We prefer to scan from your original Transparencies since it is the finest grain film, however we do work from 35mm slides, 35mm negatives (much grainier than slides), reflective photos (also grainy). Original sizes up to 12x18 Drum scanning up to 18,000 dpi. and Flatbed scanning up to 20,000 dpi.
General rule on dpi resolutions:
100dpi web applications
200dpi large format display graphics
300dpi high res offset printing.
More resolution is always better than having too low.