Resolution issues

What resolution should I use when scanning photos? What resolution should I set my digital camera for? How many mega-pixels do I need in a digital camera? Should I use my 1200 dpi printer or buy a new 3600 dpi printer? There’s a lot of confusion on this subject, so let’s talk about image resolution and printer technology so you can make an informed choice.

 

What is a pixel?
Digital images are actually a rectangular grid filled with dots called “pixels” (picture elements). A “Mega-Pixel” is a technical term for a million pixels (actually 1,048,576 due to the odd way computers store numbers). Each pixel is stored as a number that refers to a color. Numbers are stored in computers as “bits”. The more bits you use the more colors you can get. Digital cameras store their pictures using 24 bits for each pixel, giving us about 16 million colors per pixel. Less than 24 bits per pixel give fewer colors and don’t look as good. Some scanners use up to 48 bits per pixel, but most people can’t tell the difference, and it makes the file twice as big. So our job is to figure out how many of those pixels we need to get a nice print.

Digital cameras have a sensor with a fixed number of pixels. Storing those pixels take up precious space in the camera’s memory, so most cameras have the ability to reduce the size of the image by storing fewer pixels. This allows the camera to hold more images. The camera actually blends adjacent pixels together to create a lower-resolution image.

Scanners work in a different manner. Scanners have a sensor that physically moves and can vary their resolution to some degree.

First, we’ll talk about scanning a regular color photograph. Let’s say you want to make a copy of a photograph and print it the EXACT same size. Let’s assume that your printer has a resolution of, say, 300 dots per inch (dpi). If you scan at 300 dpi, you get one pixel (picture dot) per printer dot. If you scan at 150 dpi, the printer takes each pixel and prints two dots across, and two dots high. That’s easy math for the printer and your print will look as good as a 150 dpi print is going to look. But it won’t look as good a it will scanned at 300 dpi.

But let’s say you scan at 400 dpi, thinking you’ll get a better print. 400 pixels divided by 300 printer dots is 1-1/3 pixels per printer dot. That means the color of the printer dot will be a mix of the colors of the two pixels. If the adjacent pixels are the same color, there’s no problem. But imagine that one pixel is white and the adjacent pixel is black. One white pixel and 1/3 black pixel adds up to 75% grey, so now your image has changed – you don’t have pure white and pure black, you have a mixture of black, white and grey dots. The same thing happens if you scan at 200 dpi and print it at 300 dpi. The first printer dot will be the color of 2/3 of the first pixel, but the second dot will a color calculated by 1/3 of the first pixel, and 2/3 of the second pixel. The printer will not handle this well and you picture quality will suffer.

Let’s say your image has a checker board pattern of black dots and white dots. As long as the ratio of pixels-to-printer dots is a whole number (1:1, 1:2, 1:3, etc) you don’t get a color shift, because the printer just prints more dots of the same color. But if the ratio is a fraction (3:2, 2:3, etc) the printer has to calculate the colors and you will get a color shift. Your black-and-white checkerboard prints with some grey dots. Plus, you no longer get proper dot alignment and funny moiré patterns can emerge. Your image quality drops.

It’s easy to see this in our checkerboard example, but color photos have smoother transitions between colors. This will cause you to get a softer image, but doesn’t wreck it completely. In our example, scanning at 400 dpi gives us a bigger file, longer scan time and a worse image than a 300 dpi scan. So, for our 300 dpi printer, we should scan at 300 dpi (1 dot per pixel), 150 dpi (2 dots per pixel), 100 dpi (3 dots per pixel), or 75 dpi (4 dots per pixel).

What if I want to enlarge the photo? If the photo is 4” wide and we want to print it at 8” (twice as big), we should double the scan resolution to 600 dpi. Then we tell our photo-editor software (I like Photoshop) to change the image from 600 dpi to 300 dpi, but keep all the pixels. In Photoshop’s “Image Size” dialog box, I uncheck “Resample Image”, then change the “Resolution” box from 600 to 300 and click “OK”. If you allow Photoshop to resample the image, it will keep the physical size the same, but reduce the number of pixels, calculating new colors for each new pixel. But if you do not allow Photoshop to resample, it keeps the same number of pixels, and increases the size of the print. The file size won’t change, we are just telling Photoshop how to tell the printer what to do.

Here’s the math:
(Print size) X (printer resolution) = (pixels)
(Pixels) / (picture size) = Scan Resolution

To create a 2” wide print at 300 dpi you need and image to be 600 pixels wide (2 x 300 = 600). To get a 600 pixel-wide image, from a 6” wide photograph, we scan at 100 dpi (600 / 6 = 100).

But, lets say you have a 4.5” wide picture and you want to print it 8” wide. Your printer works best at a resolution of 600 dpi. You multiply 600 times 8, which equals 4800 pixels wide. To get 4800 pixels from a 4.5” photo, we divide 4800 by 4.5, which equals 1066.66. That’s not a round number so we’ll scan at 1077 dpi. Can your scanner actually do this? Some professional drum scanners will but most consumer flatbed scanners won’t. As you can see, this digital stuff is far from perfect.

My scanner has a mechanical resolution of 3600 dpi. The software will let me set it to 1077, but what it is really doing is interpolating, which slightly alters the image. To get around this problem, I run my scanner at 3600 dpi and have Photoshop resample the image. The algorithms in a $700 professional photo editing software should be better than the software that comes with a $200 scanner. Notice I say “should” – if you’re serious, you test these things out. The big boys use drum scanners, which will truly let them achieve almost any resolution they want.

