Read from here on CMYK etc http://www.scantips.com/basics15.html

extract from http://www.scantips.com/basics12b.html

A 35 mm slide frame is perhaps 1.3 x 0.84 inches if in a cardboard mount. At 600 dpi, you would get no more than a (600 x 1.3) x (600 x 0.84) = 780x506 pixel image from full frame. A 1200 dpi scanner gives images twice that size, maybe 1560x1012 pixels from mounted 35 mm slides, full frame. Either image size might be more than enough for web pages or the screen, but the number of pixels is insufficient for printing large prints.

Said another way, the ratio of (scanning resolution / printing resolution) is the enlargement factor of the film size when printed on paper. A 35 mm film frame is 36x24 mm, and medium film is called 6x6 cm, but the frame is 56 mm square. Using these numbers, and assuming we are scaling to print at 250 dpi, then we can compute this chart:

This assumes full frame, and any cropping reduces this size. You can of course always print at more or less than 250 dpi to alter the printed size that way too. The short side of 35 mm film is normally the limiting factor for printing computations

Negatives from http://www.scantips.com/basics13.html

Image size and memory cost can be quite huge when scanning film, because you are realistically able to scan at very high resolution. The huge size is the entire point, for example to create enough pixels to print full page size. You will want at least 128 megabytes of memory, and more is better. But a film scanner definitely does allow acquiring enough quality pixels to scale to print a large image.

For example, a full frame 35 mm color negative scanned at 2400 dpi will be about 3400x2200 pixels, and about 22 megabytes. Scanning at 2400 dpi and printing at 300 dpi allows enlarging that printed image 8 times more than the original film size (2400/300 = 8). Scaling by 8, so that the 1.4 x 0.9 inch film size (36 x 24 mm) prints 8X larger gives 11.2 x 7.2 inches. It will look great in regard to detail if printed at 200 to 300 dpi (assuming the printer can handle it). Scanning film originals can support this level of detail. Scanning a 6x4 inch photo will not.

Let's quickly review scaling again, to make the point about large images, and to make sure the simple arithmetic is understood. The basic fact is that dpi means "pixels per inch". The main point is that the image size in inches is computed from the image size in pixels, using resolution to space those pixels on paper.

The ratio of (scanning resolution / printing resolution) gives the enlargement factor. If scanning at 2700 dpi, and printing at 240 dpi, then the printed image is 2700/240 = 11.2 times larger than the original film. We can adjust the printed size by varying the printing resolution, maybe 200 or 300 dpi instead of 240 dpi.

Saying the same thing another way to make sure it is clear: If we scan 1.4 inches of 35 mm film at 2700 dpi, then we get (1.4 inches x 2700 dpi) = 3780 pixels. If we print 3780 pixels at 240 dpi on paper, then that image size is (3780 pixels / 240 dpi) = 15.7 inches. 15.7 inches is 11.2 times larger than 1.4 inches. Large images in pixels are needed to print large images in inches.

For example, to print 8x10 inches at 240 dpi requires (8 inches x 240 dpi) x (10 inches x 240 dpi) = 1920 x 2400 pixels. It takes (1920 pixels / 0.9 inches) = 2135 dpi to create this image from 35 mm film (full frame, so even more if it is cropped).

We do need large images to print large at high scaled resolution. Film scanners will give us those large images while retaining very good image quality.