Photography: Image size and DPI

Image size and DPI

Since there is so much confusion about image sizes and dpi for scanning and printing, I have decided to create this page to shed some light on the matter. Actually the matter is pretty straightforward and not complicated at all.

First a few basic principles
These principles are the basics of the matter. They are very simple. Of course you can complicate the matter (later on I'll show how) but this is absolutely not necessary!
* The size of 35mm film (slide or negative) is 24mm x 36mm (in inches 0.945 x 1.417)
* An inch = 25.4mm. A millimeter is 0.0394 inches.
* A DOT or PIXEL is the smallest unit of an image and each has a specific color.

DPI
The term DPI simply means dots per inch. A dot is also often called a pixel. So sometimes the term PPI (Pixels Per Inch) is also used. Actually the term pixel is only correct when referring to digital files and not when referring to printed media. But we'll come to that later. It is not important right now.

One very important thing you have to remember is that DPI is absolutely not a measure of image size. This is probably the most often made mistake. People will say: " my file is 72dpi and I have enlarged it to 300dpi". But changing dpi has nothing to do with the size of the file ! If the file you started with was 1000 by 2000 pixels, it will be exactly 1000 by 2000 pixels after changing the dpi setting. Regardless of the dpi setting, whether 10dpi or 10000dpi, the file will remain exactly 1000 by 2000 pixels. So the number of pixels is a measure of size for a digital image, dpi is not. DPI is a measure for resolution, Dots Per Inch. Once you realize this, things become much simpler. It's easy to see that DPI is a measure of resolution. The more dots per inch your (printed) image has, the sharper it will appear.

Dots and color
In a digital image each dot or pixel has exactly one color. How many different colors a pixel can have is called "color depth". Most computers have graphic cards that use 24 bit color. This color depth is also known as "true color" because it resembles the number of colors a human eye can distinguish. There are several different color systems but the one most widely used is called RGB. Each color is described as a combination of the basic colors Red, Green and Blue. Mixing these colors can create any other color. When using 24 bit color, each of the RGB values uses 8 bits, or 256 different values. So each color has three values: R, G and B each ranging from 0 to 255. Together this makes 256x256x256 or 16.777.216 different colors. A color could have the following values: R=251, G=171, B=77. In this case the color is a shade of orange. Since 24 bits equals three bytes, we can also calculate the size of a file. An image with 24 bit color depth, measuring 800 by 600 pixels will have a file size of: 800x600x3bytes or 1.440.000 bytes (approx. 1.4Mbytes). File formats that use compression, like JPEG, can reduce this file size.

DPI and computer screens
People often use DPI in regard to computer screens. The resolution of a screen being 72dpi or 96 dpi. But using DPI for computer screens is useless. The resolution of computer screens is measured in pixels. In the early days the standard for resolution was 640x480 pixels, called VGA. Later screens and graphics cards became better and the new standard was 800x600 pixels. Though still used very widely, at present many computers are set to 1024x768 pixels. But much higher resolutions (like 1600x1200) are also possible. Suppose you have an image of 300x200 pixels on your website. How large will it show on the screen of your viewer? You can't tell, since you don't know what screen, graphics card and resolution the viewer will be using. It could be an 14 inch screen set to 1024x768 but also an 19 inch screen set to 800x600. You only know for sure that the image will consist of 300x200 pixels. So on one screen it may show very big (maybe 3 inches), while on another it is very small (barely an inch). Still it is 300 pixels wide. So on the first screen it may have 300 pixels in 3 inches (or 100 pixels per inch) while on the second screen it would have 300 pixels in just one inch (or 300 pixels per inch). And this is regardless of the DPI you may have set for the image with a program like PaintShop or PhotoShop. So for screens DPI makes no sense.

Scan DPI
Another thing that confuses many people is that DPI is used both for scanning and printing. When scanning, you convert an image into a digital image, a file on your computer. In scanning both the terms DPI (Dots Per Inch) and PPI (Pixels Per Inch) are used. Here they are the same thing. When you scan an image and convert it into a digital image, DPI is a measure of resolution. DPI means how many dots (or pixels) the scanner can resolve per inch of the original image. So scanning at 100 dpi means that the scanner can resolve 100 dots in each inch of the original image. Scanning at 200 dpi simply means the scanner can resolve double the amount of dots for each inch. So the digital image will have double the number of dots (or pixels) in both length and width.

