Tip & How-To about Epson Perfection V200 Flatbed Scanner
LPI, PPI,DPI conversions
Here is a conversion chart for DPI (Dots per inch) to LPI (Lines per inch) or PPI (Pixels per inch)
180 DPI................31 lines/inch or 68 PPI 360 DPI................63 lines/inch or 138 PPI 720 DPI................127 lines/inch or 280 PPI 1440 DPI..............255 lines/inch or 561 PPI 2880 DPI..................510 lines/inch or 1122 PPI
If you plan to enlarge your image, it is better to enlarge it when you're scanning rather than in your application. Each time you modify an image, you lose some quality. Keep in mind when you enlarge a small photo, such as a 35 mm slide or wallet-size photo, you'll need to scan at a high enough resolution to retain your image quality, but not so high that your file size is too large to be efficient. You must manually adjust the resolution (dpi) to maintain the same image quality when you resize your image. For example, if you have a 300 dpi 2 x 2-inch image that you want to enlarge to 4 x 4 (a factor of 200%), increase the Scale setting to 200% or change the resolution to 600 dpi.
What is DPI PPI and Why Do They Matter
To some extent, we're all photographers these days. With a camera on every phone and digital SLRs coming down in price, we've all got a trove of photos waiting to be shared. When it comes time to share online, print, or email our favorite images, many are unsure about how to set the image's resolution...
If you've found yourself in this spot, don't worry - dots per inch (shortened to DPI from here on out) is a concept that even confounds some professional graphic artists. Here's a primer DPI so you can stop worrying about technology and start sharing your photos. Getting started
Digital photos are comprised of pixels, much like the individual boxes on a sheet of graph paper. DPI tells you how small those pixels will be when the image is printed. For example, "300 dots per inch" means that 300 pixels fit across each inch. If your photo is 600 pixels tall by 900 pixels wide, for example, it would come out at 2" x 3" inches if you were to print at 300 DPI. Keep in mind that most digital photos are several thousand pixels in either direction, but for the sake of simplicity, we'll use the more manageable 600 x 900 pixels. Separating pixels from presentation
It's important to separate DPI from the raw pixel dimensions, and this is where even the pros slip up. DPI is not an indication of image quality or clarity. When you print that 600 x 900 pixel image at 300DPI, it'll likely look pretty sharp, because every inch is densely packed with pixels.
Now imagine printing that same image, with the same number of pixels, at a mere 30 DPI. As each inch would have only 30 pixels across, the density drops immensely and the image prints much larger: 20" by 30". What was once sharp now appears blurry, because each individual pixel is now ten times larger than before. By separating DPI from actual pixel count, we can understand that raising DPI doesn't magically improve a photo. DPI simply takes the same data (the original pixels) and alters how we'll view them. Pin itIt's all about context
Another factor is viewing distance. Just think of the eye chart at your doctor's office. If you're a bit nearsighted, the tiny letters at the bottom are illegible specks, while the letters at the top are easily discerned. In actuality, each tiny letter may be half an inch tall, but the distance makes them seem microscopic. Now consider our 600 by 900 pixel image. When we printed it at 30 DPI, the giant pixels made it look blurry. Were we to look at it across the doctor's office long hallway, however, it may look just as sharp as the 300 DPI print did in our hands. This illustrates how DPI is more about context than quality.
Pin itPixels Per Inch
You'll notice I've been talking about DPI in relation to printing only. This is because while printers can produce a variety of DPI settings, a computer display's resolution is fixed - its pixel density is part of the physical hardware, and cannot be altered. When talking about displays instead of print, most use the term PPI, or "pixels per inch."
