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Little thought about resolution DPI/LPI/PPI

11.08.10 (Tech Stuff)
I am coming very often into discussions regarding image DPI, print resolution, camera resolution and so on. This is my little thought about it because number of uninformed or plain wrong people (including professionals) is almost funny sometimes.

Basics
It is a little more complicated than this, but to keep it simple, i will write it so: DPI is Dot per inch, LPI is line per inch, PPI is pixel per inch.

PPI - Tells how many (final full color) pixels are in the image per inch (2,54mm). It is most useful for computer displays to describe it's resolution (density) and other devices where it's DPI or LPI cannot change. For example - monitors have fixed pixels on their displays, cameras have fixed pixels on their sensors. It is less useful in description of printer, but can be used to avoid discussion between geeks and normal people. As a photographer you can be told that magazine is using 300PPI printing process, and regardless of what printer they use (halftone, rgb,...) you need to deliver 300 pixels for each inch of printed image.

DPI - Means dots per inch. Now question is, what is a dot? There is lot of confusion, but dot is "printing" unit. It is smallest point device can display (in multicolor). It is not single red, green or blue pixel in RGB or Bayer arrays and it is for print use only. Monitors have no DPI, DPI in monitors is PPI. Same is for digital files. Digital images cannot have DPI. They can be printed at any DPI which will change their size on paper. Read lower.

LPI - Lines per inch is basically DPI used in relation to halftone and pantone systems. It means how many different lines process can print. When unsure what your contractor meant, use DPI and let his DTP people handle it.

SPI - Samples per inch. Is same as DPI only for scanning devices. It means how many pixels per source image it can "sample". Result is image with same PPI as was scanning SPI.

It is really simple, so where the confusion comes from? Many not technology inclined people tend to misinterpret everything technology related. Photography is area accessed by lot of not technology oriented people and that is source of confusion. Important is, that now you know and we will also tell you how to keep your sanity when discussing with people lacking that knowledge.

1) Contract
"Deliver me source photograph as TIFF at 300 DPI" is often part of photography contracts. DTP people responsible for printing are usually informed, but not business people. DTP workers tell to business people that image for magazine cover need to be 300 DPI. They know that magazine size is A4 so it is all they tell. Your business contact knows no better, and requires that you send 300 DPI image, whatever that means. They even verify that in EXIF or IPTC (embedded meta data in image file) which have virtually no meaning except recommendation.
You need to know the size of final print to deliver what they need, but it is sometimes hard to ask or convince them that you need it. To keep your sanity, deliver images at 300 PPI at maximum magazine (or intended purpose of photograph - you have that in your contract usually, don't you...) size or at maximum resolution cropped to desired ratio. In Photoshop set the desired size and DPI to make sure that meta data are valid as well in case they check. DTP workers can re-size it for they real needs.

2) Printer

Your daily bubblejet printer have resolution about 1440 DPI (or even more). Good quality printers really can print this resolution (that means on appropriate paper you can see difference between black and white lines at this resolution). It is very useful for text and for CAD, lines, charts etc. Higher it is, the better. For photography, it is rather useless high, but it is good to know what resolution your printer have, for few reasons.
DPI of most printers is not same as PPI in terms of quality. While printer can print 1440 DPI, each DOT (as in DPI) cannot have so many colors as each pixel (as in PPI) of the source image. For this reason, you should print at as much as possible DPI you can, but there is a trick. With most of the colors are being lost, it makes no big deal if you print in lower resolution and let printer or Photoshop upscale the image. If your printer is 1440 DPI, you can print at 144 DPI and printer will print each pixel of source image using 10x10 dots of it's printing head. Most printers upscale automatically in higher quality photo modes. My experience is, that it makes sense to re-size image in Photoshop to at least double PPI it was originally in, but definitely in values relevant to printer's resolution. So, if your image was 88 PPI (for example 880x880px image desired to be 10x10 inches big) and your printer is 1440 DPI, resize image in photoshop to at least 144 DPI (better 288 DPI) which will result in 2880x2880 pixel image (still printed at 10x10 inches).
Reasoning why it is not big issue to print up-scaled image is, that printer throws most of the data out anyway. Important is, that dots are as small as possible (high quality printing mode) and that Photoshop re-sizes better then most printer processors.


3) Print resolution
Rule of the thumb is, min. 300dpi (288 for bubblejet) for text and vector graphics, min. 72dpi (source resolution) for photos. Lower than this, up close viewing distance is disturbed (pixelated). But there are two factors affecting feeling from the image:

Viewing distance

If you will hang your print on wall and watch it from 2m, you do not need to worry about more than 72dpi, for photos in album 144dpi is perfect. Saying that 100dpi print is not enough is dumb. Your monitor is 100dpi and you hardly can say it is pixelated, even for text. Higher the distance is, lower resolution can be. Nobody minds printing quality of 12dpi billboards but magazines or album photos needs to be printed higher, to the sense. Good luck seeing difference between 288 and 400dpi printed photo.

Sharpness

Feeling from image is often depending on sharpness. You can have 1000 dpi print but if image is not sharp, it is all in waste. 100dpi sharp print will always look better. Compare two following images, both (seeing on your monitor) are 100dpi. Right is sharp (profi lens), left is not (consumer lens). Now you can print both on the printer and compare. Left one will look worse even when printed at 200dpi (4x smaller) and right one looks good even in 100dpi.

4) Camera

Resolution of digital camera have two camps. One saying it won’t matter, second saying it does. Where is the truth? It matters! Whatever is better - matters. Saying that resolution don’t matter is like saying that bigger hard-drives makes no sense or that monitors above 19″ are useless.

However. It of course matters only if you can squeeze details in that resolution. Higher resolution means lower sensitivity and higher noise so in the end it is always product you need to compare. For cameras with few mm big sensors certainly makes no sense to increase resolution, but if sensor is improved, firmware able to do decent noise reduction, higher pixel count is advantage. Compare products, not pixels!

Few examples where higher resolution is useful:

1. 12Mpix camera with 300mm prime equals to 6Mpix camera with 300-600mm lens.
2. 12Mpix camera have 3Mpix real resolution (PPI, since each point is only single color, R,G,B,G!). Higher pixel count equals to higher scene dynamics.
3. Higher pixel count decreases appearance of moire

5) Film

35mm film resolution is very often discussed. Reality is far from what is being often said. Film resolution is depending only on film used. It can vary a lot. For example my favorite Kodak Extachrome EC EPN 100 can be scanned in 5600dpi with still visible details in pixels. This gives you about 48 Megapixels resolution. 12, 16 or 24 Mpix some sources saying are close to regular or high iso film.
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