Pixel Dimensions and Resolution:  A digital photograph does not have a fixed print size. Any digital image may be printed at any size, though the larger the print, the lower the resolution. For example, the image created by a 6 megapixel camera (3000 pixels by 2000 pixels) may be printed in the following ways:

7" x 4.6" at a resolution of 430 ppi (pixels per inch)
10" x 6.6" at 300 ppi
42" x 28" at 72 ppi.

In each case, if you multiply inches by pixels, you get roughly 3000 x 2000, or 6 million pixels. The image file on your computer is exactly the same -- the difference is only in the way it is interpreted on paper. Saying that a digital photo has such-and-such a resolution has no meaning unless the print size is also specified.

Minimum Standard for Pixel Dimensions:  The HPO minimum standard for images for both survey and National Register nominations is a pixel dimension of 1950 x 1350. This creates a print of  6.5" x 4.5" at a resolution of 300 ppi (a 7 x 5 print with margins). A 3 megapixel camera should create an image of about 2100 x 1400 pixels, and this is why it is the minimum for survey and National Register work.

File Formats:  Most digital cameras create images in a format called jpeg (identified by the extension .jpg at the end of the filename, and pronounced JAY-peg), and this is the standard for day-to-day digital photography.  There are many other image formats. Image editing software like Photoshop Elements can convert a jpeg file to other formats, and vice-versa.

The National Register requires that nomination photographs be submitted in the TIF format, a more stable format for archival purposes but one that uses many times as much disk space for the same image. If you are comfortable with image editing software, you may convert your nomination images from jpeg to TIF yourself before you submit them. Otherwise the HPO will convert your images to TIFs for you before they are printed for the nomination.

Some high-end cameras have the option of taking the photograph as a TIF. The disadvantage of this is that you won’t get nearly as many pictures on the memory card in your camera before it fills up.

File Size on Disk:  The amount of space an image file takes up on disk is expressed in kilobytes (Kb) or megabytes (Mb). Several factors determine the size of the file, including the image format, the number of pixels, and (for jpeg images) the compression ratio or quality level at which the file is saved.

Most digital cameras have three quality levels: high (or "fine"), medium (or "normal"), and low. A 6 megapixel Nikon D100 camera at the highest setting creates a jpeg photograph of 3008 x 2000 pixels with an average file size of about 2.5 Mb.  The same file converted to the TIF format would be around 18 Mb.  File size increases exponentially as the pixel dimensions increase. All other things being equal, a 3000 x 2000 pixel jpeg image is four times the size of the same image at 1500 x 1000 pixels.  See Using Digital Images in Presentations below for information about reducing image size for PowerPoint and email.

Image Editing and Management Software: A basic image editor/manager is needed to:

1. Rotate, straighten, and crop images for the best composition.
2. Adjust brightness and contrast to maximize the quality of an image
3. Create a version of the image with a smaller file size to use in PowerPoint presentations or to attach to email to share for informational purposes.
4.  Enable batch renaming and/or resizing of many photos simultaneously as a time saver.

Software Options:

Photoshop Elements (about $90) is an affordable version of the professional program Photoshop and performs these and many other operations. It is excellent for editing image quality and size. It will print proofs of six images per sheet, with the filename printed under each image. Its batch renaming function has a drawback in that it limits the size of the new filename.

IrfanView, a free download, is an image editor and with an accompanying manager program call Irfanview Thumbnails. Thumbnails is superior software for renaming large numbers of files and creating proof sheets. It does not limit the length of new filenames in batch renaming, and it enables printing of nine images per page at maximum size with the file name printed under each image. Follow this link to a privately posted site demonstrating IrfanView Thumbnails.  The address may be entered manually as http://70.154.102.35/~mtsouth/irfanview/irfanhome.html .

ACD See  (about $50, $120 for the Pro version) is another image editor and manager used by some consultants; it will print proof sheets with labels.

Picasa is an image manager from Google that is a free download. It will print proofs with at least four per sheet, and has the advantage of giving you the option of not cropping the images when they are printed as proofs. It does not appear to include file names or captions.

Windows Picture and Fax Viewer, a program included with the XP operating system for quick viewing of photographs, is neither an image editor or file manager, but it does provide for printing proof sheets, with four or six images per page, but without file names or captions. It crops photos slightly to make them fit the format.

