DPI

[HLBJ]

Hi everyone. Sorry I am newbie. Please, I have some questions about
"dpi".
I have some letuce seedlings. I would like to measure the length of
those seedlings.
Please how should i get the images? By scanner or camera? How many dpi
is correct to get it? I would really appreciate if the advanced users
could help me at the first steps.

Thanks in advance.

Hodair