- PPI, SPI and DPI: units for resolution
- Photos (paper prints)
- For digital output (screens, monitors, smart-phones etc.)
- Slides and negatives
- Adobe® Super Resolution
What would be the best resolution for digitizing analogue images (slides, negatives, photo prints etc.)? The answer is:
It depends on the original and what are you going to do with the scanned image.
Quite a few people confuse the image size with the image resolution and speak of a “high resolution” when they really only mean a file with many pixels. Resolution actually only refers to the number of total pixels on the edge of the picture, the unit of measurement is called dots per inch (dpi) or pixels per inch (ppi). The resolution provides information about the fineness of the individual pixels – not about their quantity.
The resolution and the actual number of pixels of an image are decisive properties of images. The amount of pixels in an image cannot easily be changed, especially not enlarged.
PPI, SPI and DPI: units for resolution
The resolution of a scanner is specified in PPI, SPI or DPI. All
these terms represent units for resolution based on an inch
(2.54 cm). The difference lies in the area in which the unit is used.
PPI = pixels per inch
This unit is used when displaying an image on a monitor. It indicates the number of pixels per inches of an image when displayed on the screen.
A pixel is the smallest picture element that a screen can display.
SPI = samples per inch
SPI represent the number of samples (sampling points) a scanner can read out from an original image. A sample is the smallest picture element that a scanner can detect.
DPI = dots per inch
DPI determines the resolution of a printer. This is the number of dots a printer can apply per inch on the print medium. In inkjet printers, one dot corresponds to one ink droplet. Less than a drop cannot be applied.
An inkjet printer with a high resolution must therefore be able to produce very small droplets.
Unfortunately, hardly anyone adheres to these theoretical definitions of resolution. It’s just of academic interest, whether the resolution of a scanner is stated 4,000 DPI, PPI or correctly SPI. Even the units used in common image editing programs are all in a tumble. Therefore, I am going to use DPI on this website.
Photos (paper prints)
Paper prints are usually printed with a resolution of 300 DPI, which means if you scan a photo 6×8″ with 300 DPI, it will give you an exact copy of your original 6×8″ photo.
But you can’t easily print e.g. a 8×10″ photo with that scan. The scan produces 1,800 (6×300) x 2,400 (8×300) = 4,320,000 pixel.
A 8×10 photo though consists of 2,400 (8×300) x 3,000 (10×300) = 7,200,000 pixels.
If you scanned that 6×8″ photo with 400 DPI you would have enough pixels for a 8×10″ print:
(6×400) x (8×400) = 2,400 x 3,200 = 7,680,000 pixels.
But even if you scan a photo in a higher resolution you won’t get more details than the original, since it was printed using 300 dots per inch.
When you scan your photos at 400 DPI or higher, you are not picking up any more detail off your photo. You are just making it bigger.
Therefore, a scan with a higher resolution enables you to have bigger prints but not with more details. A higher resolution in this regard will just pick up and show other “details” like scratches or blemishes.
For digital output (screens, monitors, smart-phones etc.)
Let’s say you don’t intend to reprint the photo and just want to watch it on your favourite screen. In that case, the dimension of your screen is the relevant number.
A Full HD television has a dimension of 1080 x 1920 pixel (2.073.600 pixel). Since a 300 DPI scan of your 6×8″ photo would result in 1,800 (6×300) x 2,400 (8×300) = 4,320,000 pixels, it would have a dimension bigger than required. It would therefore be excellent for a Full HD screen. The television would crop the image to 1080 x 1440, so it will fit in its dimension on the screen with the full height and reduced width.
But nowadays a lot of people have a television with a 4K resolution, which is 2160 x 3840 = 8.294.400. It comprises four times the pixel of Full HD. The 300 DPI scan of the 6×8″ photo would not be enough to fill a 4K-screen.
