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You Are Here:  Applying "DPI" to Photo Licensing

Applying "DPI" to Photo Licensing

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When you want to license an image for some print use, most people ask, "Can you deliver the image at 300dpi?" But this doesn't make sense—"DPI" has nothing to do with image resolution, which is really what you need to ask for. Figuring that out is the point of this article.

What most people get hung up on is the fallasy that they need images at 300DPI—or "dots per inch." They don't really understand what that means because that value doesn't tell you how large the file is. I can give you an image that's 500 pixels wide, and another that's 5000 pixels wide, and both can be set at 300DPI. But obviously, one can print onto a large piece of paper, and the other cannot.

To determine the best size image for your printing needs, first, determine the size of the area you want the image to cover, and then determine what the best "dpi" for the printer itself and the materials you are going to print on. Not all materials "require" 300dpi.

The following table includes commonly used sizes and papers.

20-50dpi
Areas larger than 10 feet, such as billboards, wrapped materials for trucks, wall paper
100-150dpi
Areas larger than 20", such as posters, signs, banners, calendars, product packaging, retail signs
150-200dpi
Plain paper, newsletters, books; 70# Matte Paper
250dpi
Basic magazines, textbooks, most everything; 80# Matte Paper
300dpi
High-density catalogs or covers; 100# Matte or Gloss Paper

The term "300dpi" is a historical expression that refers to to a technical the maximum resolution density that a printer could accept before it throws away the excess pixels. But this "maximum" dpi isn't the same as a printer's optimal dpi. In almost all cases, most modern, high-end printing devices will throw away even those excess pixels as well in order to achieve the "optimal" dpi for the paper being used. People don't notice this because the system will produce exactly the same visual result from images at 250dpi or 300dpi. Most mid-range printing systems will produce the same results from images scanned at 200dpi. (That is, the results are slightly worse, but that's because of the printer, not the quality of image.)

Don't be deceived by "home printers" that say they produce ink droplets at 1440dpi. A single droplet of ink does not correlate directly to a pixel in an image. Ink droplets are small for the sole purpose of mixing proper quantities of different colors to yield more accurate colors represented by the pixels in the image. For example, if the pixel in the image requires 3 parts blue ink, 10 parts red ink, and 20 parts yellow ink, and 3 parts black ink, the printer is going to be able to reproduce that because it can mix lots of ink droplets into a single point on the page. This has nothing to do with the dpi of the image. Most (if not all) home printers will downscale all images to about 200-250 dpi during the printing process. You can try to print images that are at 300dpi and higher, but you won't get any different results.

So, putting all this together, say you're laying out a catalog page where an image will be used. First, know the physical dimensions of how much space the image will take. Not things like "¼-page" or "½-page." Actually measure the amount of space in inches. (All page-layout software programs will tell you this information.)

If you don't know the "optimum dpi" of a device, you can almost assuredly assume that 200dpi will work just fine. That value can drop further if you cover more area, such as posters, etc.

Film Scans

Though most people get images from digital cameras these days, there's still an awful lot of old film images being sold and delivered. However, people have a misunderstanding about film, thinking that it's just a continuous tone of of color that can be scanned at higher and higher resolutions to get finer detail in the photo. But film isn't a continuous tone of colors. Rather, it's more like digital cameras, in that it has its own pixels; here, called film grain. Therefore, all film has a "maximum" resolution: the a point at which you don't get more detail in higher-resolution scans, you just get a lot of "grain." In fact, scanning film beyond it's maximum resolution will yield worse digital renditions, not better. Also, because different films have different grains, each has its own "maximum" density as well.

Most professional 35mm color films has a maximum scan yield of about 5500 pixels in the long dimension. (This has the equivalent of a 21 megapixel digital camera.) Yes, there are scanners that will go up to 8000 pixels, but the digital image you get as a result is actually poorer, because it will only get more of the grainy surface of the film, degrading the colors. Additionally, there are scanners that only yield 4000 pixels in the long dimension, but they produce better digital results due to having a better imaging sensor (the CCD). So, the quality of a film scan is not measured simply by its resolution.

