File Formats for Output
JPG - The jpg file as an output format allows the photographer to compress the file information in 12 optional quality settings after other processing steps have been taken in the editing software. Compressing the information, even in the highest quality setting, substantially reduces the size of the file. This makes the format popular for web and email images, but less desirable as an archiving medium as it removes image information in the process of compression. Even so, files for output to the printer can be made into jpg files as the initial step reveals little loss either on screen or on a print. Jpg files should be made in addition to full sized Tif files or native Photoshop PSD files for archiving purposes, not in place of them.
TIF - A preferred format for archiving and transfer of files as all of the original file information is retained. While a lossless compression option is available, it is rarely used for files to be transferred to another person as it may be less compatible with the receiver's software. Otherwise, it is the most usable file format. A tif file contains as much information as possible. Note that there can be layered tif files. For distribution to others an uncompressed flattened tif is preferable.
GIF - A file with limited (256) color used primarily for graphics, and logos. A well made jpg may actually be smaller and will be definitely better at rendering the color and continuous tones of a photograph. Gif files are less used than in the past except for some web purposes like logos.
PSD - An Adobe format, literally a PhotoShop Document, that preserves the state of the information in a file being edited, including layers and channels. This makes it possible to reopen and continue the editing process at another time. Not generally a format shared with others, but primarily a production format. Very useful for the photographer. Maximize compatibility should be set to always in file handling preferences to facilitate transfer to Lightroom. The image (right) shows a portion of a layer stack typical of a psd file.
PNG - The png was ignored for a long time as it was initally seen as a GIF replacement. However, it supports transparency and 24 bit color making it more image friendly than the GIF. The logos on my web site are PNG files as they are against transparent backgrounds and can be placed over any color without having a field of their own around the text.
Other Formats - There are other special formats available for specific needs that are of little value to everyday photography, and you may never use more than what are mentioned above. Outside of the above formats, there are also proprietary formats that come with some imaging software provided free by some less expensive cameras. They are best avoided as they will not be usable by others, and may not even be usable by you as you move into better software. Keep it simple and stick with the formats described above.
Output resolution for monitor display
If our output is the monitor, such as an image for the web or an email, we only need to be concerned with how it will display on the monitor. If your monitor resolution is actually 72 ppi, a 4 x 6 inch image on screen would only have to be 288 x 432 pixels. On a monitor with a 100 ppi resolution a 4 x 6 inch image would have to be 400 x 600 pixels. Compared to an original image captured by most current digital cameras this is a very small image. Now you know why yourimages directly from the camera look huge on your monitor (displayed at 100%, or full size). If your image pixel size is larger than your monitor resolution it will display bigger than the screen, unless you are viewing it with software that reduces the image size. An image embedded in an email will display at full size unless the email software renders it differently. Current iPhones, for example, will resize images to fit the screen, but older phones may not.
We can also modify the pixel resolution of our images with imaging programs to make the image size more appropriate to the intention of the image. For emails and monitor display, such as web pages, use either absolute pixel numbers or calculate the approximate size needed using any convenient number for monitor resolution (100 ppi is really easy) and you will be close enough. Precision is not necessary.
Output Resolution for Printing
Printing an image on a desktop printer, sending it to a kiosk for printing or providing an image for professional offset printing requires looking at the file in a different way. Now we need to attach the output resolution numbers to the image to determine how large the image will be. The more you know about the device that will used to make the print, the better you will be able to provide an image of the appropriate size. This will mean the best quality print will be produced from your image file. The key is to think in terms of the size of the image in pixels.