If you scan an image with a size of 2 by 3 inches with 100 dpi, the resulting file will be: 2x100 by 3x100 pixels. Or 200x300 pixels. Scanning the same original image with 200 dpi will result in a file of 2x200 by 3x200 pixels, or 400x600 pixels. Remind that since you double both length and width, the total image size will not double but quadruple (200x300= 60.000 pixels and 400x600=240.000pixels) !

At what resolutions you can scan depends of course on your scanner. Flatbed scanners generally have resolutions ranging from 75dpi to 600 dpi or even 1200dpi. However often the higher resolutions are called "digital" and not "optical". The highest optical resolution is what the scanner hardware can resolve. Anything further is just software extrapolation. The image will then first be scanned at the highest optical resolution and after that, the software will just increase the number of pixels. But since there is no additional information on these pixels from the original, the software will "guess" how they should look in order to make the new image. In fact you are just being fooled about these "digital" resolutions.

Slide scanners will usually have a maximum resolution of around 2400 dpi. Some may have exactly 2400 dpi, while others have maximum resolutions of 2700 dpi or 2880 dpi. This is because a 35mm slide (or negative) scanned at such a resolution, creates a file big enough to be published in "photo quality" at A4 size (approx. 8 by12 inches). We'll get into that later. There are also slide scanners with resolutions of around 4000 dpi, but these are much more expensive.

Print DPI
In printing the term DPI may cause some confusion. But you don't need to get confused at all. This confusion is mainly created by the marketing guys from the printer companies, in an attempt to highlight the superior qualities of the printer they want to sell. An image printed with 300dpi is often called "photo quality". But there are many printers on the market with print resolutions of 1200 dpi or even 2400 dpi. Why is that?

As shown above, a pixel can have many millions of different colors and a computer screen can show all of these different colors. However a printer can not. Most (inkjet) color printers use only a small number of different inks. For example: black, red, green, blue and yellow. To create an illusion of many millions of colors, these inks have to be mixed. So to print the single pixel (that on a screen had a color depth of 16 million colors) many tiny drops of ink (or dots) are required. To accurately show the color, it may require as many as 16 drops of ink. (e.g. 4 red, 3 blue, 5 green, 2 yellow and 2 black)

This number of ink drops a printer can put on paper is called the printer resolution dpi. So if a printer has a resolution of 1200 dpi but requires 16 drops of ink to display a color, the actual resolution is much lower. Since ink drops are used in both directions (length and width: 16 drops made up of 4 rows of 4 drops), you could say that the effective resolution would be 1200 divided by 4 (or 300) dpi. Practically this isn't entirely true since modern inkjet printer can spray tiny ink drops that overlap each other. Roughly you can say that the higher the printer dpi, the higher the print quality, but it also depends on how the printer forms the dots on paper.

But the general picture is that since a printer can't display all colors directly, it needs many more dots to create the illusion of that color. Therefore the dpi in printer specifications should not be used for calculating file sizes.

Effective DPI
To calculate the resolution of printed images I would rather use the term effective dpi. Generally speaking an effective dpi of about 300 is considered photographic quality. So to print an image on a piece of paper in photographic quality, just multiply the paper size in inches by 300. For a print the size of 5x7 inches, you will need a file the size of 5x300 by 7x300 pixels (being 1500x2100 pixels). Quite easy, isn't it?

So at what DPI should I scan my slides to make prints?
If you have understood the above part, it shouldn't be very difficult for you to figure this out. Just a simple calculation. Remind, a 35mm slide or negative has a size of 24x36mm (0.945 x 1.417 inches) and photo quality requires an effective 300dpi.

Suppose you want a print sized 6 by 9 inches in photo quality. What steps to calculate the required scan dpi?
1. A print of 6x9 inches in 300dpi requires a file of 6x300 by 9x300 pixels, or 1800 by 2700 pixels.
2. To get 1800 pixels out of 0.945 inches (or 2700 out of 1.417 inches), you would need to scan at 1905 pixels per inch. Divide 1800 by 0.945, giving 1905 pixels. Or divide 2700 by 1.417, giving the same result of 1905.
3. So scan your entire slide at 1905 dpi scan resolution.

That's all there is to it. You can even do it easier. Since you might not precisely want to scan your entire slide (or maybe your scanner can't even do this) and you might not be able to set a resolution of exactly 1905 dpi, you could say that a slide measures roughly 1 by 1,5 inches. Printing an image at 8 by 12 inches is an enlargement of 8 times, compared to your slide. Also enlarge the effective dpi with this factor, giving you the required scan dpi. So a print of 8x12 inches requires a file scanned from a 35mm slide at 8x300dpi, or 2400 dpi. (300dpi effective, 8 times enlarged compared to 35mm slide)

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