Backing Up Image Files:  After you copy your image files from your camera to your computer, you will need to delete the files from the camera's memory card to make room for more pictures. If the files on your computer's hard drive are your only copies, there is a danger that you could lose all of your photographs permanently in the event your hard drive crashes. Make periodic backups of your pictures onto CDs, DVDs, or an external hard drive that connects to your computer with a USB cable. External hard drives have become quite affordable -- around $1 per gigabyte of storage.

II. Digital Images in Presentations

Printing Versus Screen Presentation:  As a general rule, you want the most pixels your camera can create for printing the best quality images on paper with the highest level of detail. The larger the pixel dimensions of your image, the more detail it will contain, and the better will be prints made from it.  Printers can typically print at 600 or 720 dots per inch, which means that every pixel will be distinguished, giving the greatest detail in the print.

But it’s different when you look at a photo on your computer screen. Most screens display only 72 pixels per inch, and a typical display is only 1024 x 768 pixels. This means that if you display a photo that is 3000 x 2000 pixels on your screen so you can see the whole photo, your computer selectively eliminates many of the pixels from what it displays. If you tell it to display every pixel, you will see only a portion of the photo. If you make a copy of the photo and use image editing software to reduce the size to 1000 x 666 pixels, it will look exactly like the original version on the screen when displayed whole, but will be only about one ninth the size of the original.  However, you would see a loss in quality in a print made from the smaller image.

The Problem of Image Size in PowerPoint Presentations: This is important to understand when you are using photos in PowerPoint presentations or sending them as attachments to email when you are sharing them for informational purposes. The smaller images take much less disk space and load faster, but look the same on the screen. This means you may want to have two versions of some of your images – one for prints, and one for presentations and email. Images taken directly from a digital camera and placed in a PowerPoint Presentation will quickly swell the size of the PowerPoint file if it contains very many images. An image taken directly from the camera may be 2 Mb or larger and a presentation with 50 images easily exceeds 100 Mb. If the PowerPoint presentation is too large, it may take a long time to load and will run slowly. Older computers or laptops may fail to run the presentation at all, or may crash. This can be a matter of concern if you are taking a presentation on CD to run on someone else's equipment at a public meeting.

For PowerPoint presentations, photographs don't need to be over 1000 x 700 pixels because the resolution of most computer screens and digital projectors is 1024 x 768 pixels. So if your original image is 3000 x 2000 pixels, a copy that is no larger than 1000 x 666 pixels for PowerPoint will look as good as the original image when projected. The smaller versions can also be used as attachments to email. They take up less space on recipient's email server, download more quickly, and the recipient can see the complete image on the monitor in their email without having to copy the image to their computer to view in other software.

Using PowerPoint to Reduce the Size of the Images: It is possible to reduce the size of photographs from within PowerPoint after all of the photographs have been loaded. This is done with the program's Compress Pictures function (Office 2003 version and later). This can greatly reduce the overall size of the presentation without reducing the quality. The changes will not affect your original photographs. This method gives you limited control and will not provide you with reduced versions of the photos to use for other purposes unless you copy the images back out of PowerPoint one at a time and name them as you save them.

To access the Compress Picture function, click on any photo in the presentation to select it.  Then go to the photo editing toolbar (which also includes brightness, cropping, and other controls) and find and click the Compress Pictures icon. Select “all pictures in document” and set the resolution for “web/screen.”  Then click OK.

Using Photoshop Elements (or Other Image Editing Software) to Reduce Image Size

Do not resize your original image. Open the original image, and use “Save As” to copy it as new file with a slightly different name. When you save a jpeg image, you will be asked what quality level you would like, with a range between 1 and 12. Select 7 or 8, which is high enough quality but which further reduces the size of the file.

Modify the file size of the copy. This is most easily done by changing one of the pixel dimensions, for example changing the width from 3000 to 1000 pixels, making sure to keep the width-to-height ratio constant.

In Photoshop Elements, with the photo displayed, select Image, Resize, Image Size. In the next window, make sure the Resample Image box is checked. Then reduce the pixel width or height, and save the file.

Elements also enables you to resize many photos at once with the Process Multiple Files function in the File menu.  You should use this function before you otherwise alter any of the images in the target folder, such as changing the orientation from landscape to portrait for vertically oriented photos.  The batch process will apply the same pixel width dimension to every photo in the folder.