Sizing digital images for various uses may be one of the most puzzling problems that the average digital photographer runs across these days. Do your eyes glaze over when trying to understand terms like resolution, pixels and dpi? Have you ever sent an image of your dog in an email that is so large that it fills up the recipient’s email in-box? Have you ever had an image enlarged and printed only to find out that the file size was too small and the printed photo is so pixilated that it looks like a graphic from an old Atari video game? If you answered yes to any of these questions then read on. Hopefully I can shed a little light on the matter.
In a previous article I provided some pointers for sizing images for email and the web. You can check out those tips by clicking HERE
Sizing images for printing is very different than sizing images for a computer screen. A concept that is difficult to wrap the mind around is the fact that the size you see an image on your screen is not the size at which it will print, unless you print at screen resolution, which generally does not have enough detail for good quality prints. For example, if I size an image to 4″ by 6″ inches at 80 dpi (dots per inch) it measures 4″x6″ on my monitor (this will be different from monitor to monitor depending on what the resolution of the monitor is set at). Screen resolution is usually between 72 and 96 dpi. At 80 dpi the 4″x6″ image looks great on my screen. When I print it, the print does indeed measure 4″x6″, but with only 80 dpi it is very blocky and pixilated. In order to get a detailed 4″x6″ print I would need to set the dpi much higher. When viewed on my 19″ screen, a 4″x6″ image at 300 dpi is too large to fit, even though it still prints as a 4″x6″ print (with much greater detail than the 4×6 @ 80 dpi). So, as you can see, how big an image appears on your screen does not necessarily indicate how large a print it will make.
Most cameras come with software that provide some help with sizing. However, this software is often oversimplified, giving the user very little control over the size and resolution of the final print. I prefer to use Photoshop, or some other advanced image editing software to size my images so that I have the most control. However, more contol also means more complexity, not something everyone wants. However you size your images, the requirements for getting good results remain the same.
Different methods of printing require different amounts of resolution detail, or dots per inch. If you don’t care that your print has jagged edges and visible pixels, then go ahead and print at 72 dpi. Some tests indicate that 150 dpi to be the minimum for good print results, but there are many factors that come in to play. For chemical process printing (non ink jet printing) 200 dpi is generally plenty of resolution. Most publishers (magazines, newspapers, etc.) have their printing standardized to 300 dpi and want all submissions at this resolution. The current generation of high quality ink jet photo printers are able to resolve detail in the 300 to 360 dpi range, although tests have shown that 240 dpi is almost indistinguishable and that 200 dpi gives great results. Knowing how the image will be printed will help you determine what dpi resolution to select when sizing the image.
Another consideration in selecting the dpi resolution at which to print has to do with the distance the print will be viewed at. Small prints that will be viewed close up benefit from higher print resolutions (240-300 dpi), while large prints that will be viewed from a few feet away can make due with lower print resolutions (150-200 dpi). Billboards that are viewed from hundreds of feet away are printed at 72 dpi and lower.
Once you know what dpi resolution you will select, the next thing to do before sizing is determine the native size of the image; in other words, the size at which it came out of the camera. In Photoshop it is easy to determine this. Open an image and then go to Image>Image Size. A dialog box will open that indicates the pixel dimensions as well as the dimensions in inches and the dpi (dots per inch). Next to the pixel dimensions it also indicates the total pixel count in thousands (k). It takes one million pixels (1000 k) to equal one megapixel. If the number in the dpi box already matches your desired print resolution, simply look at the inch measurements and they will show the height and width that the image will print. If the dpi is not correct for your desired print method, make sure that the “Resample Image” box is un-checked and then change the dpi to what you want. Once the dpi matches your desired print resolution you can now look at the image dimensions in inches to determine how large it is. For example, if you set the dpi to 200 and the dimensions in inches are 10 inches by 15 inches, the file will make a 10″x15″ print without any changes to its size. If you want to make a print that is 10″x15″ or smaller, then you are in good shape. Downsizing an image doesn’t create any resolution problems. Let’s say you want to make an 8″x12″ print. Simply re-check the “Resample Image” box, leave the dpi at 200 and then type 12 into the box that currently indicates 15 inches. If the “Constrain Proportions” box is also checked (which it should be) then the box indicating 10 inches will automatically change to 8. Click “OK” and your image will automatically be sized to print at 8″x12″ at 200 dpi. Remember not to save the image now unless you do so with a different name. Otherwise you will have permanently saved the image at the smaller size.
Now, let’s suppose that you want to enlarge the image so it prints bigger than its native size of 10″x15″ at 200 dpi. When enlarging, or upsizing, images you need to be a little more careful than when downsizing. To downsize, the software can simply remove pixels to shrink the image, which will not degrade how the smaller image appears to your eye. In order to enlarge an image, the software must add pixels. The act of removing or adding pixels is called interpolation. When interpolating an enlargement, the program uses complex math to decide how best to add pixels to make the image bigger. The current generation of interpolation software contained in Photoshop or third party applications, such as Genuine Fractals, is amazingly good at doing this. Lots of pixels can be added to enlarge an image without a noticeable decline in image quality. However, the computer can not add detail that wasn’t recorded by the camera, so at some point an enlargement will begin to show digital artifacts left by the computer doing its best to add large numbers of new pixels.
To enlarge an image in Photoshop, follow the same steps as when downsizing. Open the image in Photoshop and go to Image>Image Size. Uncheck the “Resample Image” box and enter the desired dpi resolution. This will show you how big the image will print at that dpi without any change in size. Using our previous hypothetic example, the image will print at 10″x15″ at 200 dpi. If you wish to print the image at 16″x24″ at 200 dpi, you know you will need to use interpolation to enlarge the print. As before, simply recheck the “Resample Image” box and change 10 inches to 16 inches. With the proportions constrained, the 15 inch box will automatically change to 24 inches. Click OK and Photoshop will enlarge the image accordingly.
How much can you enlarge an image and still maintain fine image detail? The answer is largely dependant on your own personal taste, the material the image is being printed on and the distance at which the image will be viewed. I have enlarged some 8×12 inch @ 300dpi images to 20×30 inches @ 300dpi with very good results, especially when viewed at normal viewing distance. The master print lab that I use has a very handy chart that helps visualize just how much various sized image files can be enlarged with good results. The chart can be viewed by clicking HERE.
So far, all of the sizing examples I have given have assumed that the image is being enlarged or reduced while maintaining the same dimensional proportions of the original. For example, an 8″x12″ image will perfectly size to 4″x6″, 10″x15″ or 12″x18″ because the proportions have not changed. However, it will not cleanly size to 5″x7″, 8″x10″ or 11″x14″ because the proportions are different. In order to size an image for printing and also crop the image to new proportions at the same time, use the Crop tool. Open an image in Photoshop and then click the Crop tool on the tools palette (or press “c” on the keyboard). With the Crop tool selected some cropping control boxes will appear below the menu tabs at the top of the page. Here you can enter your desired print dimensions and dpi resolution. If you wish to print an image at 8″x10″ at 300 dpi, simply enter those figures in the appropriate boxes. Then, move the crop tool to one corner of the image and drag to select the area that will be cropped. Once you have created the cropping area, you can use your mouse to move it around and fine tune your selection. You will notice that because you are changing the proportions of the image, some of the image will be cropped out. Once you are happy with your cropping selection, double click inside the selection or press the Return key and the image will be cropped and sized to your specifications.
