A question about reducing dpi of a scan

[Joran]

I own a Plustek OpticFilm 8100 (7200dpi) scanner as well as an Epson V550 (6400dpi). Tests on filmscanner.info reveal that both scanners effective resolution is less than whats advertised on the box. For the Plustek they advice to scan at 7200dpi and reduce to 3800dpi afterwards. For the Epson they state scan at 3200dpi and reduce to 1560dpi.

When you just alter the dpi setting in Photoshop nothing really happens (image doesn't get smaller). In case of the Plustek, for the new width of the image, should I be use the following formula for resizing:

target_width = source_width / (source_dpi / target_dpi)

And then also set the correct dpi to 3800. Which resampling method should I be using?

 

[jbielikowski]

In Photoshop ou need to check the box next to resampling method.

 

[Joran]

Thanks!

Found out why it wasn't working at first. You need to set the units to inches instead of pixels. :bang:

 

[Scapevision]

still wrong, all you need to do is tick the resample box like the previous poster noted. Also use the "sharper" option "best for reduction".
If you don't tick the resample box then all it does is change your dpi value without affecting anything. This is useful if you want to set your image dpi to 300 for printing. As from the scan software it usually comes out embedded as whatever dpi you scan at, unless you specify to save file with output resolution of something else.

 

[jbielikowski]

I never change pixels to inches and it works. Also I find pointless to downsize pictures to size other than the final size you want. When scanning with flatbed I scan @3200dpi for 35mm and @1200dpi for 120. Then in PS apply USM 300% 1px 0 threshold. No USM for dedicated scanners and scan @3600dpi. For resampling I prefer "bicubic (smoothing gradients)" and then slight USM i.e. for web size 33% 0.3px 0 threshold.