Monday, October 09, 2006

Simplifying Maximum Print Size

One thing that almost always earns respect from me is when a seemingly complex concept is simplified or clarified in a way that gives normal laypeople like me the 'a-ha!' moment. The converse of course, is taking a relatively simple concept and overcomplicating it. I came across one such concept in my feed aggregator today - it involved printing.

The core concept of the article was about figuring out the maximum print size that can be made given a digital camera's megapixel rating. There's a bit of a meaty write-up accompanied by a colourful chart correlating print size to megapixel rating for a given print resolution. There is a significant amount of supporting information provided (none of which is wrong).

But does it all have to be so complicated? There are only 3 simple steps between you and the answer:

1. Find out the camera's maximum image size (in pixels). This is infinitely more useful than the camera's megapixel rating.

2. Pick a print resolution between 300 and 200 ppi (pixels-per-inch). 300 is generally accepted as 'true' photographic quality. Going higher than 300 gains you nothing except increased filesize. You can go below 300 but the lower you go, the greater the chances that you will see the pixels of your image on the final print. I've printed with perfectly satisfactory results down at 200 ppi, but 240ppi is my normal bottom limit.

3. Divide your maximum image size dimensions by your chosen print resolution to get the maximum print size.


Step 1. My Canon Rebel XT has a maximum image size of 3456x2304 pixels.

Step 2. I'm going to take 300 ppi as my desired print resolution.

Step 3.
            3456/300 = 11.52in
            2304/300 =  7.68in

So my maximum print size for 300ppi is 11.52in x 7.68in. If I want a bigger image, I've got to use a lower print resolution (like 240ppi).

Further notes:

While the above steps are useful if you are searching for the maximum print size a given camera can make, it becomes much more practical when you want to crop one of your original photos and still need to assure yourself that you'll be able to make a serviceable print out of it. This is how I approach it:

- I'd have an idea of how I want to crop the photo. Maybe it's to remove some telephone poles, or remove some litter in the foreground, or maybe even to improve the composition of the photograph in some way.

- I'd decide on what maximum final print size I'd like to make of the shot. Let's say it's 5x7 inches.

- I'd pick a print resolution and do the math. Let's say I'm perfectly happy with 240 ppi. So 5x240 = 1200, and 7x240 = 1680. So I know that the final size of the cropped image better be at least 1200pixels x 1680pixels.

Also, don't be fooled by printer specs like 5680x2880 resolution. This is the resolution of the nozzle dots, many of which overlap to form colours on your print. What you care about is the pixel size of the image you're printing. This is what determines the maximum print size you can make with it and still maintain photo quality.