 

Digital Cameras
My 4 megapixel digital camera can be set for different resolutions. I created a cheat-sheet for each resolution setting by dividing the camera resolution by my printer dpi. That tells me what size prints I can get for each resolution. An example:

Resolution - 1024x768
150 dpi print = 6.8” x 5.12”
300 dpi print = 3.4” x 2.56”
600 dpi print = 1.7” x 1.28
1200 dpi print = 0.85” x 0.64”

Resolution – 2272 x 1704
150 dpi print = 15.15” x 11.36”
300 dpi print = 7.57” x 5.68”
600 dpi print = 3.78” x 2.84”
1200 dpi print = 1.89” x 1.42”

So, if I want 300dpi prints that are about 7”x 5”, I set my camera to resolution to 2272 x 1704, then crop the image in Photoshop to get the exact size.

 

Digital Cameras vs. Scanners
In the above chart, look how small a 1200 dpi print is – its less than 2” wide. To get a 7”x 5” photo at maximum camera resolution, I can only print at 300 dpi. An 8” x 10” print at 1200 dpi requires slightly over 112 mega-pixels. So selling your 4 mega-pixel camera to buy a new 5 mega-pixel seems silly to me. If you double your print resolution, you double the width and double the height. That means you need four-times the number of pixels. So the next jump from a 4 mega-pixel camera is 16 mega-pixels, then 64 mega-pixels. We are a long way off from having consumer cameras of that size.

On the other hand, my scanner has an optical resolution of 3600 dpi, which allows me to scan at 1200 dpi and print full size at the maximum printer resolution. Or, I can scan at 3600 dpi, and blow it up 3 times bigger. You see why so many professionals still shoot 35mm film cameras and then scan the photograph. A full size 8.5” x 11” scan at 3600 dpi is over 1.2 billion pixels – over a million mega-pixels! And I could print that at 1200 dpi for a print size of 33” x 25.5”. At 300 dpi, it would be 132” x 76.5”. You can see why a $200 Epson 3170 scanner and a $200 Nikon 35mm camera are tough for any digital camera beat.

So, if my printer is only 8.5” x 11”, why did I buy a scanner with 3600 dpi? Two reasons. First, a high-resolution scanner allows me to scan a small area of an old photo and greatly enlarge it. I can take a 4”x 5” photo of a wedding and crop at down to a headshot of a single person. Second, I have a lot of 30-year old slides I wanted to scan. Slides are very small; you need a much higher resolution scan to get a decent sized print. I scanned a slide and got a 4171 x 2613 pixel image. I can crop the image and still get a 10” x 8” print. If you are really serious about photography, you get a special slide scanner, like a Nikon.

 

Printer Resolution
Is there any advantage to higher resolution printers? It depends on a lot of factors. A lot of professional high-resolution image setters max out at about 1200 dpi. Some go as high as 3600, but most people can’t tell much difference. Pros use magnifiers to really look close. But at arm’s length, most people can’t tell the difference between a good 300 dpi print and a good 1200 dpi print. If you don’t get too close, you can get away with 150 dpi, but that’s pushing it.

 

Are all printer dots the same?
No. Some printers can mix ink on the same dot to achieve a wide range of colors for each dot. Other printers can’t mix the ink on a per-dot basis, and use a process called dithering. Look close at a newspaper. You’ll see the photographs are made of various size dots of black ink. If the dots are small enough for a given viewing distance, your brain blends the dots with the white page and creates an illusion of gray shades. This is known as “dithering”. Magazines are printed with a process known as “four-color”. They print four separate color images (black, cyan, magenta and yellow) with separate dots, and your eye perceives this as a full color image.

Early ink-jet printers did the same thing, and created horrible images. A pixel from a scanner or digital camera could be any of 16.7 million colors. But the printer could only print one of four colors for each dot. So if you tried to print a 300 dpi image at 300 dpi, the printer would pick the closest ink color and you had a rotten print. But using the principle of dithering, you could print at half the printer resolution, printing each pixel with four printer dots (2 dots wide times 2 dots high). The eye would blend those four dots, creating the illusion of 16 colors. Printing at one-fourth of the printer resolution created the illusion of 256 colors. To print a 300 dpi image, we needed printers with at least 1200 dpi to get an adequate image. Not good, just ok. When HP first come out with their 36” wide DesignJet 600 (designed for printing line drawing), customers complained that large image prints were ugly. I suggested they print at 150 dpi, not 600. They thought I was nuts, but when they tried it they were amazed.

Manufacturers began to develop better printers that could drop multiple color inks on the same spot before the ink dried. This caused the inks to mix, giving the ability to truly have dots of more than just 4 colors. That was when “photo-realistic” ink-jet printers became a reality. Newer printers now use 6 (or more) color inks for even better color fidelity. Once you have a 6-color 1200 dpi printer, there’s not much point for non-professionals to spend more money. HP has 6 color 1200 dpi printers for about $100. You can get better than that, but the quality curve starts to flatten after that. Pros will still spend more money for larger print size, archival quality, the ability to color match, and other things they need.

What am I using?
Digital Camera - Nikon D200 (10 MP), Olympus Camedia C4040 (4 MP)
Film Cameras- Nikon F, Nikon F2, Nikon F4, Nikon FM, Leica IIIc, Leica IIIf
Scanner - Epson Perfection 3170 (3600 dpi optical with built-in slide scanner
Printer - HP Deskjet 5550 (4 or 6 color, 1200 dpi)
Software - Photoshop, Fireworks, various plug-ins.

How do I know all this stuff?
I used to sell 36" wide scanners and printers, was a professional photographer, and have been involved in photography, computers and digital imaging for over 30 years.

For more information, I suggest this site http://www.scantips.com/