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Resizing Digital Photographs
Digital Photographs: Sizes and Resolutions (Finally) Explained
Every digital image is made up of little squares called pixels. Every digital image has an actual size.
Look at the image of the soldier above - just the portion on the right. Its size is 28 pixels across by 28 pixels down. The full portrait on the left? It is 473 by 600 pixels; total pixels: 283,800. The more the better, right?
If you want to know the exact pixel size on a PC, just right click and select "properties" and you will see the size in pixels. On a Mac, control-click and select "Get Info".
What about the file size, in kilobytes (KB) or megabytes (MB)? Important right? No. Ignore it. Seriously. There is a relationship between the number of pixels and file size, but it's complicated by things like levels of JPEG (or even PNG) compression - which vary from photo to photo. So ignore file size and concentrate on pixels.
And the photo's resolution? You can mostly ignore that too. The question of resolution is seldom ever important when viewing or publishing digital images (screens and websites ignore the resolution and just look at pixel size). The only time you should care about image resolution is if you are printing something; in that case you need 300 pixels for every inch of print you want.
Get Smart with Your Digital Photo Resizing
If you use digital images, you need a way to enlarge (or shrink) them that achieves the maximum potential in the photo. It is not enough to just "trust to the system" and hope for the best. You want a technique that doesn't leave you with stair-step or jaggy images or with unwanted artifacts. Here are your options to resize digital photographs:
- Rescan the original at a higher or lower resolution;
- Go online and use a free online resizing service like PicResize;
- Download your own resizing program - either SmillaEnlarger (free) or BenVista ($);
- Contract a professional photo editor or restorer; or
- Go pro and use a pro photo editing program like Gimp (free); Photoshop Elements ($) or Photoshop ($$$).
When resizing, keep in mind that unless you rescan you are going to be creating brand new pixels pretty much out of thin air. How good the end result looks will depend on the original image and the genius of the resizing algorithm you use.
1. Rescan the Original Photo
If you happen to have the original photograph, and you are unhappy with the size or quality of the digital image, then you should rescan at a bigger size.
These days, computer monitors are getting pretty high resolution. A 27" Apple monitor is 2560 pixels by 1440 pixels. Even smart phones are becoming high resolution: The size of the Apple iPhone 6 screen is 1334 pixels by 750 pixels wide.
So, make sure you clean the glass and set the scanner to about 300 dpi. Click "preview" and adjust those crop lines so you are only getting the photograph. Then "scan" and save as either jpeg or png. If the original image is 6"x4", then the digital file will be 1800 pixels by 1200 pixels - perfect for modern screens. If the image is smaller than 6"x4" then scan at 600 dpi. If it's around 8x10 then scan at 200 dpi. And don't be afraid to play with your scanning options - experimentation is the best way to learn!
Digital images come in two types: "Bitmap" images and "vector" images. "Bitmap" images are the normal images you take with your digital camera or that you scan or download - an example is this old soldier above (killed in WWII, BTW). Bitmap images may be JPEG or PNG or GIFF or TIFF or any one of a number of other proprietary file formats. The critical point of Bitmap images is that they have a defined size - measured in pixels across and pixels down.
Vector images are mostly used in graphic design and have no defined size. Company logos, product packaging and almost all non-photographic commercial images are “vector” images. They are usually complex line, shape and text compositions and are infinitely scalable to any size at all, with the requisite software (like Adobe Illustrator). Resizing vector images presents no problem so long as you are in the creating software.
2. Online Image Resizing
If you are in a hurry, it is hard to beat PicResize. Just browse your computer for the image, upload, then choose a resize setting.
You also have other options like batch editing, adding special effects like black and white, "Polaroid", rounded corners, or frames. You can crop and rotate, flip horizontal or vertical, and save in one of four different formats.
PicResize will rename your file, which may be a nuisance, and you will needs to endure some ads on the site - buy hey, they gotta pay the bills somehow, right? And they don't make you register first or anything like that.
3. Downloadable Photo Resizing Programs
The downloadable photo resizing programs give you a little more privacy and a little more control. SmillaEnlarger is well reviewed, quick to get and easy to use.
With the SmillaEnlarger, you can drag and drop images from your desktop, zoom in to enlarge, choose a custom size, or fit to a particular aspect ratio like 4:3 or 16:9. You can't change the look much, but you can "sharpen" the image if it's a bit soft (or blurred) and reduce noise.
The Smilla interface (shown right) also limits your "save as" options to those that match the file - jpeg or png during my tests.
If you are determined to have your own, desktop-ready image resizing program, and you want superior functionality, you should also consider non-free programs such as BenVista's "Photo Zoom Pro".
Photo Zoom Pro
Here is a pro-grade, photo resizing program. It is also pro-grade in price - $199 at time of writing. Photo Zoom Pro boasts the world's best resizing algorithm - "S-Spline Max" - and has a basket more features than even full strength Photoshop.
Lab tests here show that this program is very good indeed (see below). The question for the casual user will be whether they need such a powerful program, and if they have the need for so many features. For example, Photoshop offers six different algorithms and no fine tuning for its resizing; Photo Zoom Pro offers a choice of twelve algorithms and over twenty sub-adjustments.
4. Use Pro Services
There are many photo technicians who can easily resize an image to maximize the available image elements using professional software (including my own business of photo restorations and repair). If you are reading this article though, you probably want to solve the problem yourself. So keep reading.
The solution that most appeals to me:
5. Pro Grade Photo Editing Software
GIMP, free pro-grade photo editing software (and the subject of an earlier article: "DIY Photo Restorations") offers an excellent tool for photo resizing, with three different algorithms (linear, cubic and "Sinc (Lanczos3)" - used in the above comparison). The final size can be dialed in, and the file can be saved in over 40 different file formats.
Photoshop (and the entry level Photoshop Elements) is what most graphic professionals would use for photo resizing. The test image above shows a very satisfactory result. One clear advantage of Photoshop (or Photoshop Elements) for the average user is that is recommends which algorithm suits an enlargement, which suits a reduction - and what is best if you are working with smooth gradients or hard edges. GIMP and Photo Zoom Pro also have multiple algorithms - but no quick guidance on which to use in what circumstances.