Our theoretical camera capture provided us with 2000 x 3000 pixels. If we need to provide someone with a 300 ppi (remember that means pixels per inch) image, we need to divide the number of pixels by 300 to get the numbers of inches the file will provide. 2000 / 300 = 6.666 and 3000 / 300 = 10, so our 6 Mp camera will provide an image 6.666 x 10 inches at 300 ppi. By comparison, a 72 ppi monitor display of that image was 2000 / 72 =27.777 x 3000 / 72 = 41.666 inches. An Epson printer can be used at 240 ppi or 360 ppi, and the kiosk at your local Costco sends files to the printer at 300 or 320 ppi. Dpi (dots per inch) is often used to describe what should be called ppi. Dpi is a description of the screen resolution of offset printing plates. Since printers typically request images with a resolution relative to their printing plates, photographers have incorrectly come to use the terms interchangeably.
So, the same number of pixels results in different size images depending on the output resolution, which is the resolution of the device on which the file will be displayed or printed. A smaller file with fewer pixels will produce a smaller image. Our on screen 4x6 only had to be 288 x 432 pixels. If we try to print that file at 300 ppi it will only be .96 by 1.44 inches. The number of pixels doesn't change, just the relative size of the pixels depending on the output device, and therefore the size in inches. How does a printer like Costco change the size of your file to make different sized prints if the print size is a matter of how many pixels you provide? The answer is software, and the printer at the store can interpolate or modify the file to satisfy your order for larger or smaller prints, just as you can with imaging software. In their case the software for their printers is much more sophisticated and resizing images is easily handled by the printer. This is accomplished by varying the resolution of the image sent to the printer, or an interpolation of the pixeks by sophisticated software called a rip (rasterized image processor).
The two images to the right represent the same file seen in two different resolutions. Note that the pixel dimensions and the image size are the same in both. By changing the resolution we change only the effective width and height in inches. Note that the Resample box is NOT checked meaning we do not want to modify the file. The three factors Width, Height, and Resolution are shown locked together indicating that changing one will change the others, but the image dimensions in pixels will remain the same.
Resizing images yourself
First, let me emphasize that making larger or smaller images from your file should be left to the printer. Most fine art photographers currently print using Lightroom, which has very sophisticated printing capabilities. Outside printing services may or may not use Lightroom, but will provide the best print if you follow the same basic concepts in cropping your images.
This means you should crop to the aspect ratio you need and not the specific size of the image. For example, a 4x5 aspect ratio will give you output files of 8x10 or 16x20 as they respect the 4x5 aspect ratio. Intermediate sizes will also respect the ratio, so a print specified to be 16 inches wide will print 12.8 inches tall.
The crop tool in Photoshop offers you cropping Ratios as the first choice. These represent most commonly used ratios so if you need to conform to something specific the options are already there for you. If you do not care what the aspect ratio is you can simply click the clear button to the right and the crop tool becomes a free form tool that allows you to make whatever rectangular shape you feel is appropriate to your image. This is how I crop my images as aspect ratio is of little concern to me.
Your inclination is to crop to a specific size and resolution, especially if you have been asked to provide an image to someone. Unfortunately, most people do not fully understand resolution and even if you give them a file that is accurate in terms of pixel dimensions, if the specified resolution is not what they expect they will challenge you on the file. Exporting files to customers or anyone requesting an image from you may require that you crop to specific dimensions and include a resolution. The second choice down in the crop tool allows you to make those specifications is you need them. Be careful here as common terminology can be misleading. Giving someone " an 8x10 of that picture" does not specifically mean 8x10 inches but is rather a means of telling you approximately what they expect in terms of size. Nobody asks for an 8 and a half by 11 when they want a copy of a text document, but you know what they mean.
Resizing images for specific purposes, such as inclusion in a website, is a different story. There you need to work with specific pixel dimensions, but again, the resolution is not important. While 72 ppi is assumed by most designers for the web, the only dimensions specified in the code are the width and height in pixels, never the resolution. The displayed size will be accurate in terms of pixels but will actually vary slightly based on the resolution of the monitor.
If you learn to understand images in terms of the pixel dimensions you will be ahead of the game. If you need to talk in terms of inches, for example, simply multiply the inches by the desired resolution to determine the number of pixels needed. A 4x6 inch print at 300 ppi therefore requires 1200 x 1800 pixels. It is rather simple once you think about it a little.