If you intend to put your 600 x 900 pixel image online, switching the resolution to 30, 300, or 3000 PPI is completely arbitrary, because the computer display can't change its density. As modern desktop displays usually have a PPI in the low 100s, the 600 x 900 pixel image will appear around 6" by 9" (mobile displays may be much higher). Of course, your web browser could display the image smaller if need be, but it will do so by averaging and eliminating pixels, not squeezing them to be physically smaller. This is why it's always important to keep your end goal in mind when working with images. In summary:
• An image is defined by its pixel dimensions - # pixels tall by # pixels wide
• DPI/PPI determines the scale and pixel density at which image will be displayed
• What appears blurry from close up may look fine at a distance, so consider how an image will be seen
• Printers can produce a range of DPIs, while displays have fixed resolution
Whether you're a blogger dealing with an upload limit or are just trying to print a photo to hang on the wall, understanding DPI/PPI can go a long way. I hope these tips help you feel more in control of your images and how you share them with the world!
DPI is short for Dots Per Inch and it actually refers to printer dots per inch. These days, when used in photo software, it means PPI, which are pixels per inch. DPI and PPI are two different things, yet often DPI is used when PPI is meant, so read its usage in context.
Dpi (actually ppi) is only important in case of printing. When you open a picture in Photoshop a dpi-number is mentioned. Don't pay attention to it. It doesn't tell you anything, you can change it in any number you want, it doesn't change anything of the picture. But beware: you have to uncheck the resample-box! Resampling means calculating a new amount of pixels. The moment resampling is off, you can change the dpi-number, the according printing-measure is given. It also works the other way around: change the printing-measure, the dpi-number of the picture will be shown.
Printer DPI and PPI Ratings, General
Dots per inch stands for the maximum number of tiny spots of ink that the printer can place in a straight line where the spots are theoretically small enough (i.e. ignoring spreading or smearing effects of ink on paper) that if placed in every other such dot position leaving white space between them, the spots can be individually distinguished.
Pixels per inch stands for the maximum number of unique positions in a straight line that the printer can place an ink spot under control from the outside world, namely from a computer connected to the printer.
Lines per inch stands for how close thin parallel lines can be printed and still be distinguished in the finished printout. The spaces between the lines count as "lines".
Pixels per inch and dots per inch originally referred to the same thing. The printer mechanism was under the direct control of the computer and was physically positioned and placed dots as directed by the computer. Back then, most printer mechanisms were limited to placing dots only in positions suggested by a grid of dots X per inch horizontally and Y per inch vertically, for example 100x100 dpi
Nowadays, many printers put dots "wherever they want" as opposed to in positions suggestive of a horizontal/vertical grid. Still there is a minimum dot size and a minimum dot spacing.
A picture file (image file) represents pixels in a uniform horizontal/vertical grid pattern. And the printer needs to make a finished picture of the size, say 5x7 inches, that the user chose regardless of the number of pixels in the picture file. To simplify the process of relating the pixel count in the picture file to the possibly non-uniformly spaced dots on the paper, the printer or its supporting software may generate a temporary intermediate picture file with a set number of pixels per inch. The printer may have, internally, several choices of ratio of pixels to dots and the published rating can be the largest ratio except that the published rating may not exceed the dpi rating. Therefore there might be three "per inch" values involved at a given time, the pixels of the original picture file, the pixels per inch that the printer works with, and the dots per inch of the printer mechanism.
Pixels per inch is usually not mentioned with printers. All printers come with their own software (including parts called drivers) to install on your computer. Usually the software does not let you exercise control over individual dots using your picture file. Rather the printer takes your picture file or data file and uses its own built in logic to lay down the dots and create the printed output. We are led to believe that a printer's ppi is usually a fraction such as a half or a third of its dpi rating.
When a temporary picture file is created, there are at least two levels of software in use. High level software (which may run in your computer) takes your picture file and creates the temporary file. Low level software runs in the printer, takes the temporary file and controls the dot size and dot placement on the paper.
Sometimes a printer is advertised using a phrase such as "300 dpi 1200 dpi quality". This means that the printer has some way of making dark edges on a light background appear smoother than the first number would otherwise suggest. A printer with 300 dpi 1200 dpi quality definitely cannot resolve alternating dark and light pixels less than 1/300'th inch each. But curved and diagonal lines and color boundaries should not have jagged edges suggesting individual dots rigidly positioned on a grid with a 1/300'th inch pitch.