scanning slide resolution and dpi

[rusmi]

I'm a very beginner in scanning and I found so many infos abot dpi, ppi, resolution that I got confused. I got a flim scanner - Plustek 7200 - and I want scan 300 slides, I use Silverfast SE 6 and I need to setup the dpi for the scan and the dimension of the output in pixel. I was thinking to scan the slide using similar format of a 10mpix DSLR - say 3872x2592. I tried to change the dpi from 300 to 7200 keeping the same dimension 3872x2592, it look like the output is identical for 300 and 7200, I'm confused, any help appreciated!

[LSI_Noack]

Dear rusmi

SilverFast displays the output/printing DPI value in its box.
If you keep either CTRL or F5 (depends on your system and SF version) is pressed, you'll see the input/scanning DPI value.

Well, there a different ways of inputting things into SilverFast's scan control window, so that persons with different backgrounds in imaging can easily use whatever suits them bests.
However, you basically always generate an array of pixels (bitmap image) measuring X pixels width and Y pixels breadth, so don't you worry to much by the options and don't let you get too confused about this.

Best regards
Sonny Noack
- Manager Technical Support, LaserSoft Imaging AG -

[rusmi]

Little bit more clear but still confused, I have silverfast SE 6.5 (demo-waiting for serial from customer service), I tried input 4000pixel and press control it comes 3600dpi. Now if I open a file from 10mpix DSLR is is rough same size in pixel and dimension but 300dpi (check its property by right click on the file). And indeed if I scan the same file using 300 dpi it takes same time as 3600 dpi and result is identical.
1. Does it mean if I want an output of 4000pixel I should ignore the dpi box?
2. If I right click on the file to check the property the file scanned at 300 show 300dpi and that at 3600 show 3600 dpi, but why are the file identical?

TOM

[LSI_Muenier]

Dear Tom,

the DSLR (and in fact any image) may well have the same pixel content at a different resolution. The difference then is the physical (!) dimension of the image.

Examples:
(1) 3000 pixels at 300 dpi amount to 3000 / 300 = 10 inch
(2) 3000 pixels at 3000 dpi amount to 3000 / 3000 = 1 inch

Although the same pixel exent here, the resolution difference results in different physical sizes of 1 or 10 inch.

For the DSLR image this physical dimension is totally arbitrary and (maybe for printing purposes) chosen to be 300dpi by the manufacturer.
Your scan comes from a real (two-dimensional) physical object, and the physical extent of it is known (and preserved in the scan by its resolution).

Hope this clarifies it to some extent.

Best regards
Martin