Aspect Ratio, Image size and Cropping for Print
A subject that causes a lot of confusion is aspect ratio, image size and resolution. Understanding these concepts is key to being able to size images correctly for printing.
Pixels and PPI
Lets start with image size. This is the physical number of pixels that the image has, say 800 pixels on the long edge and 600 pixels on the short edge so the image size would be 800x600 which would be 480,000 pixels. If you like this is the amount of stuff that your image contains.
Now lets look at resolution. The resolution of an image is referred to in terms of ppi or pixels per inch. This is referring to how many pixels will be used in every inch of the output print. It has no bearing on the size of the image existing on the computer.
If you export and image at a resolution of 72ppi then export the same image with a resolution of 300ppi both images will be identical in image size.
The images below have been exported with different resolutions, 72 and 300ppi but the image size has not changed. If you are saving an image for use on the internet then resolution is not important. If you are saving an image for print then it is important as this figure is embedded in the file for use by the printer in determining how big a print will be.
For example of you have an image 900 pixels x 600 pixels with a print resolution of 300ppi then the final print size will be 900/300 = 3 inches wide by 600/300 = 2 inches tall.
When you crop an image you are throwing away pixels and making the image smaller, this will impact on the maximum print you can achieve. A common recommendation for a good print resolution is 300ppi. So an image with say 3000 x 2400 pixels would have a dpi of 300 when printed at 10 x 8(3000/10=300, 2400/8 = 300). For a 20x16 print the resolution would fall to 150ppi(3000/20 = 150, 2400/16 = 150)
Before printing always make sure that you have enough resolution to make your prints look great.
Aspect Ratio is a comparison of the length and width of your image. For most DSLR cameras this ratio is 3:2. For most point and shoot cameras this ratio is 4:3. This is a ratio, it is not related to physical pixels in an image.
If you don’t know the aspect ratio of your camera you can work it out from your image dimensions. For example my camera has image dimensions of 5760 pixels x 3840 pixels. So my aspect ratio is 5760/3840 = 1.5.
It is important to know the aspect ratio because different print sizes have different aspect ratios.
If your camera shoots with an aspect ratio of 3: 2 and you want to print your photograph at 6 x 4 inch then your printed image will match your original image.(3:2 ratio is the same as 6:4)
However if you wanted to print at say 10 x 8 inch, this is an aspect ratio of 10/8 = 1.25. The original image would become a 12 x 8 inch print
On the image below you can see this would mean losing two inches or an inch from each side of the image.
This can be important if important if key parts of your image are close to the edge as they may get cropped off.
The effect of aspect ratio is even more pronounced if you want to print a canvas.
In the example above we would lose the shaded areas with the crop, but also the area outside the black line would be needed to wrap around the sides of the canvas so here part of the cottage and the chimney would wrap around the edge, which would not look good.
So shooting for canvases it is important to leave plenty of space around your main subject matter when you frame up your shot.