The good answer for creating large images to print is to use a film scanner to scan the film instead of the prints. These are often called slide scanners, but they scan both slides or negatives. Most are for 35 mm film.
Scanning film is better than scanning prints, because in the first place, scanning the film is using the original image, but the print is a second generation copy. Making a copy of a negative onto photo print paper is like making a copy of a music CD onto a cassette tape. The cassette tape is not so bad, it is very usable, but it sure is not a CD. We would not choose the cassette tape as the master to make yet another copy if the CD were available.
In the second place, film has greatly more detail and contrast available. Without quibbling about the numbers, most film is capable of over 3000 dpi, compared to only about 300 dpi for color photo print paper. The 35 mm film is a smaller original, so it must be enlarged more, about 4 times more than a scanned 6×4 inch print, to get the same image size at the printer. However the film and film scanner has well over 10 times more capability to do it.
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 3400×2200 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 6×4 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.
Click button below to convert inches in centimetres
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 8×10 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.