About Image Size

In film photography, understanding the size of an image was simple - it was simply the size of the print. With digital photography, things are a little more complicated. There are at least three different aspects to the "size" of a digital photograph:

•	File Size - Typically measured in megabytes (MB), this measures to the amount of disk or memory card space consumed by the image file.

•	Pixel Size - Measured in pixels or megapixels, this is measures the number of individual pixels or colored dots that comprise the image.

•	Print Size - Measured inches, millimeters, or other physical units of measure. Any time a measure of length, like inches or centimeters, is used to describe a digital image, they are referring to Print Size.

Every digital photograph from a given camera will start out with the exact same Pixel Size, which will be something like 3000x2400 pixles, or about 7 megapixels and corresponds to the number of photo-sites or pixels your camera's sensor has. However, the File Size for different photos from the same camera will vary. This is based on image content and is due to compression techniques used to make the file as small as possible. Generally speaking, a higher megapixel camera will produce larger files.

Print size is determined by two things: Pixel Size and how many pixels to print in a given physical size, commonly called DPI or dots per inch or PPI, pixels per inch. For example, the 3000x2400 pixel image from the example above, when printed at 300 DPI will be exactly 10 inches by 8 inches (3000 pixels / 300 pixels per inch = 10 inches). Changing only the DPI in no way affects the file size on the photo or the pixel size, it only changes the Print Size. For example, changing that same image to 600 DPI, which would yield a 4 inch by 5 print.

DPI and Print Size only matter when printing an photo; DPI is irrelevant to images on screen, in email, or otherwise viewed, edited or used on a computer. Pixel Size alone determines how much information or "quality" in stored in the image file.

Cropping an image is used to re-compose the shot to include only portions of the original image, or to change the Aspect Ratio - the height relative to the width - of the photo. If you have several images from the same camera that you want to print the same size, some after cropping and some without cropping, then the DPI of those images must be different.

Resizing allows you to make the Pixel Size larger or smaller than the original image. To continue the example from above, suppose you cropped a portion of the image from the center of the frame, say 1500 by 1200 pixels. A "Full Size" output image from this cropped image would be 1500 by 1200 pixels - much smaller than your original image. You could still print this image at 10 inches by 8 inches - but that would be at 150 DPI instead of 300. If you wanted to make an output image in Bibble 5 that had the same pixel dimensions as the original file, you would need to upscale - or stretch - the image back to 3000x2400 pixels. This does not add information or quality to the photo, and it will not print significantly better now than before it was resized.

If you print from within Bibble 5, then DPI and Print Size will be handled for you automatically by our Print Output system. If your images are only shown on computer screens, in email or in other electronic forms, then DPI and Print Size are meaningless.

DPI and Print Size only matter when you output images from Bibble to be printed by someone else.