In digital photography, most of us have settings on our cameras allowing us to select the quality of the images we record.

Some cameras allow you to record Small, Medium or Large JPEGs, plus one size Raw.  Other cameras allow you to choose from multiple Raw sizes as well.

My Canon 7D offers the following settings for Raw or JPEG files:

  • 18 megapixels – 5184 x 3456 pixels
  • 10 megapixels – 3888×2592 pixels
  • 8 megapixels – 3456×2304 pixels
  • 4.5 megapixels – 2592×1728 pixels

Note how the sizes are calculated:  5184×3456=17.9 million pixels.  Rounded up, that’s 18 megapixels.  That number is simply a count of the number of pixels (or data points) recorded by your camera for each image.  It’s not the same as the image’s weight (the space it takes up on your hard drive).  Each of my 18 megapixel images weights somewhere around 25 megabytes.

When you export an image from your camera, it doesn’t have an inherent size in inches – it does have an aspect ratio, however.  Each of those pixel calculations above has the same aspect ratio – 2×3.  That means that the short side is 2/3 the length of the long size.

And you can print those images without cropping on any sized paper that has the same 2:3 aspect ratio.  Like:

  • 2×3
  • 4×6
  • 8×12
  • 16×24
  • 24×36

If I had an 18 megapixel image that I wanted to print at 16×24, I would calculate the pixels per inch by dividing the number of pixels on one side by the number of inches I wanted that side to be.  So, 5184 pixels divided by 24 inches equals 216 pixels per inch – that resolution is not bad, and the print would probably look pretty good.  (Dots per inch is technically more correct than pixels per inch when referring to a printed image, but that’s another story.)

If, however, I had started with the smallest 4.5 megapixel file, dividing 2592 pixels by 24 inches would give me 108 pixels per inch on the print.  And that is a relatively low resolution – the pixels on the printed image would probably be large enough to see, giving the image a pixelated look.

Ok, so you get it now.  Take a large-sized photo if you want a large print, right?

But that’s not the end of the story.  What happens if you need to zoom in?  A lot?  Like from this:

To this:

You’ve got to have lots of megapixels in your image to make that work.  After cropping, my 18 megapixel image went down to 2 megapixels.  If I want to print that image, my maximum print size is probably going to be 4×6.  How do I know?  Well, the long size is 1018 pixels tall after cropping – divide that by 6 inches and and I get 170 pixels per inch.  Even that won’t be the best quality print.

That’s ok though – cropped to that level, the noise and blur is really obvious in the image anyway, so it’s not something I’d want to print.

As a matter of habit, I always record Large Raw images.  But I don’t always take advantage of all those pixels.  However, this is one situation where I’m using every last one of them.  So, if you will be shooting in a situation when you know you’re going to crop your images down significantly, make sure you are recording your images at the highest possible on camera quality.

Need more help with cropping, resizing and aspect ratio?  These articles might help: