the "dots per inch" the printer is laying down. An offset printing press used by a newspaper usually is around 90 to 120 dpi. A high quality magazine or book might be printed at 170 to 190 dpi. A photo quality inkjet usually prints at 720 dpi to 1440 dpi.

"ppi" is a whole different animal... I'm sure you're aware it means "pixels per inch". Think of it as digital, versus dpi is analog.

When you make a print from a digital image, you are basically converting it from digital to analog. Scanning an image is the opposite. You are taking an analog image and making it into digital.

BTW, you mention 300 ppi, which is what a lot of people (me included) use for printing. In fact, the recommended resolution for printing with an inkjet is 240 ppi. An inkjet printer basically uses three to six colors to render each pixel... mixing those colors visually... hence 720 ppi (240 x 3) or 1440 ppi (240 x 6). In simplest terms, the printer "mixes" the color of each pixel by how much of each color it sprays onto the paper. It's actually a lot more complex and the best photo quality inkjets use more colors than just the three colors. An RGB digital image "becomes" CMYK when printed (CMYK = cyan, magenta, yellow and "key", which is black).

srfmhg Loc: Marin County, CA

Thanks so much for the info and the link Don.