For this you would need a scan with 360 DPI:
(6×360) x (8×360) = 2,160 x 2,880 = 6.220.800 pixel
The television would show it with the matching height (2,160) and on both sides black areas like in the old days on the top and bottom of wide-screen movies you watched on tv.
Nevertheless, the scan with the higher resolution enables you to facilitate the full potential of your 4K-TV, but will not let you have more details of your photo (remember the physical limitation of the original 300 DPI). Just like when you go from a 55″ 4K-TV to a 75″ 4K-TV, you do not get more picture detail, you get a bigger screen.
A scan with 600 DPI of the 6×8″ photo will result in dimensions of 3,600 x 4.800 pixel, which still be enough for even a 5K-resolution.
Slides and negatives
There is a slightly different story with slides and negatives. Slides and negatives don’t consist of pixels or dots.
Scholars, experts, photographers, scan operators and many others who have anything to do with photography and image processing are arguing about the question of how many pixels can be extracted from a small picture. It is obvious that the number of pixels on a 35mm slide depends on the type of film. A film with 200 ASA is of course coarser than a film with 50 ASA; There are also differences between the individual film types within the respective film speed.
A very good film can theoretically distinguish 40-60 million dots on the surface of a small picture. In order to be able to expose these individual dots differently, the film must be in a top of the range camera with an excellent lens. Simple standard lenses hardly manage to bring more than 10 million pixels (corresponds to 2800 DPI) onto a 35 mm slide. Professional lenses or prime lenses reach the 20 million dots limit (corresponds to 4000 dpi).
For slides and negatives taken with an average camera equipment, a scan with 2800 dpi is enough to get about 10 million pixels. If one used very high-quality equipment and excellent films, one can get around 20 million pixels from a 35 mm slide with a 4000 DPI scan.
Does it make any sense to generally have a 4000 DPI-scan? The answer is yes. When scanning an image, effects such as aliasing, interferences etc. occur, which are all related to the interaction between the scanner resolution used and the actual image resolution. The higher the scan resolution, the less interference you get. Therefore, we generally recommend scanning images with 4000 DPI.
Scanning slides and negatives for prints
“Standard” slides and negatives have a dimension of 24 x 36 mm, which is 0.94 x 1.42″. If you assume that a photo is printed with 300 DPI than you can realize the following sizes:
That shows you, that a scan with 2800 DPI is generally sufficient for most photo prints but a scan with 4000 DPI will expand your opportunities, e.g. of you want a larger print at a later stage.
Scanning slides and negatives for digital output
When scanning for digital output, the dimension of the screen is the decisive factor. The maximum resolution of a modern 4K television is 2160 x 3840 DPI, therefore a scan with 2800 DPI would be sufficient to fill the whole screen.
Nevertheless, again we recommend a scan with 4000 DPI. Scanning slides and negatives with a higher resolution normally gives you also a better image. The television will downscale the DPI to its native resolution by usually keeping the better image achieved with the higher resolution.
Adobe® Super Resolution
If you have old or low-resolution photos you want to print, we can apply Adobes® latest Super Resolution feature. Super Resolution creates a single image with two times the linear resolution. That means the enhanced image will have twice the width and twice the height of the original image, or four times the total pixel count. Super Resolution uses an artificial intelligence upsampling algorithm that Adobe says was trained on “millions of photos” in order to give better results than usual.
In addition, If you’ve taken photos on an older camera or a lower-resolution phone camera, one can scale up those shots to match today’s higher-resolution image quality standards (e.g. for a 4k TV).
Furthermore, as we did see above, large prints require more pixels. With Adobe’s Super Resolution, one can meet rigorous print standards to upsample a 12MP image to 48MP — equivalent to a 16” x 24” photo.
However, a 12-megapixel image that has been enhanced to 48 megapixels with Super Resolution won’t match an original photo from a 48-megapixel camera. Super Resolution can’t work wonders. But since one can’t bring one of those modern camera bodies back in time to re-shoot our favourite images, and so for many situations, this feature will absolutely breathe new life into older files.