This then begs the questions of what do you do if you have a 35mm slide and want to print a 60" image. At 300dpi, 5500 pixels is only 18", a far cry from 60". If you try to print this digital file at that size, you'd get an effective dpi of only 91! Can you get a reasonable result from an image printed at 91dpi?

Yes. It's almost universally true that most good, high quality printing systems will render perfectly acceptable results from a 91dpi image, provided that 1) the scanner that was used had a high quality CCD sensor, 2) the operator used the right color-management workflow to assure the proper translation between source and destination media types (film to paper, for example), and 3) the final material used was of sufficient quality.

By contrast, one can get a very poor result from that same image if it were scanned at 300dpi at 60" because the digital file would have consisted of mostly film grain, not color. This also makes proper color management nearly impossible since the original source colors would be too noisy from the grain.

Testing the Theory

Look at the two photos below. The one on the left was shot on film and scanned at a resolution that yielded 4000 pixels horizontally. The 35mm frame of film has tiny dots, and the truck is covered with a huge sheet of paper that has big dots (that are also spaced apart). Where "DPI" comes is when you're talking about how many of those dots represent an inch.


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The output device that prints the paper that wraps around the truck will tells how many "dots" make up an inch. The whole reason for specifying a DPI value is to adjust your image to match the output specifications to that of the output/printing device. It turns out that, for the paper that goes on this truck, 20dpi was sufficient.

People who know this stuff don't just ask for a high-res image or a 300dpi image, they ask for an image either at a specific resolution, or indicate the final output size along with the desired DPI. For example, a company that makes postcards might request images at 4x6" at 330dpi because they know that most postcard printers print image optimally at that resolution. If a European company ask for an image at size A4 at 200dpi, you can determine that size by converting the metric system to inches. (Although Americans have to first figure out what the heck "A4" means.) This translation is easily done in Photoshop's "New Image" dialog.

Summary

Most people never actually need images at 300dpi because it's an overkill. It rarely hurts to have an image larger than what is needed because it can be reduced. But that's a technicality. In today's image-licensing world, higher-resolution images cost more, which needs to be taken into account. You don't want an image sized at 300dpi if it costs twice as much as the "proper" image size. Therefore, it's smart to know exactly what you need, and not just expect 300dpi.

Though most people will still hand-wave this technicality aside, they will butt up to the problem noted above: what do you do if you think you need an image that simply can't be delivered? Digital cameras can only produce images up to a certain size, and film's highest-resolution scan usually tops out at 5500 pixels. If you do the math, these won't print on posters or other larger sizes at 300dpi. Well, by the simple fact that we've seen these posters forever, and they were printed just fine with lower-dpi images, it disproves the theory that you "need" 300dpi. You can feel perfectly comfortable printing images using lower-resolution film scans.

Of course, business is business, and services often deliver clients what they ask for, especially if it translates to more money, or to appease an "insistent" client. Invariably what happens is that such a client asks (and pays for) a higher resolution image than what is technically possible, so the provider simply opens that same image in photoshop, and "resamples" the image to be larger. In essence, it simply doubles the pixels (or some magnification factor) to yield what the client asks for. This is actually a lower-quality image, but by the time the printing system gets it, it's going to simply throw away those new pixels anyway, effectively printing the same thing it would have had the image not been unnecessarily altered.

In summary, most people don't need to worry about this stuff. Do the simple math to figure out the total image size you need and ignore the "dpi" value entirely. For example, if you're printing in a typical magazine, newspaper, or poster material, assume you can get a good print at dpi values of anywhere from 150-200. Therefore, if you want to print up to 30", a 200dpi image would be 6000 pixels wide. If the photographer wants to give you an image that's 5000 pixels, that would be 166dpi (for a 30" area), which is perfectly fine.

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